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/* Malloc implementation for multiple threads without lock contention.
Copyright (C) 1996-2017 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Wolfram Gloger <>
and Doug Lea <>, 2001.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2.1 of the
License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; see the file COPYING.LIB. If
not, see <>. */
This is a version (aka ptmalloc2) of malloc/free/realloc written by
Doug Lea and adapted to multiple threads/arenas by Wolfram Gloger.
There have been substantial changes made after the integration into
glibc in all parts of the code. Do not look for much commonality
with the ptmalloc2 version.
* Version ptmalloc2-20011215
based on:
VERSION 2.7.0 Sun Mar 11 14:14:06 2001 Doug Lea (dl at gee)
* Quickstart
In order to compile this implementation, a Makefile is provided with
the ptmalloc2 distribution, which has pre-defined targets for some
popular systems (e.g. "make posix" for Posix threads). All that is
typically required with regard to compiler flags is the selection of
the thread package via defining one out of USE_PTHREADS, USE_THR or
USE_SPROC. Check the thread-m.h file for what effects this has.
Many/most systems will additionally require USE_TSD_DATA_HACK to be
defined, so this is the default for "make posix".
* Why use this malloc?
This is not the fastest, most space-conserving, most portable, or
most tunable malloc ever written. However it is among the fastest
while also being among the most space-conserving, portable and tunable.
Consistent balance across these factors results in a good general-purpose
allocator for malloc-intensive programs.
The main properties of the algorithms are:
* For large (>= 512 bytes) requests, it is a pure best-fit allocator,
with ties normally decided via FIFO (i.e. least recently used).
* For small (<= 64 bytes by default) requests, it is a caching
allocator, that maintains pools of quickly recycled chunks.
* In between, and for combinations of large and small requests, it does
the best it can trying to meet both goals at once.
* For very large requests (>= 128KB by default), it relies on system
memory mapping facilities, if supported.
For a longer but slightly out of date high-level description, see
You may already by default be using a C library containing a malloc
that is based on some version of this malloc (for example in
linux). You might still want to use the one in this file in order to
customize settings or to avoid overheads associated with library
* Contents, described in more detail in "description of public routines" below.
Standard (ANSI/SVID/...) functions:
malloc(size_t n);
calloc(size_t n_elements, size_t element_size);
free(void* p);
realloc(void* p, size_t n);
memalign(size_t alignment, size_t n);
valloc(size_t n);
mallopt(int parameter_number, int parameter_value)
Additional functions:
independent_calloc(size_t n_elements, size_t size, void* chunks[]);
independent_comalloc(size_t n_elements, size_t sizes[], void* chunks[]);
pvalloc(size_t n);
cfree(void* p);
malloc_trim(size_t pad);
malloc_usable_size(void* p);
* Vital statistics:
Supported pointer representation: 4 or 8 bytes
Supported size_t representation: 4 or 8 bytes
Note that size_t is allowed to be 4 bytes even if pointers are 8.
You can adjust this by defining INTERNAL_SIZE_T
Alignment: 2 * sizeof(size_t) (default)
(i.e., 8 byte alignment with 4byte size_t). This suffices for
nearly all current machines and C compilers. However, you can
define MALLOC_ALIGNMENT to be wider than this if necessary.
Minimum overhead per allocated chunk: 4 or 8 bytes
Each malloced chunk has a hidden word of overhead holding size
and status information.
Minimum allocated size: 4-byte ptrs: 16 bytes (including 4 overhead)
8-byte ptrs: 24/32 bytes (including, 4/8 overhead)
When a chunk is freed, 12 (for 4byte ptrs) or 20 (for 8 byte
ptrs but 4 byte size) or 24 (for 8/8) additional bytes are
needed; 4 (8) for a trailing size field and 8 (16) bytes for
free list pointers. Thus, the minimum allocatable size is
16/24/32 bytes.
Even a request for zero bytes (i.e., malloc(0)) returns a
pointer to something of the minimum allocatable size.
The maximum overhead wastage (i.e., number of extra bytes
allocated than were requested in malloc) is less than or equal
to the minimum size, except for requests >= mmap_threshold that
are serviced via mmap(), where the worst case wastage is 2 *
sizeof(size_t) bytes plus the remainder from a system page (the
minimal mmap unit); typically 4096 or 8192 bytes.
Maximum allocated size: 4-byte size_t: 2^32 minus about two pages
8-byte size_t: 2^64 minus about two pages
It is assumed that (possibly signed) size_t values suffice to
represent chunk sizes. `Possibly signed' is due to the fact
that `size_t' may be defined on a system as either a signed or
an unsigned type. The ISO C standard says that it must be
unsigned, but a few systems are known not to adhere to this.
Additionally, even when size_t is unsigned, sbrk (which is by
default used to obtain memory from system) accepts signed
arguments, and may not be able to handle size_t-wide arguments
with negative sign bit. Generally, values that would
appear as negative after accounting for overhead and alignment
are supported only via mmap(), which does not have this
Requests for sizes outside the allowed range will perform an optional
failure action and then return null. (Requests may also
also fail because a system is out of memory.)
Thread-safety: thread-safe
Compliance: I believe it is compliant with the 1997 Single Unix Specification
Also SVID/XPG, ANSI C, and probably others as well.
* Synopsis of compile-time options:
People have reported using previous versions of this malloc on all
versions of Unix, sometimes by tweaking some of the defines
below. It has been tested most extensively on Solaris and Linux.
People also report using it in stand-alone embedded systems.
The implementation is in straight, hand-tuned ANSI C. It is not
at all modular. (Sorry!) It uses a lot of macros. To be at all
usable, this code should be compiled using an optimizing compiler
(for example gcc -O3) that can simplify expressions and control
paths. (FAQ: some macros import variables as arguments rather than
declare locals because people reported that some debuggers
otherwise get confused.)
Compilation Environment options:
Changing default word sizes:
Configuration and functionality options:
Options for customizing MORECORE:
Tuning options that are also dynamically changeable via mallopt:
DEFAULT_MXFAST 64 (for 32bit), 128 (for 64bit)
There are several other #defined constants and macros that you
probably don't want to touch unless you are extending or adapting malloc. */
void* is the pointer type that malloc should say it returns
#ifndef void
#define void void
#endif /*void*/
#include <stddef.h> /* for size_t */
#include <stdlib.h> /* for getenv(), abort() */
#include <unistd.h> /* for __libc_enable_secure */
#include <_itoa.h>
#include <atomic.h>
#include <bits/wordsize.h>
#include <sys/sysinfo.h>
#include <ldsodefs.h>
#include <errno.h>
#include <stdio.h> /* needed for malloc_stats */
#include <unistd.h>
#include <shlib-compat.h>
/* For uintptr_t. */
#include <stdint.h>
/* For va_arg, va_start, va_end. */
#include <stdarg.h>
/* For MIN, MAX, powerof2. */
#include <sys/param.h>
/* For ALIGN_UP et. al. */
#include <libc-internal.h>
#include <malloc/malloc-internal.h>
Because freed chunks may be overwritten with bookkeeping fields, this
malloc will often die when freed memory is overwritten by user
programs. This can be very effective (albeit in an annoying way)
in helping track down dangling pointers.
If you compile with -DMALLOC_DEBUG, a number of assertion checks are
enabled that will catch more memory errors. You probably won't be
able to make much sense of the actual assertion errors, but they
should help you locate incorrectly overwritten memory. The checking
is fairly extensive, and will slow down execution
noticeably. Calling malloc_stats or mallinfo with MALLOC_DEBUG set
will attempt to check every non-mmapped allocated and free chunk in
the course of computing the summmaries. (By nature, mmapped regions
cannot be checked very much automatically.)
Setting MALLOC_DEBUG may also be helpful if you are trying to modify
this code. The assertions in the check routines spell out in more
detail the assumptions and invariants underlying the algorithms.
Setting MALLOC_DEBUG does NOT provide an automated mechanism for
checking that all accesses to malloced memory stay within their
bounds. However, there are several add-ons and adaptations of this
or other mallocs available that do this.
#define MALLOC_DEBUG 0
#ifdef NDEBUG
#define assert(expr) ((void) 0)
#define assert(expr) \
((expr) ? ((void) 0) : __malloc_assert(#expr, __FILE__, __LINE__, __func__))
extern const char *__progname;
static void __malloc_assert(const char *assertion, const char *file,
unsigned int line, const char *function) {
(void) __fxprintf(NULL, "%s%s%s:%u: %s%sAssertion `%s' failed.\n",
__progname, __progname[ 0 ] ? ": " : "", file, line,
function ? function : "", function ? ": " : "",
REALLOC_ZERO_BYTES_FREES should be set if a call to
realloc with zero bytes should be the same as a call to free.
This is required by the C standard. Otherwise, since this malloc
returns a unique pointer for malloc(0), so does realloc(p, 0).
TRIM_FASTBINS controls whether free() of a very small chunk can
immediately lead to trimming. Setting to true (1) can reduce memory
footprint, but will almost always slow down programs that use a lot
of small chunks.
Define this only if you are willing to give up some speed to more
aggressively reduce system-level memory footprint when releasing
memory in programs that use many small chunks. You can get
essentially the same effect by setting MXFAST to 0, but this can
lead to even greater slowdowns in programs using many small chunks.
TRIM_FASTBINS is an in-between compile-time option, that disables
only those chunks bordering topmost memory from being placed in
/* Definition for getting more memory from the OS. */
#define MORECORE (*__morecore)
void *__default_morecore(ptrdiff_t);
void *(*__morecore)(ptrdiff_t) = __default_morecore;
#include <string.h>
MORECORE-related declarations. By default, rely on sbrk
MORECORE is the name of the routine to call to obtain more memory
from the system. See below for general guidance on writing
alternative MORECORE functions, as well as a version for WIN32 and a
sample version for pre-OSX macos.
#ifndef MORECORE
#define MORECORE sbrk
MORECORE_FAILURE is the value returned upon failure of MORECORE
as well as mmap. Since it cannot be an otherwise valid memory address,
and must reflect values of standard sys calls, you probably ought not
try to redefine it.
If MORECORE_CONTIGUOUS is true, take advantage of fact that
consecutive calls to MORECORE with positive arguments always return
contiguous increasing addresses. This is true of unix sbrk. Even
if not defined, when regions happen to be contiguous, malloc will
permit allocations spanning regions obtained from different
calls. But defining this when applicable enables some stronger
consistency checks and space efficiencies.
Define MORECORE_CANNOT_TRIM if your version of MORECORE
cannot release space back to the system when given negative
arguments. This is generally necessary only if you are using
a hand-crafted MORECORE function that cannot handle negative arguments.
/* MORECORE_CLEARS (default 1)
The degree to which the routine mapped to MORECORE zeroes out
memory: never (0), only for newly allocated space (1) or always
(2). The distinction between (1) and (2) is necessary because on
some systems, if the application first decrements and then
increments the break value, the contents of the reallocated space
are unspecified.
MMAP_AS_MORECORE_SIZE is the minimum mmap size argument to use if
sbrk fails, and mmap is used as a backup. The value must be a
multiple of page size. This backup strategy generally applies only
when systems have "holes" in address space, so sbrk cannot perform
contiguous expansion, but there is still space available on system.
On systems for which this is known to be useful (i.e. most linux
kernels), this occurs only when programs allocate huge amounts of
memory. Between this, and the fact that mmap regions tend to be
limited, the size should be large, to avoid too many mmap calls and
thus avoid running out of kernel resources. */
#define MMAP_AS_MORECORE_SIZE (1024 * 1024)
Define HAVE_MREMAP to make realloc() use mremap() to re-allocate
large blocks.
#define HAVE_MREMAP 0
/* We may need to support __malloc_initialize_hook for backwards
compatibility. */
#if SHLIB_COMPAT(libc, GLIBC_2_0, GLIBC_2_24)
This version of malloc supports the standard SVID/XPG mallinfo
routine that returns a struct containing usage properties and
statistics. It should work on any SVID/XPG compliant system that has
a /usr/include/malloc.h defining struct mallinfo. (If you'd like to
install such a thing yourself, cut out the preliminary declarations
as described above and below and save them in a malloc.h file. But
there's no compelling reason to bother to do this.)
The main declaration needed is the mallinfo struct that is returned
(by-copy) by mallinfo(). The SVID/XPG malloinfo struct contains a
bunch of fields that are not even meaningful in this version of
malloc. These fields are are instead filled by mallinfo() with
other numbers that might be of interest.
/* ---------- description of public routines ------------ */
malloc(size_t n)
Returns a pointer to a newly allocated chunk of at least n bytes, or null
if no space is available. Additionally, on failure, errno is
set to ENOMEM on ANSI C systems.
If n is zero, malloc returns a minumum-sized chunk. (The minimum
size is 16 bytes on most 32bit systems, and 24 or 32 bytes on 64bit
systems.) On most systems, size_t is an unsigned type, so calls
with negative arguments are interpreted as requests for huge amounts
of space, which will often fail. The maximum supported value of n
differs across systems, but is in all cases less than the maximum
representable value of a size_t.
void *__libc_malloc(size_t);
free(void* p)
Releases the chunk of memory pointed to by p, that had been previously
allocated using malloc or a related routine such as realloc.
It has no effect if p is null. It can have arbitrary (i.e., bad!)
effects if p has already been freed.
Unless disabled (using mallopt), freeing very large spaces will
when possible, automatically trigger operations that give
back unused memory to the system, thus reducing program footprint.
void __libc_free(void *);
calloc(size_t n_elements, size_t element_size);
Returns a pointer to n_elements * element_size bytes, with all locations
set to zero.
void *__libc_calloc(size_t, size_t);
realloc(void* p, size_t n)
Returns a pointer to a chunk of size n that contains the same data
as does chunk p up to the minimum of (n, p's size) bytes, or null
if no space is available.
The returned pointer may or may not be the same as p. The algorithm
prefers extending p when possible, otherwise it employs the
equivalent of a malloc-copy-free sequence.
If p is null, realloc is equivalent to malloc.
If space is not available, realloc returns null, errno is set (if on
ANSI) and p is NOT freed.
if n is for fewer bytes than already held by p, the newly unused
space is lopped off and freed if possible. Unless the #define
REALLOC_ZERO_BYTES_FREES is set, realloc with a size argument of
zero (re)allocates a minimum-sized chunk.
Large chunks that were internally obtained via mmap will always
be reallocated using malloc-copy-free sequences unless
the system supports MREMAP (currently only linux).
The old unix realloc convention of allowing the last-free'd chunk
to be used as an argument to realloc is not supported.
void *__libc_realloc(void *, size_t);
memalign(size_t alignment, size_t n);
Returns a pointer to a newly allocated chunk of n bytes, aligned
in accord with the alignment argument.
The alignment argument should be a power of two. If the argument is
not a power of two, the nearest greater power is used.
8-byte alignment is guaranteed by normal malloc calls, so don't
bother calling memalign with an argument of 8 or less.
Overreliance on memalign is a sure way to fragment space.
void *__libc_memalign(size_t, size_t);
valloc(size_t n);
Equivalent to memalign(pagesize, n), where pagesize is the page
size of the system. If the pagesize is unknown, 4096 is used.
void *__libc_valloc(size_t);
mallopt(int parameter_number, int parameter_value)
Sets tunable parameters The format is to provide a
(parameter-number, parameter-value) pair. mallopt then sets the
corresponding parameter to the argument value if it can (i.e., so
long as the value is meaningful), and returns 1 if successful else
0. SVID/XPG/ANSI defines four standard param numbers for mallopt,
normally defined in malloc.h. Only one of these (M_MXFAST) is used
in this malloc. The others (M_NLBLKS, M_GRAIN, M_KEEP) don't apply,
so setting them has no effect. But this malloc also supports four
other options in mallopt. See below for details. Briefly, supported
parameters are as follows (listed defaults are for "typical"
Symbol param # default allowed param values
M_MXFAST 1 64 0-80 (0 disables fastbins)
M_TRIM_THRESHOLD -1 128*1024 any (-1U disables trimming)
M_TOP_PAD -2 0 any
M_MMAP_THRESHOLD -3 128*1024 any (or 0 if no MMAP support)
M_MMAP_MAX -4 65536 any (0 disables use of mmap)
int __libc_mallopt(int, int);
Returns (by copy) a struct containing various summary statistics:
arena: current total non-mmapped bytes allocated from system
ordblks: the number of free chunks
smblks: the number of fastbin blocks (i.e., small chunks that
have been freed but not use resused or consolidated)
hblks: current number of mmapped regions
hblkhd: total bytes held in mmapped regions
usmblks: always 0
fsmblks: total bytes held in fastbin blocks
uordblks: current total allocated space (normal or mmapped)
fordblks: total free space
keepcost: the maximum number of bytes that could ideally be released
back to system via malloc_trim. ("ideally" means that
it ignores page restrictions etc.)
Because these fields are ints, but internal bookkeeping may
be kept as longs, the reported values may wrap around zero and
thus be inaccurate.
struct mallinfo __libc_mallinfo(void);
pvalloc(size_t n);
Equivalent to valloc(minimum-page-that-holds(n)), that is,
round up n to nearest pagesize.
void *__libc_pvalloc(size_t);
malloc_trim(size_t pad);
If possible, gives memory back to the system (via negative
arguments to sbrk) if there is unused memory at the `high' end of
the malloc pool. You can call this after freeing large blocks of
memory to potentially reduce the system-level memory requirements
of a program. However, it cannot guarantee to reduce memory. Under
some allocation patterns, some large free blocks of memory will be
locked between two used chunks, so they cannot be given back to
the system.
The `pad' argument to malloc_trim represents the amount of free
trailing space to leave untrimmed. If this argument is zero,
only the minimum amount of memory to maintain internal data
structures will be left (one page or less). Non-zero arguments
can be supplied to maintain enough trailing space to service
future expected allocations without having to re-obtain memory
from the system.
Malloc_trim returns 1 if it actually released any memory, else 0.
On systems that do not support "negative sbrks", it will always
return 0.
int __malloc_trim(size_t);
malloc_usable_size(void* p);
Returns the number of bytes you can actually use in
an allocated chunk, which may be more than you requested (although
often not) due to alignment and minimum size constraints.
You can use this many bytes without worrying about
overwriting other allocated objects. This is not a particularly great
programming practice. malloc_usable_size can be more useful in
debugging and assertions, for example:
p = malloc(n);
assert(malloc_usable_size(p) >= 256);
size_t __malloc_usable_size(void *);
Prints on stderr the amount of space obtained from the system (both
via sbrk and mmap), the maximum amount (which may be more than
current if malloc_trim and/or munmap got called), and the current
number of bytes allocated via malloc (or realloc, etc) but not yet
freed. Note that this is the number of bytes allocated, not the
number requested. It will be larger than the number requested
because of alignment and bookkeeping overhead. Because it includes
alignment wastage as being in use, this figure may be greater than
zero even when no user-level chunks are allocated.
The reported current and maximum system memory can be inaccurate if
a program makes other calls to system memory allocation functions
(normally sbrk) outside of malloc.
malloc_stats prints only the most commonly interesting statistics.
More information can be obtained by calling mallinfo.
void __malloc_stats(void);
Returns the state of all malloc variables in an opaque data
void *__malloc_get_state(void);
malloc_set_state(void* state);
Restore the state of all malloc variables from data obtained with
int __malloc_set_state(void *);
posix_memalign(void **memptr, size_t alignment, size_t size);
POSIX wrapper like memalign(), checking for validity of size.
int __posix_memalign(void **, size_t, size_t);
/* mallopt tuning options */
M_MXFAST is the maximum request size used for "fastbins", special bins
that hold returned chunks without consolidating their spaces. This
enables future requests for chunks of the same size to be handled
very quickly, but can increase fragmentation, and thus increase the
overall memory footprint of a program.
This malloc manages fastbins very conservatively yet still
efficiently, so fragmentation is rarely a problem for values less
than or equal to the default. The maximum supported value of MXFAST
is 80. You wouldn't want it any higher than this anyway. Fastbins
are designed especially for use with many small structs, objects or
strings -- the default handles structs/objects/arrays with sizes up
to 8 4byte fields, or small strings representing words, tokens,
etc. Using fastbins for larger objects normally worsens
fragmentation without improving speed.
M_MXFAST is set in REQUEST size units. It is internally used in
chunksize units, which adds padding and alignment. You can reduce
M_MXFAST to 0 to disable all use of fastbins. This causes the malloc
algorithm to be a closer approximation of fifo-best-fit in all cases,
not just for larger requests, but will generally cause it to be
/* M_MXFAST is a standard SVID/XPG tuning option, usually listed in malloc.h */
#ifndef M_MXFAST
#define M_MXFAST 1
#define DEFAULT_MXFAST (64 * SIZE_SZ / 4)
M_TRIM_THRESHOLD is the maximum amount of unused top-most memory
to keep before releasing via malloc_trim in free().
Automatic trimming is mainly useful in long-lived programs.
Because trimming via sbrk can be slow on some systems, and can
sometimes be wasteful (in cases where programs immediately
afterward allocate more large chunks) the value should be high
enough so that your overall system performance would improve by
releasing this much memory.
The trim threshold and the mmap control parameters (see below)
can be traded off with one another. Trimming and mmapping are
two different ways of releasing unused memory back to the
system. Between these two, it is often possible to keep
system-level demands of a long-lived program down to a bare
minimum. For example, in one test suite of sessions measuring
the XF86 X server on Linux, using a trim threshold of 128K and a
mmap threshold of 192K led to near-minimal long term resource
If you are using this malloc in a long-lived program, it should
pay to experiment with these values. As a rough guide, you
might set to a value close to the average size of a process
(program) running on your system. Releasing this much memory
would allow such a process to run in memory. Generally, it's
worth it to tune for trimming rather tham memory mapping when a
program undergoes phases where several large chunks are
allocated and released in ways that can reuse each other's
storage, perhaps mixed with phases where there are no such
chunks at all. And in well-behaved long-lived programs,
controlling release of large blocks via trimming versus mapping
is usually faster.
However, in most programs, these parameters serve mainly as
protection against the system-level effects of carrying around
massive amounts of unneeded memory. Since frequent calls to
sbrk, mmap, and munmap otherwise degrade performance, the default
parameters are set to relatively high values that serve only as
The trim value It must be greater than page size to have any useful
effect. To disable trimming completely, you can set to
(unsigned long)(-1)
Trim settings interact with fastbin (MXFAST) settings: Unless
TRIM_FASTBINS is defined, automatic trimming never takes place upon
freeing a chunk with size less than or equal to MXFAST. Trimming is
instead delayed until subsequent freeing of larger chunks. However,
you can still force an attempted trim by calling malloc_trim.
Also, trimming is not generally possible in cases where
the main arena is obtained via mmap.
Note that the trick some people use of mallocing a huge space and
then freeing it at program startup, in an attempt to reserve system
memory, doesn't have the intended effect under automatic trimming,
since that memory will immediately be returned to the system.
#define DEFAULT_TRIM_THRESHOLD (128 * 1024)
M_TOP_PAD is the amount of extra `padding' space to allocate or
retain whenever sbrk is called. It is used in two ways internally:
* When sbrk is called to extend the top of the arena to satisfy
a new malloc request, this much padding is added to the sbrk
* When malloc_trim is called automatically from free(),
it is used as the `pad' argument.
In both cases, the actual amount of padding is rounded
so that the end of the arena is always a system page boundary.
The main reason for using padding is to avoid calling sbrk so
often. Having even a small pad greatly reduces the likelihood
that nearly every malloc request during program start-up (or
after trimming) will invoke sbrk, which needlessly wastes
Automatic rounding-up to page-size units is normally sufficient
to avoid measurable overhead, so the default is 0. However, in
systems where sbrk is relatively slow, it can pay to increase
this value, at the expense of carrying around more memory than
the program needs.
#define M_TOP_PAD -2
#define DEFAULT_TOP_PAD (0)
MMAP_THRESHOLD_MAX and _MIN are the bounds on the dynamically
#define DEFAULT_MMAP_THRESHOLD_MIN (128 * 1024)
/* For 32-bit platforms we cannot increase the maximum mmap
threshold much because it is also the minimum value for the
maximum heap size and its alignment. Going above 512k (i.e., 1M
for new heaps) wastes too much address space. */
#if __WORDSIZE == 32
#define DEFAULT_MMAP_THRESHOLD_MAX (512 * 1024)
#define DEFAULT_MMAP_THRESHOLD_MAX (4 * 1024 * 1024 * sizeof(long))
M_MMAP_THRESHOLD is the request size threshold for using mmap()
to service a request. Requests of at least this size that cannot
be allocated using already-existing space will be serviced via mmap.
(If enough normal freed space already exists it is used instead.)
Using mmap segregates relatively large chunks of memory so that
they can be individually obtained and released from the host
system. A request serviced through mmap is never reused by any
other request (at least not directly; the system may just so
happen to remap successive requests to the same locations).
Segregating space in this way has the benefits that:
1. Mmapped space can ALWAYS be individually released back
to the system, which helps keep the system level memory
demands of a long-lived program low.
2. Mapped memory can never become `locked' between
other chunks, as can happen with normally allocated chunks, which
means that even trimming via malloc_trim would not release them.
3. On some systems with "holes" in address spaces, mmap can obtain
memory that sbrk cannot.
However, it has the disadvantages that:
1. The space cannot be reclaimed, consolidated, and then
used to service later requests, as happens with normal chunks.
2. It can lead to more wastage because of mmap page alignment
3. It causes malloc performance to be more dependent on host
system memory management support routines which may vary in
implementation quality and may impose arbitrary
limitations. Generally, servicing a request via normal
malloc steps is faster than going through a system's mmap.
The advantages of mmap nearly always outweigh disadvantages for
"large" chunks, but the value of "large" varies across systems. The
default is an empirically derived value that works well in most
Update in 2006:
The above was written in 2001. Since then the world has changed a lot.
Memory got bigger. Applications got bigger. The virtual address space
layout in 32 bit linux changed.
In the new situation, brk() and mmap space is shared and there are no
artificial limits on brk size imposed by the kernel. What is more,
applications have started using transient allocations larger than the
128Kb as was imagined in 2001.
The price for mmap is also high now; each time glibc mmaps from the
kernel, the kernel is forced to zero out the memory it gives to the
application. Zeroing memory is expensive and eats a lot of cache and
memory bandwidth. This has nothing to do with the efficiency of the
virtual memory system, by doing mmap the kernel just has no choice but
to zero.
In 2001, the kernel had a maximum size for brk() which was about 800
megabytes on 32 bit x86, at that point brk() would hit the first
mmaped shared libaries and couldn't expand anymore. With current 2.6
kernels, the VA space layout is different and brk() and mmap
both can span the entire heap at will.
Rather than using a static threshold for the brk/mmap tradeoff,
we are now using a simple dynamic one. The goal is still to avoid
fragmentation. The old goals we kept are
1) try to get the long lived large allocations to use mmap()
2) really large allocations should always use mmap()
and we're adding now:
3) transient allocations should use brk() to avoid forcing the kernel
having to zero memory over and over again
The implementation works with a sliding threshold, which is by default
limited to go between 128Kb and 32Mb (64Mb for 64 bitmachines) and starts
out at 128Kb as per the 2001 default.
This allows us to satisfy requirement 1) under the assumption that long
lived allocations are made early in the process' lifespan, before it has
started doing dynamic allocations of the same size (which will
increase the threshold).
The upperbound on the threshold satisfies requirement 2)
The threshold goes up in value when the application frees memory that was
allocated with the mmap allocator. The idea is that once the application
starts freeing memory of a certain size, it's highly probable that this is
a size the application uses for transient allocations. This estimator
is there to satisfy the new third requirement.
M_MMAP_MAX is the maximum number of requests to simultaneously
service using mmap. This parameter exists because
some systems have a limited number of internal tables for
use by mmap, and using more than a few of them may degrade
The default is set to a value that serves only as a safeguard.
Setting to 0 disables use of mmap for servicing large requests.
#define M_MMAP_MAX -4
#define DEFAULT_MMAP_MAX (65536)
#include <malloc.h>
/* On some platforms we can compile internal, not exported functions better.
Let the environment provide a macro and define it to be empty if it
is not available. */
#ifndef internal_function
#define internal_function
/* Forward declarations. */
struct malloc_chunk;
typedef struct malloc_chunk *mchunkptr;
/* Internal routines. */
static void *_int_malloc(mstate, size_t);
static void _int_free(mstate, mchunkptr, int);
static void *_int_realloc(mstate, mchunkptr, INTERNAL_SIZE_T, INTERNAL_SIZE_T);
static void *_int_memalign(mstate, size_t, size_t);
static void *_mid_memalign(size_t, size_t, void *);
static void malloc_printerr(int action, const char *str, void *ptr, mstate av);
static void *internal_function mem2mem_check(void *p, size_t sz);
static int internal_function top_check(void);
static void internal_function munmap_chunk(mchunkptr p);
static mchunkptr internal_function mremap_chunk(mchunkptr p, size_t new_size);
static void *malloc_check(size_t sz, const void *caller);
static void free_check(void *mem, const void *caller);
static void *realloc_check(void *oldmem, size_t bytes, const void *caller);
static void *memalign_check(size_t alignment, size_t bytes, const void *caller);
/* ------------------ MMAP support ------------------ */
#include <fcntl.h>
#include <sys/mman.h>
#if !defined(MAP_ANONYMOUS) && defined(MAP_ANON)
#define MMAP(addr, size, prot, flags) \
__mmap((addr), (size), (prot), (flags) | MAP_ANONYMOUS | MAP_PRIVATE, -1, 0)
----------------------- Chunk representations -----------------------
This struct declaration is misleading (but accurate and necessary).
It declares a "view" into memory allowing access to necessary
fields at known offsets from a given base. See explanation below.
struct malloc_chunk {
INTERNAL_SIZE_T mchunk_prev_size; /* Size of previous chunk (if free). */
INTERNAL_SIZE_T mchunk_size; /* Size in bytes, including overhead. */
struct malloc_chunk *fd; /* double links -- used only if free. */
struct malloc_chunk *bk;
/* Only used for large blocks: pointer to next larger size. */
struct malloc_chunk *fd_nextsize; /* double links -- used only if free. */
struct malloc_chunk *bk_nextsize;
malloc_chunk details:
(The following includes lightly edited explanations by Colin Plumb.)
Chunks of memory are maintained using a `boundary tag' method as
described in e.g., Knuth or Standish. (See the paper by Paul
Wilson for a
survey of such techniques.) Sizes of free chunks are stored both
in the front of each chunk and at the end. This makes
consolidating fragmented chunks into bigger chunks very fast. The
size fields also hold bits representing whether chunks are free or
in use.
An allocated chunk looks like this:
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size of previous chunk, if unallocated (P clear) |
| Size of chunk, in bytes |A|M|P|
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User data starts here... .
. .
. (malloc_usable_size() bytes) .
. |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (size of chunk, but used for application data) |
| Size of next chunk, in bytes |A|0|1|
Where "chunk" is the front of the chunk for the purpose of most of
the malloc code, but "mem" is the pointer that is returned to the
user. "Nextchunk" is the beginning of the next contiguous chunk.
Chunks always begin on even word boundaries, so the mem portion
(which is returned to the user) is also on an even word boundary, and
thus at least double-word aligned.
Free chunks are stored in circular doubly-linked lists, and look like this:
chunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size of previous chunk, if unallocated (P clear) |
`head:' | Size of chunk, in bytes |A|0|P|
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forward pointer to next chunk in list |
| Back pointer to previous chunk in list |
| Unused space (may be 0 bytes long) .
. .
. |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
`foot:' | Size of chunk, in bytes |
| Size of next chunk, in bytes |A|0|0|
The P (PREV_INUSE) bit, stored in the unused low-order bit of the
chunk size (which is always a multiple of two words), is an in-use
bit for the *previous* chunk. If that bit is *clear*, then the
word before the current chunk size contains the previous chunk
size, and can be used to find the front of the previous chunk.
The very first chunk allocated always has this bit set,
preventing access to non-existent (or non-owned) memory. If
prev_inuse is set for any given chunk, then you CANNOT determine
the size of the previous chunk, and might even get a memory
addressing fault when trying to do so.
The A (NON_MAIN_ARENA) bit is cleared for chunks on the initial,
main arena, described by the main_arena variable. When additional
threads are spawned, each thread receives its own arena (up to a
configurable limit, after which arenas are reused for multiple
threads), and the chunks in these arenas have the A bit set. To
find the arena for a chunk on such a non-main arena, heap_for_ptr
performs a bit mask operation and indirection through the ar_ptr
member of the per-heap header heap_info (see arena.c).
Note that the `foot' of the current chunk is actually represented
as the prev_size of the NEXT chunk. This makes it easier to
deal with alignments etc but can be very confusing when trying
to extend or adapt this code.
The three exceptions to all this are:
1. The special chunk `top' doesn't bother using the
trailing size field since there is no next contiguous chunk
that would have to index off it. After initialization, `top'
is forced to always exist. If it would become less than
MINSIZE bytes long, it is replenished.
2. Chunks allocated via mmap, which have the second-lowest-order
bit M (IS_MMAPPED) set in their size fields. Because they are
allocated one-by-one, each must contain its own trailing size
field. If the M bit is set, the other bits are ignored
(because mmapped chunks are neither in an arena, nor adjacent
to a freed chunk). The M bit is also used for chunks which
originally came from a dumped heap via malloc_set_state in
3. Chunks in fastbins are treated as allocated chunks from the
point of view of the chunk allocator. They are consolidated
with their neighbors only in bulk, in malloc_consolidate.
---------- Size and alignment checks and conversions ----------
/* conversion from malloc headers to user pointers, and back */
#define chunk2mem(p) ((void *) ((char *) (p) + 2 * SIZE_SZ))
#define mem2chunk(mem) ((mchunkptr)((char *) (mem) -2 * SIZE_SZ))
/* The smallest possible chunk */
#define MIN_CHUNK_SIZE (offsetof(struct malloc_chunk, fd_nextsize))
/* The smallest size we can malloc is an aligned minimal chunk */
#define MINSIZE \
(unsigned long) (((MIN_CHUNK_SIZE + MALLOC_ALIGN_MASK) & \
/* Check if m has acceptable alignment */
#define aligned_OK(m) (((unsigned long) (m) &MALLOC_ALIGN_MASK) == 0)
#define misaligned_chunk(p) \
((uintptr_t)(MALLOC_ALIGNMENT == 2 * SIZE_SZ ? (p) : chunk2mem(p)) & \
Check if a request is so large that it would wrap around zero when
padded and aligned. To simplify some other code, the bound is made
low enough so that adding MINSIZE will also not wrap around zero.
#define REQUEST_OUT_OF_RANGE(req) \
((unsigned long) (req) >= (unsigned long) (INTERNAL_SIZE_T)(-2 * MINSIZE))
/* pad request bytes into a usable size -- internal version */
#define request2size(req) \
/* Same, except also perform argument check */
#define checked_request2size(req, sz) \
if (REQUEST_OUT_OF_RANGE(req)) { \
__set_errno(ENOMEM); \
return 0; \
} \
(sz) = request2size(req);
--------------- Physical chunk operations ---------------
/* size field is or'ed with PREV_INUSE when previous adjacent chunk in use */
#define PREV_INUSE 0x1
/* extract inuse bit of previous chunk */
#define prev_inuse(p) ((p)->mchunk_size & PREV_INUSE)
/* size field is or'ed with IS_MMAPPED if the chunk was obtained with mmap() */
#define IS_MMAPPED 0x2
/* check for mmap()'ed chunk */
#define chunk_is_mmapped(p) ((p)->mchunk_size & IS_MMAPPED)
/* size field is or'ed with NON_MAIN_ARENA if the chunk was obtained
from a non-main arena. This is only set immediately before handing
the chunk to the user, if necessary. */
#define NON_MAIN_ARENA 0x4
/* Check for chunk from main arena. */
#define chunk_main_arena(p) (((p)->mchunk_size & NON_MAIN_ARENA) == 0)
/* Mark a chunk as not being on the main arena. */
#define set_non_main_arena(p) ((p)->mchunk_size |= NON_MAIN_ARENA)
Bits to mask off when extracting size
Note: IS_MMAPPED is intentionally not masked off from size field in
macros for which mmapped chunks should never be seen. This should
cause helpful core dumps to occur if it is tried by accident by
people extending or adapting this malloc.
/* Get size, ignoring use bits */
#define chunksize(p) (chunksize_nomask(p) & ~(SIZE_BITS))
/* Like chunksize, but do not mask SIZE_BITS. */
#define chunksize_nomask(p) ((p)->mchunk_size)
/* Ptr to next physical malloc_chunk. */
#define next_chunk(p) ((mchunkptr)(((char *) (p)) + chunksize(p)))
/* Size of the chunk below P. Only valid if prev_inuse (P). */
#define prev_size(p) ((p)->mchunk_prev_size)
/* Set the size of the chunk below P. Only valid if prev_inuse (P). */
#define set_prev_size(p, sz) ((p)->mchunk_prev_size = (sz))
/* Ptr to previous physical malloc_chunk. Only valid if prev_inuse (P). */
#define prev_chunk(p) ((mchunkptr)(((char *) (p)) - prev_size(p)))
/* Treat space at ptr + offset as a chunk */
#define chunk_at_offset(p, s) ((mchunkptr)(((char *) (p)) + (s)))
/* extract p's inuse bit */
#define inuse(p) \
((((mchunkptr)(((char *) (p)) + chunksize(p)))->mchunk_size) & PREV_INUSE)
/* set/clear chunk as being inuse without otherwise disturbing */
#define set_inuse(p) \
((mchunkptr)(((char *) (p)) + chunksize(p)))->mchunk_size |= PREV_INUSE
#define clear_inuse(p) \
((mchunkptr)(((char *) (p)) + chunksize(p)))->mchunk_size &= ~(PREV_INUSE)
/* check/set/clear inuse bits in known places */
#define inuse_bit_at_offset(p, s) \
(((mchunkptr)(((char *) (p)) + (s)))->mchunk_size & PREV_INUSE)
#define set_inuse_bit_at_offset(p, s) \
(((mchunkptr)(((char *) (p)) + (s)))->mchunk_size |= PREV_INUSE)
#define clear_inuse_bit_at_offset(p, s) \
(((mchunkptr)(((char *) (p)) + (s)))->mchunk_size &= ~(PREV_INUSE))
/* Set size at head, without disturbing its use bit */
#define set_head_size(p, s) \
((p)->mchunk_size = (((p)->mchunk_size & SIZE_BITS) | (s)))
/* Set size/use field */
#define set_head(p, s) ((p)->mchunk_size = (s))
/* Set size at footer (only when chunk is not in use) */
#define set_foot(p, s) \
(((mchunkptr)((char *) (p) + (s)))->mchunk_prev_size = (s))
#pragma GCC poison mchunk_size
#pragma GCC poison mchunk_prev_size
-------------------- Internal data structures --------------------
All internal state is held in an instance of malloc_state defined
below. There are no other static variables, except in two optional
* If USE_MALLOC_LOCK is defined, the mALLOC_MUTEx declared above.
* If mmap doesn't support MAP_ANONYMOUS, a dummy file descriptor
for mmap.
Beware of lots of tricks that minimize the total bookkeeping space
requirements. The result is a little over 1K bytes (for 4byte
pointers and size_t.)
An array of bin headers for free chunks. Each bin is doubly
linked. The bins are approximately proportionally (log) spaced.
There are a lot of these bins (128). This may look excessive, but
works very well in practice. Most bins hold sizes that are
unusual as malloc request sizes, but are more usual for fragments
and consolidated sets of chunks, which is what these bins hold, so
they can be found quickly. All procedures maintain the invariant
that no consolidated chunk physically borders another one, so each
chunk in a list is known to be preceeded and followed by either
inuse chunks or the ends of memory.
Chunks in bins are kept in size order, with ties going to the
approximately least recently used chunk. Ordering isn't needed
for the small bins, which all contain the same-sized chunks, but
facilitates best-fit allocation for larger chunks. These lists
are just sequential. Keeping them in order almost never requires
enough traversal to warrant using fancier ordered data
Chunks of the same size are linked with the most
recently freed at the front, and allocations are taken from the
back. This results in LRU (FIFO) allocation order, which tends
to give each chunk an equal opportunity to be consolidated with
adjacent freed chunks, resulting in larger free chunks and less
To simplify use in double-linked lists, each bin header acts
as a malloc_chunk. This avoids special-casing for headers.
But to conserve space and improve locality, we allocate
only the fd/bk pointers of bins, and then use repositioning tricks
to treat these as the fields of a malloc_chunk*.
typedef struct malloc_chunk *mbinptr;
/* addressing -- note that bin_at(0) does not exist */
#define bin_at(m, i) \
(mbinptr)(((char *) &((m)->bins[ ((i) -1) * 2 ])) - \
offsetof(struct malloc_chunk, fd))
/* analog of ++bin */
#define next_bin(b) ((mbinptr)((char *) (b) + (sizeof(mchunkptr) << 1)))
/* Reminders about list directionality within bins */
#define first(b) ((b)->fd)
#define last(b) ((b)->bk)
/* Take a chunk off a bin list */
#define unlink(AV, P, BK, FD) \
{ \
FD = P->fd; \
BK = P->bk; \
if (__builtin_expect(FD->bk != P || BK->fd != P, 0)) \
malloc_printerr(check_action, "corrupted double-linked list", P, \
AV); \
else { \
FD->bk = BK; \
BK->fd = FD; \
if (!in_smallbin_range(chunksize_nomask(P)) && \
__builtin_expect(P->fd_nextsize != NULL, 0)) { \
if (__builtin_expect(P->fd_nextsize->bk_nextsize != P, 0) || \
__builtin_expect(P->bk_nextsize->fd_nextsize != P, 0)) \
malloc_printerr( \
check_action, \
"corrupted double-linked list (not small)", P, AV); \
if (FD->fd_nextsize == NULL) { \
if (P->fd_nextsize == P) \
FD->fd_nextsize = FD->bk_nextsize = FD; \
else { \
FD->fd_nextsize = P->fd_nextsize; \
FD->bk_nextsize = P->bk_nextsize; \
P->fd_nextsize->bk_nextsize = FD; \
P->bk_nextsize->fd_nextsize = FD; \
} \
} else { \
P->fd_nextsize->bk_nextsize = P->bk_nextsize; \
P->bk_nextsize->fd_nextsize = P->fd_nextsize; \
} \
} \
} \
Bins for sizes < 512 bytes contain chunks of all the same size, spaced
8 bytes apart. Larger bins are approximately logarithmically spaced:
64 bins of size 8
32 bins of size 64
16 bins of size 512
8 bins of size 4096
4 bins of size 32768
2 bins of size 262144
1 bin of size what's left
There is actually a little bit of slop in the numbers in bin_index
for the sake of speed. This makes no difference elsewhere.
The bins top out around 1MB because we expect to service large
requests via mmap.
Bin 0 does not exist. Bin 1 is the unordered list; if that would be
a valid chunk size the small bins are bumped up one.
#define NBINS 128
#define NSMALLBINS 64
#define in_smallbin_range(sz) \
((unsigned long) (sz) < (unsigned long) MIN_LARGE_SIZE)
#define smallbin_index(sz) \
((SMALLBIN_WIDTH == 16 ? (((unsigned) (sz)) >> 4) \
: (((unsigned) (sz)) >> 3)) + \
#define largebin_index_32(sz) \
(((((unsigned long) (sz)) >> 6) <= 38) \
? 56 + (((unsigned long) (sz)) >> 6) \
: ((((unsigned long) (sz)) >> 9) <= 20) \
? 91 + (((unsigned long) (sz)) >> 9) \
: ((((unsigned long) (sz)) >> 12) <= 10) \
? 110 + (((unsigned long) (sz)) >> 12) \
: ((((unsigned long) (sz)) >> 15) <= 4) \
? 119 + (((unsigned long) (sz)) >> 15) \
: ((((unsigned long) (sz)) >> 18) <= 2) \
? 124 + (((unsigned long) (sz)) >> 18) \
: 126)
#define largebin_index_32_big(sz) \
(((((unsigned long) (sz)) >> 6) <= 45) \
? 49 + (((unsigned long) (sz)) >> 6) \
: ((((unsigned long) (sz)) >> 9) <= 20) \
? 91 + (((unsigned long) (sz)) >> 9) \
: ((((unsigned long) (sz)) >> 12) <= 10) \
? 110 + (((unsigned long) (sz)) >> 12) \
: ((((unsigned long) (sz)) >> 15) <= 4) \
? 119 + (((unsigned long) (sz)) >> 15) \
: ((((unsigned long) (sz)) >> 18) <= 2) \
? 124 + (((unsigned long) (sz)) >> 18) \
: 126)
// XXX It remains to be seen whether it is good to keep the widths of
// XXX the buckets the same or whether it should be scaled by a factor
// XXX of two as well.
#define largebin_index_64(sz) \
(((((unsigned long) (sz)) >> 6) <= 48) \
? 48 + (((unsigned long) (sz)) >> 6) \
: ((((unsigned long) (sz)) >> 9) <= 20) \
? 91 + (((unsigned long) (sz)) >> 9) \
: ((((unsigned long) (sz)) >> 12) <= 10) \
? 110 + (((unsigned long) (sz)) >> 12) \
: ((((unsigned long) (sz)) >> 15) <= 4) \
? 119 + (((unsigned long) (sz)) >> 15) \
: ((((unsigned long) (sz)) >> 18) <= 2) \
? 124 + (((unsigned long) (sz)) >> 18) \
: 126)
#define largebin_index(sz) \
(SIZE_SZ == 8 ? largebin_index_64(sz) : MALLOC_ALIGNMENT == 16 \
? largebin_index_32_big(sz) \
: largebin_index_32(sz))
#define bin_index(sz) \
((in_smallbin_range(sz)) ? smallbin_index(sz) : largebin_index(sz))
Unsorted chunks
All remainders from chunk splits, as well as all returned chunks,
are first placed in the "unsorted" bin. They are then placed
in regular bins after malloc gives them ONE chance to be used before
binning. So, basically, the unsorted_chunks list acts as a queue,
with chunks being placed on it in free (and malloc_consolidate),
and taken off (to be either used or placed in bins) in malloc.
The NON_MAIN_ARENA flag is never set for unsorted chunks, so it
does not have to be taken into account in size comparisons.
/* The otherwise unindexable 1-bin is used to hold unsorted chunks. */
#define unsorted_chunks(M) (bin_at(M, 1))
The top-most available chunk (i.e., the one bordering the end of
available memory) is treated specially. It is never included in
any bin, is used only if no other chunk is available, and is
released back to the system if it is very large (see
M_TRIM_THRESHOLD). Because top initially
points to its own bin with initial zero size, thus forcing
extension on the first malloc request, we avoid having any special
code in malloc to check whether it even exists yet. But we still
need to do so when getting memory from system, so we make
initial_top treat the bin as a legal but unusable chunk during the
interval between initialization and the first call to
sysmalloc. (This is somewhat delicate, since it relies on
the 2 preceding words to be zero during this interval as well.)
/* Conveniently, the unsorted bin can be used as dummy top on first call */
#define initial_top(M) (unsorted_chunks(M))
To help compensate for the large number of bins, a one-level index
structure is used for bin-by-bin searching. `binmap' is a
bitvector recording whether bins are definitely empty so they can
be skipped over during during traversals. The bits are NOT always
cleared as soon as bins are empty, but instead only
when they are noticed to be empty during traversal in malloc.
/* Conservatively use 32 bits per map word, even if on 64bit system */
#define idx2block(i) ((i) >> BINMAPSHIFT)
#define idx2bit(i) ((1U << ((i) & ((1U << BINMAPSHIFT) - 1))))
#define mark_bin(m, i) ((m)->binmap[ idx2block(i) ] |= idx2bit(i))
#define unmark_bin(m, i) ((m)->binmap[ idx2block(i) ] &= ~(idx2bit(i)))
#define get_binmap(m, i) ((m)->binmap[ idx2block(i) ] & idx2bit(i))
An array of lists holding recently freed small chunks. Fastbins
are not doubly linked. It is faster to single-link them, and
since chunks are never removed from the middles of these lists,
double linking is not necessary. Also, unlike regular bins, they
are not even processed in FIFO order (they use faster LIFO) since
ordering doesn't much matter in the transient contexts in which
fastbins are normally used.
Chunks in fastbins keep their inuse bit set, so they cannot
be consolidated with other free chunks. malloc_consolidate
releases all chunks in fastbins and consolidates them with
other free chunks.
typedef struct malloc_chunk *mfastbinptr;
#define fastbin(ar_ptr, idx) ((ar_ptr)->fastbinsY[ idx ])
/* offset 2 to use otherwise unindexable first 2 bins */
#define fastbin_index(sz) \
((((unsigned int) (sz)) >> (SIZE_SZ == 8 ? 4 : 3)) - 2)
/* The maximum fastbin request size we support */
#define MAX_FAST_SIZE (80 * SIZE_SZ / 4)
#define NFASTBINS (fastbin_index(request2size(MAX_FAST_SIZE)) + 1)
FASTBIN_CONSOLIDATION_THRESHOLD is the size of a chunk in free()
that triggers automatic consolidation of possibly-surrounding
fastbin chunks. This is a heuristic, so the exact value should not
matter too much. It is defined at half the default trim threshold as a
compromise heuristic to only attempt consolidation if it is likely
to lead to trimming. However, it is not dynamically tunable, since
consolidation reduces fragmentation surrounding large chunks even
if trimming is not used.
Since the lowest 2 bits in max_fast don't matter in size comparisons,
they are used as flags.
FASTCHUNKS_BIT held in max_fast indicates that there are probably
some fastbin chunks. It is set true on entering a chunk into any
fastbin, and cleared only in malloc_consolidate.
The truth value is inverted so that have_fastchunks will be true
upon startup (since statics are zero-filled), simplifying
initialization checks.
#define have_fastchunks(M) (((M)->flags & FASTCHUNKS_BIT) == 0)
#define clear_fastchunks(M) catomic_or(&(M)->flags, FASTCHUNKS_BIT)
#define set_fastchunks(M) catomic_and(&(M)->flags, ~FASTCHUNKS_BIT)
NONCONTIGUOUS_BIT indicates that MORECORE does not return contiguous
regions. Otherwise, contiguity is exploited in merging together,
when possible, results from consecutive MORECORE calls.
The initial value comes from MORECORE_CONTIGUOUS, but is
changed dynamically if mmap is ever used as an sbrk substitute.
#define contiguous(M) (((M)->flags & NONCONTIGUOUS_BIT) == 0)
#define noncontiguous(M) (((M)->flags & NONCONTIGUOUS_BIT) != 0)
#define set_noncontiguous(M) ((M)->flags |= NONCONTIGUOUS_BIT)
#define set_contiguous(M) ((M)->flags &= ~NONCONTIGUOUS_BIT)
/* ARENA_CORRUPTION_BIT is set if a memory corruption was detected on the
arena. Such an arena is no longer used to allocate chunks. Chunks
allocated in that arena before detecting corruption are not freed. */
#define arena_is_corrupt(A) (((A)->flags & ARENA_CORRUPTION_BIT))
#define set_arena_corrupt(A) ((A)->flags |= ARENA_CORRUPTION_BIT)
Set value of max_fast.
Use impossibly small value if 0.
Precondition: there are no existing fastbin chunks.
Setting the value clears fastchunk bit but preserves noncontiguous bit.
#define set_max_fast(s) \
global_max_fast = \
(((s) == 0) ? SMALLBIN_WIDTH : ((s + SIZE_SZ) & ~MALLOC_ALIGN_MASK))
#define get_max_fast() global_max_fast
----------- Internal state representation and initialization -----------
struct malloc_state {
/* Serialize access. */
__libc_lock_define(, mutex);
/* Flags (formerly in max_fast). */
int flags;
/* Fastbins */
mfastbinptr fastbinsY[ NFASTBINS ];
/* Base of the topmost chunk -- not otherwise kept in a bin */
mchunkptr top;
/* The remainder from the most recent split of a small request */
mchunkptr last_remainder;
/* Normal bins packed as described above */
mchunkptr bins[ NBINS * 2 - 2 ];
/* Bitmap of bins */
unsigned int binmap[ BINMAPSIZE ];
/* Linked list */
struct malloc_state *next;
/* Linked list for free arenas. Access to this field is serialized
by free_list_lock in arena.c. */
struct malloc_state *next_free;
/* Number of threads attached to this arena. 0 if the arena is on
the free list. Access to this field is serialized by
free_list_lock in arena.c. */
INTERNAL_SIZE_T attached_threads;
/* Memory allocated from the system in this arena. */
INTERNAL_SIZE_T system_mem;
INTERNAL_SIZE_T max_system_mem;
struct malloc_par {
/* Tunable parameters */
unsigned long trim_threshold;
INTERNAL_SIZE_T mmap_threshold;
INTERNAL_SIZE_T arena_test;
INTERNAL_SIZE_T arena_max;
/* Memory map support */
int n_mmaps;
int n_mmaps_max;
int max_n_mmaps;
/* the mmap_threshold is dynamic, until the user sets
it manually, at which point we need to disable any
dynamic behavior. */
int no_dyn_threshold;
/* Statistics */
INTERNAL_SIZE_T mmapped_mem;
INTERNAL_SIZE_T max_mmapped_mem;
/* First address handed out by MORECORE/sbrk. */
char *sbrk_base;
/* There are several instances of this struct ("arenas") in this
malloc. If you are adapting this malloc in a way that does NOT use
a static or mmapped malloc_state, you MUST explicitly zero-fill it
before using. This malloc relies on the property that malloc_state
is initialized to all zeroes (as is true of C statics). */
static struct malloc_state main_arena = {.mutex = _LIBC_LOCK_INITIALIZER,
.next = &main_arena,
.attached_threads = 1};
/* These variables are used for undumping support. Chunked are marked
as using mmap, but we leave them alone if they fall into this
range. NB: The chunk size for these chunks only includes the
initial size field (of SIZE_SZ bytes), there is no trailing size
field (unlike with regular mmapped chunks). */
static mchunkptr dumped_main_arena_start; /* Inclusive. */
static mchunkptr dumped_main_arena_end; /* Exclusive. */
/* True if the pointer falls into the dumped arena. Use this after
chunk_is_mmapped indicates a chunk is mmapped. */
((p) >= dumped_main_arena_start && (p) < dumped_main_arena_end)
/* There is only one instance of the malloc parameters. */
static struct malloc_par mp_ = {.top_pad = DEFAULT_TOP_PAD,
.n_mmaps_max = DEFAULT_MMAP_MAX,
.mmap_threshold = DEFAULT_MMAP_THRESHOLD,
.trim_threshold = DEFAULT_TRIM_THRESHOLD,
#define NARENAS_FROM_NCORES(n) ((n) * (sizeof(long) == 4 ? 2 : 8))
.arena_test = NARENAS_FROM_NCORES(1)};
/* Maximum size of memory handled in fastbins. */
static INTERNAL_SIZE_T global_max_fast;
Initialize a malloc_state struct.
This is called only from within malloc_consolidate, which needs
be called in the same contexts anyway. It is never called directly
outside of malloc_consolidate because some optimizing compilers try
to inline it at all call points, which turns out not to be an
optimization at all. (Inlining it in malloc_consolidate is fine though.)
static void malloc_init_state(mstate av) {
int i;
mbinptr bin;
/* Establish circular links for normal bins */
for (i = 1; i < NBINS; ++i) {
bin = bin_at(av, i);
bin->fd = bin->bk = bin;
if (av != &main_arena)
if (av == &main_arena) set_max_fast(DEFAULT_MXFAST);
av->flags |= FASTCHUNKS_BIT;
av->top = initial_top(av);
Other internal utilities operating on mstates
static void *sysmalloc(INTERNAL_SIZE_T, mstate);
static int systrim(size_t, mstate);
static void malloc_consolidate(mstate);
/* -------------- Early definitions for debugging hooks ---------------- */
/* Define and initialize the hook variables. These weak definitions must
appear before any use of the variables in a function (arena.c uses one). */
#ifndef weak_variable
/* In GNU libc we want the hook variables to be weak definitions to
avoid a problem with Emacs. */
#define weak_variable weak_function
/* Forward declarations. */
static void *malloc_hook_ini(size_t sz, const void *caller) __THROW;
static void *realloc_hook_ini(void *ptr, size_t sz, const void *caller) __THROW;
static void *memalign_hook_ini(size_t alignment, size_t sz,
const void *caller) __THROW;
void weak_variable (*__malloc_initialize_hook)(void) = NULL;
compat_symbol(libc, __malloc_initialize_hook, __malloc_initialize_hook,
void weak_variable (*__free_hook)(void *__ptr, const void *) = NULL;
void *weak_variable (*__malloc_hook)(size_t __size,
const void *) = malloc_hook_ini;
void *weak_variable (*__realloc_hook)(void *__ptr, size_t __size,
const void *) = realloc_hook_ini;
void *weak_variable (*__memalign_hook)(size_t __alignment, size_t __size,
const void *) = memalign_hook_ini;
void weak_variable (*__after_morecore_hook)(void) = NULL;
/* ---------------- Error behavior ------------------------------------ */
static int check_action = DEFAULT_CHECK_ACTION;
/* ------------------ Testing support ----------------------------------*/
static int perturb_byte;
static void alloc_perturb(char *p, size_t n) {
if (__glibc_unlikely(perturb_byte)) memset(p, perturb_byte ^ 0xff, n);
static void free_perturb(char *p, size_t n) {
if (__glibc_unlikely(perturb_byte)) memset(p, perturb_byte, n);
#include <stap-probe.h>
/* ------------------- Support for multiple arenas -------------------- */
#include "arena.c"
Debugging support
These routines make a number of assertions about the states
of data structures that should be true at all times. If any
are not true, it's very likely that a user program has somehow
trashed memory. (It's also possible that there is a coding error
in malloc. In which case, please report it!)
#define check_chunk(A, P)
#define check_free_chunk(A, P)
#define check_inuse_chunk(A, P)
#define check_remalloced_chunk(A, P, N)
#define check_malloced_chunk(A, P, N)
#define check_malloc_state(A)
#define check_chunk(A, P) do_check_chunk(A, P)
#define check_free_chunk(A, P) do_check_free_chunk(A, P)
#define check_inuse_chunk(A, P) do_check_inuse_chunk(A, P)
#define check_remalloced_chunk(A, P, N) do_check_remalloced_chunk(A, P, N)
#define check_malloced_chunk(A, P, N) do_check_malloced_chunk(A, P, N)
#define check_malloc_state(A) do_check_malloc_state(A)
Properties of all chunks
static void do_check_chunk(mstate av, mchunkptr p) {
unsigned long sz = chunksize(p);
/* min and max possible addresses assuming contiguous allocation */
char *max_address = (char *) (av->top) + chunksize(av->top);
char *min_address = max_address - av->system_mem;
if (!chunk_is_mmapped(p)) {
/* Has legal address ... */
if (p != av->top) {
if (contiguous(av)) {
assert(((char *) p) >= min_address);
assert(((char *) p + sz) <= ((char *) (av->top)));
} else {
/* top size is always at least MINSIZE */
assert((unsigned long) (sz) >= MINSIZE);
/* top predecessor always marked inuse */
} else if (!DUMPED_MAIN_ARENA_CHUNK(p)) {
/* address is outside main heap */
if (contiguous(av) && av->top != initial_top(av)) {
assert(((char *) p) < min_address || ((char *) p) >= max_address);
/* chunk is page-aligned */
assert(((prev_size(p) + sz) & (GLRO(dl_pagesize) - 1)) == 0);
/* mem is aligned */
Properties of free chunks
static void do_check_free_chunk(mstate av, mchunkptr p) {
mchunkptr next = chunk_at_offset(p, sz);
do_check_chunk(av, p);
/* Chunk must claim to be free ... */
/* Unless a special marker, must have OK fields */
if ((unsigned long) (sz) >= MINSIZE) {
assert((sz & MALLOC_ALIGN_MASK) == 0);
/* ... matching footer field */
assert(prev_size(p) == sz);
/* ... and is fully consolidated */
assert(next == av->top || inuse(next));
/* ... and has minimally sane links */
assert(p->fd->bk == p);
assert(p->bk->fd == p);
} else /* markers are always of size SIZE_SZ */
assert(sz == SIZE_SZ);
Properties of inuse chunks
static void do_check_inuse_chunk(mstate av, mchunkptr p) {
mchunkptr next;
do_check_chunk(av, p);
if (chunk_is_mmapped(p)) return; /* mmapped chunks have no next/prev */
/* Check whether it claims to be in use ... */
next = next_chunk(p);
/* ... and is surrounded by OK chunks.
Since more things can be checked with free chunks than inuse ones,
if an inuse chunk borders them and debug is on, it's worth doing them.
if (!prev_inuse(p)) {
/* Note that we cannot even look at prev unless it is not inuse */
mchunkptr prv = prev_chunk(p);
assert(next_chunk(prv) == p);
do_check_free_chunk(av, prv);
if (next == av->top) {
assert(chunksize(next) >= MINSIZE);
} else if (!inuse(next))
do_check_free_chunk(av, next);
Properties of chunks recycled from fastbins
static void do_check_remalloced_chunk(mstate av, mchunkptr p,
if (!chunk_is_mmapped(p)) {
assert(av == arena_for_chunk(p));
if (chunk_main_arena(p))
assert(av == &main_arena);
assert(av != &main_arena);
do_check_inuse_chunk(av, p);
/* Legal size ... */
assert((sz & MALLOC_ALIGN_MASK) == 0);
assert((unsigned long) (sz) >= MINSIZE);
/* ... and alignment */
/* chunk is less than MINSIZE more than request */
assert((long) (sz) - (long) (s) >= 0);
assert((long) (sz) - (long) (s + MINSIZE) < 0);
Properties of nonrecycled chunks at the point they are malloced
static void do_check_malloced_chunk(mstate av, mchunkptr p, INTERNAL_SIZE_T s) {
/* same as recycled case ... */
do_check_remalloced_chunk(av, p, s);
... plus, must obey implementation invariant that prev_inuse is
always true of any allocated chunk; i.e., that each allocated
chunk borders either a previously allocated and still in-use
chunk, or the base of its memory arena. This is ensured
by making all allocations from the `lowest' part of any found
chunk. This does not necessarily hold however for chunks
recycled via fastbins.
Properties of malloc_state.
This may be useful for debugging malloc, as well as detecting user
programmer errors that somehow write into malloc_state.
If you are extending or experimenting with this malloc, you can
probably figure out how to hack this routine to print out or
display chunk addresses, sizes, bins, and other instrumentation.
static void do_check_malloc_state(mstate av) {
int i;
mchunkptr p;
mchunkptr q;
mbinptr b;
unsigned int idx;
unsigned long total = 0;
int max_fast_bin;
/* internal size_t must be no wider than pointer type */
assert(sizeof(INTERNAL_SIZE_T) <= sizeof(char *));
/* alignment is a power of 2 */
/* cannot run remaining checks until fully initialized */
if (av->top == 0 || av->top == initial_top(av)) return;
/* pagesize is a power of 2 */
/* A contiguous main_arena is consistent with sbrk_base. */
if (av == &main_arena && contiguous(av))
assert((char *) mp_.sbrk_base + av->system_mem ==
(char *) av->top + chunksize(av->top));
/* properties of fastbins */
/* max_fast is in allowed range */
assert((get_max_fast() & ~1) <= request2size(MAX_FAST_SIZE));
max_fast_bin = fastbin_index(get_max_fast());
for (i = 0; i < NFASTBINS; ++i) {
p = fastbin(av, i);
/* The following test can only be performed for the main arena.
While mallopt calls malloc_consolidate to get rid of all fast
bins (especially those larger than the new maximum) this does
only happen for the main arena. Trying to do this for any
other arena would mean those arenas have to be locked and
malloc_consolidate be called for them. This is excessive. And
even if this is acceptable to somebody it still cannot solve
the problem completely since if the arena is locked a
concurrent malloc call might create a new arena which then
could use the newly invalid fast bins. */
/* all bins past max_fast are empty */
if (av == &main_arena && i > max_fast_bin) assert(p == 0);
while (p != 0) {
/* each chunk claims to be inuse */
do_check_inuse_chunk(av, p);
total += chunksize(p);
/* chunk belongs in this bin */
assert(fastbin_index(chunksize(p)) == i);
p = p->fd;
if (total != 0)
else if (!have_fastchunks(av))
assert(total == 0);
/* check normal bins */
for (i = 1; i < NBINS; ++i) {
b = bin_at(av, i);
/* binmap is accurate (except for bin 1 == unsorted_chunks) */
if (i >= 2) {
unsigned int binbit = get_binmap(av, i);
int empty = last(b) == b;
if (!binbit)
else if (!empty)
for (p = last(b); p != b; p = p->bk) {
/* each chunk claims to be free */
do_check_free_chunk(av, p);
size = chunksize(p);
total += size;
if (i >= 2) {
/* chunk belongs in bin */
idx = bin_index(size);
assert(idx == i);
/* lists are sorted */
assert(p->bk == b ||
(unsigned long) chunksize(p->bk) >=
(unsigned long) chunksize(p));
if (!in_smallbin_range(size)) {
if (p->fd_nextsize != NULL) {
if (p->fd_nextsize == p)
assert(p->bk_nextsize == p);
else {
if (p->fd_nextsize == first(b))
assert(chunksize(p) <
assert(chunksize(p) >
if (p == first(b))
assert(chunksize(p) >
assert(chunksize(p) <
} else
assert(p->bk_nextsize == NULL);
} else if (!in_smallbin_range(size))
assert(p->fd_nextsize == NULL && p->bk_nextsize == NULL);
/* chunk is followed by a legal chain of inuse chunks */
for (q = next_chunk(p); (q != av->top && inuse(q) &&
(unsigned long) (chunksize(q)) >= MINSIZE);
q = next_chunk(q))
do_check_inuse_chunk(av, q);
/* top chunk is OK */
check_chunk(av, av->top);
/* ----------------- Support for debugging hooks -------------------- */
#include "hooks.c"
/* ----------- Routines dealing with system allocation -------------- */
sysmalloc handles malloc cases requiring more memory from the system.
On entry, it is assumed that av->top does not have enough
space to service request for nb bytes, thus requiring that av->top
be extended or replaced.
static void *sysmalloc(INTERNAL_SIZE_T nb, mstate av) {
mchunkptr old_top; /* incoming value of av->top */
INTERNAL_SIZE_T old_size; /* its size */
char * old_end; /* its end address */
long size; /* arg to first MORECORE or mmap call */
char *brk; /* return value from MORECORE */
long correction; /* arg to 2nd MORECORE call */
char *snd_brk; /* 2nd return val */
INTERNAL_SIZE_T front_misalign; /* unusable bytes at front of new space */
INTERNAL_SIZE_T end_misalign; /* partial page left at end of new space */
char * aligned_brk; /* aligned offset into brk */
mchunkptr p; /* the allocated/returned chunk */
mchunkptr remainder; /* remainder from allocation */
unsigned long remainder_size; /* its size */
size_t pagesize = GLRO(dl_pagesize);
bool tried_mmap = false;
If have mmap, and the request size meets the mmap threshold, and
the system supports mmap, and there are few enough currently
allocated mmapped regions, try to directly map this request
rather than expanding top.
if (av == NULL ||
((unsigned long) (nb) >= (unsigned long) (mp_.mmap_threshold) &&
(mp_.n_mmaps < mp_.n_mmaps_max))) {
char *mm; /* return value from mmap call*/
Round up size to nearest page. For mmapped chunks, the overhead
is one SIZE_SZ unit larger than for normal chunks, because there
is no following chunk whose prev_size field could be used.
See the front_misalign handling below, for glibc there is no
need for further alignments unless we have have high alignment.
size = ALIGN_UP(nb + SIZE_SZ, pagesize);
size = ALIGN_UP(nb + SIZE_SZ + MALLOC_ALIGN_MASK, pagesize);
tried_mmap = true;
/* Don't try if size wraps around 0 */
if ((unsigned long) (size) > (unsigned long) (nb)) {
mm = (char *) (MMAP(0, size, PROT_READ | PROT_WRITE, 0));
if (mm != MAP_FAILED) {
The offset to the start of the mmapped region is stored
in the prev_size field of the chunk. This allows us to adjust
returned start address to meet alignment requirements here
and in memalign(), and still be able to compute proper
address argument for later munmap in free() and realloc().
/* For glibc, chunk2mem increases the address by 2*SIZE_SZ
MALLOC_ALIGN_MASK is 2*SIZE_SZ-1. Each mmap'ed area is
aligned and therefore definitely
assert(((INTERNAL_SIZE_T) chunk2mem(mm) &
front_misalign = 0;
} else
front_misalign =
if (front_misalign > 0) {
correction = MALLOC_ALIGNMENT - front_misalign;
p = (mchunkptr)(mm + correction);
set_prev_size(p, correction);
set_head(p, (size - correction) | IS_MMAPPED);
} else {
p = (mchunkptr) mm;
set_prev_size(p, 0);
set_head(p, size | IS_MMAPPED);
/* update statistics */
int new = atomic_exchange_and_add(&mp_.n_mmaps, 1) + 1;
atomic_max(&mp_.max_n_mmaps, new);
unsigned long sum;
sum = atomic_exchange_and_add(&mp_.mmapped_mem, size) + size;
atomic_max(&mp_.max_mmapped_mem, sum);
check_chunk(av, p);
return chunk2mem(p);
/* There are no usable arenas and mmap also failed. */
if (av == NULL) return 0;
/* Record incoming configuration of top */
old_top = av->top;
old_size = chunksize(old_top);
old_end = (char *) (chunk_at_offset(old_top, old_size));
brk = snd_brk = (char *) (MORECORE_FAILURE);
If not the first time through, we require old_size to be
at least MINSIZE and to have prev_inuse set.
assert((old_top == initial_top(av) && old_size == 0) ||
((unsigned long) (old_size) >= MINSIZE && prev_inuse(old_top) &&
((unsigned long) old_end & (pagesize - 1)) == 0));
/* Precondition: not enough current space to satisfy nb request */
assert((unsigned long) (old_size) < (unsigned long) (nb + MINSIZE));
if (av != &main_arena) {
heap_info *old_heap, *heap;
size_t old_heap_size;
/* First try to extend the current heap. */
old_heap = heap_for_ptr(old_top);
old_heap_size = old_heap->size;
if ((long) (MINSIZE + nb - old_size) > 0 &&
grow_heap(old_heap, MINSIZE + nb - old_size) == 0) {
av->system_mem += old_heap->size - old_heap_size;
(((char *) old_heap + old_heap->size) - (char *) old_top) |
} else if ((heap = new_heap(nb + (MINSIZE + sizeof(*heap)),
mp_.top_pad))) {
/* Use a newly allocated heap. */
heap->ar_ptr = av;
heap->prev = old_heap;
av->system_mem += heap->size;
/* Set up the new top. */
top(av) = chunk_at_offset(heap, sizeof(*heap));
set_head(top(av), (heap->size - sizeof(*heap)) | PREV_INUSE);
/* Setup fencepost and free the old top chunk with a multiple of
/* The fencepost takes at least MINSIZE bytes, because it might
become the top chunk again later. Note that a footer is set
up, too, although the chunk is marked in use. */
old_size = (old_size - MINSIZE) & ~MALLOC_ALIGN_MASK;
set_head(chunk_at_offset(old_top, old_size + 2 * SIZE_SZ),
if (old_size >= MINSIZE) {
set_head(chunk_at_offset(old_top, old_size),
set_foot(chunk_at_offset(old_top, old_size), (2 * SIZE_SZ));
set_head(old_top, old_size | PREV_INUSE | NON_MAIN_ARENA);
_int_free(av, old_top, 1);
} else {
set_head(old_top, (old_size + 2 * SIZE_SZ) | PREV_INUSE);
set_foot(old_top, (old_size + 2 * SIZE_SZ));
} else if (!tried_mmap)
/* We can at least try to use to mmap memory. */
goto try_mmap;
} else /* av == main_arena */
{ /* Request enough space for nb + pad + overhead */
size = nb + mp_.top_pad + MINSIZE;
If contiguous, we can subtract out existing space that we hope to
combine with new space. We add it back later only if
we don't actually get contiguous space.
if (contiguous(av)) size -= old_size;
Round to a multiple of page size.
If MORECORE is not contiguous, this ensures that we only call it
with whole-page arguments. And if MORECORE is contiguous and
this is not first time through, this preserves page-alignment of
previous calls. Otherwise, we correct to page-align below.
size = ALIGN_UP(size, pagesize);
Don't try to call MORECORE if argument is so big as to appear
negative. Note that since mmap takes size_t arg, it may succeed
below even if we cannot call MORECORE.
if (size > 0) {
brk = (char *) (MORECORE(size));
LIBC_PROBE(memory_sbrk_more, 2, brk, size);
if (brk != (char *) (MORECORE_FAILURE)) {
/* Call the `morecore' hook if necessary. */
void (*hook)(void) = atomic_forced_read(__after_morecore_hook);
if (__builtin_expect(hook != NULL, 0)) (*hook)();
} else {
If have mmap, try using it as a backup when MORECORE fails or
cannot be used. This is worth doing on systems that have "holes"
address space, so sbrk cannot extend to give contiguous space,
space is available elsewhere. Note that we ignore mmap max count
and threshold limits, since the space will not be used as a
segregated mmap region.
/* Cannot merge with old top, so add its size back in */
if (contiguous(av)) size = ALIGN_UP(size + old_size, pagesize);
/* If we are relying on mmap as backup, then use larger units */
if ((unsigned long) (size) <
(unsigned long) (MMAP_AS_MORECORE_SIZE))
/* Don't try if size wraps around 0 */
if ((unsigned long) (size) > (unsigned long) (nb)) {
char *mbrk =
(char *) (MMAP(0, size, PROT_READ | PROT_WRITE, 0));
if (mbrk != MAP_FAILED) {
/* We do not need, and cannot use, another sbrk call to find
* end */
brk = mbrk;
snd_brk = brk + size;
Record that we no longer have a contiguous sbrk region.
After the first time mmap is used as backup, we do not
ever rely on contiguous space since this could
bridge regions.
if (brk != (char *) (MORECORE_FAILURE)) {
if (mp_.sbrk_base == 0) mp_.sbrk_base = brk;
av->system_mem += size;
If MORECORE extends previous space, we can likewise extend top
if (brk == old_end && snd_brk == (char *) (MORECORE_FAILURE))
set_head(old_top, (size + old_size) | PREV_INUSE);
else if (contiguous(av) && old_size && brk < old_end) {
/* Oops! Someone else killed our space.. Can't touch anything.
malloc_printerr(3, "break adjusted to free malloc space", brk,
Otherwise, make adjustments:
* If the first time through or noncontiguous, we need to call sbrk
just to find out where the end of memory lies.
* We need to ensure that all returned chunks from malloc will meet
* If there was an intervening foreign sbrk, we need to adjust sbrk
request size to account for fact that we will not be able to
combine new space with existing space in old_top.
* Almost all systems internally allocate whole pages at a time, in
which case we might as well use the whole last page of request.
So we allocate enough more memory to hit a page boundary now,
which in turn causes future contiguous calls to page-align.
else {
front_misalign = 0;
end_misalign = 0;
correction = 0;
aligned_brk = brk;
/* handle contiguous cases */
if (contiguous(av)) {
/* Count foreign sbrk as system_mem. */
if (old_size) av->system_mem += brk - old_end;
/* Guarantee alignment of first new chunk made from this
* space */
front_misalign =
if (front_misalign > 0) {
Skip over some bytes to arrive at an aligned
We don't need to specially mark these wasted front
They will never be accessed anyway because
prev_inuse of av->top (and any chunk created from its
is always true after initialization.
correction = MALLOC_ALIGNMENT - front_misalign;
aligned_brk += correction;
If this isn't adjacent to existing space, then we will
be able to merge with old_top space, so must add to 2nd
correction += old_size;
/* Extend the end address to hit a page boundary */
end_misalign = (INTERNAL_SIZE_T)(brk + size + correction);
correction +=
(ALIGN_UP(end_misalign, pagesize)) - end_misalign;
assert(correction >= 0);
snd_brk = (char *) (MORECORE(correction));
If can't allocate correction, try to at least find out
brk. It might be enough to proceed without failing.
Note that if second sbrk did NOT fail, we assume that
is contiguous with first sbrk. This is a safe assumption
program is multithreaded but doesn't use locks and a
foreign sbrk
occurred between our first and second calls.
if (snd_brk == (char *) (MORECORE_FAILURE)) {
correction = 0;
snd_brk = (char *) (MORECORE(0));
} else {
/* Call the `morecore' hook if necessary. */
void (*hook)(void) =
if (__builtin_expect(hook != NULL, 0)) (*hook)();
/* handle non-contiguous cases */
else {
/* MORECORE/mmap must correctly align */
assert(((unsigned long) chunk2mem(brk) &
else {
front_misalign = (INTERNAL_SIZE_T) chunk2mem(brk) &
if (front_misalign > 0) {
Skip over some bytes to arrive at an aligned
We don't need to specially mark these wasted
front bytes.
They will never be accessed anyway because
prev_inuse of av->top (and any chunk created from
its start)
is always true after initialization.
aligned_brk += MALLOC_ALIGNMENT - front_misalign;
/* Find out current end of memory */
if (snd_brk == (char *) (MORECORE_FAILURE)) {
snd_brk = (char *) (MORECORE(0));
/* Adjust top based on results of second sbrk */
if (snd_brk != (char *) (MORECORE_FAILURE)) {
av->top = (mchunkptr) aligned_brk;
(snd_brk - aligned_brk + correction) | PREV_INUSE);
av->system_mem += correction;
If not the first time through, we either have a
gap due to foreign sbrk or a non-contiguous region.
Insert a
double fencepost at old_top to prevent consolidation with
we don't own. These fenceposts are artificial chunks that
marked as inuse and are in any case too small to use. We
two to make sizes and alignments work out.
if (old_size != 0) {
Shrink old_top to insert fenceposts, keeping size a
multiple of MALLOC_ALIGNMENT. We know there is at
enough space in old_top to do this.
old_size =
(old_size - 4 * SIZE_SZ) & ~MALLOC_ALIGN_MASK;
set_head(old_top, old_size | PREV_INUSE);
Note that the following assignments completely
old_top when old_size was previously MINSIZE. This
intentional. We need the fencepost, even if old_top
otherwise gets
set_head(chunk_at_offset(old_top, old_size),
chunk_at_offset(old_top, old_size + 2 * SIZE_SZ),
/* If possible, release the rest. */
if (old_size >= MINSIZE) {
_int_free(av, old_top, 1);
} /* if (av != &main_arena) */
if ((unsigned long) av->system_mem > (unsigned long) (av->max_system_mem))
av->max_system_mem = av->system_mem;
/* finally, do the allocation */
p = av->top;
size = chunksize(p);
/* check that one of the above allocation paths succeeded */
if ((unsigned long) (size) >= (unsigned long) (nb + MINSIZE)) {
remainder_size = size - nb;
remainder = chunk_at_offset(p, nb);
av->top = remainder;
set_head(p, nb | PREV_INUSE | (av != &main_arena ? NON_MAIN_ARENA : 0));
set_head(remainder, remainder_size | PREV_INUSE);
check_malloced_chunk(av, p, nb);
return chunk2mem(p);
/* catch all failure paths */
return 0;
systrim is an inverse of sorts to sysmalloc. It gives memory back
to the system (via negative arguments to sbrk) if there is unused
memory at the `high' end of the malloc pool. It is called
automatically by free() when top space exceeds the trim
threshold. It is also called by the public malloc_trim routine. It
returns 1 if it actually released any memory, else 0.
static int systrim(size_t pad, mstate av) {
long top_size; /* Amount of top-most memory */
long extra; /* Amount to release */
long released; /* Amount actually released */
char * current_brk; /* address returned by pre-check sbrk call */
char * new_brk; /* address returned by post-check sbrk call */
size_t pagesize;
long top_area;
pagesize = GLRO(dl_pagesize);
top_size = chunksize(av->top);
top_area = top_size - MINSIZE - 1;
if (top_area <= pad) return 0;
/* Release in pagesize units and round down to the nearest page. */
extra = ALIGN_DOWN(top_area - pad, pagesize);
if (extra == 0) return 0;
Only proceed if end of memory is where we last set it.
This avoids problems if there were foreign sbrk calls.
current_brk = (char *) (MORECORE(0));
if (current_brk == (char *) (av->top) + top_size) {
Attempt to release memory. We ignore MORECORE return value,
and instead call again to find out where new end of memory is.
This avoids problems if first call releases less than we asked,
of if failure somehow altered brk value. (We could still
encounter problems if it altered brk in some very bad way,
but the only thing we can do is adjust anyway, which will cause
some downstream failure.)
/* Call the `morecore' hook if necessary. */
void (*hook)(void) = atomic_forced_read(__after_morecore_hook);
if (__builtin_expect(hook != NULL, 0)) (*hook)();
new_brk = (char *) (MORECORE(0));
LIBC_PROBE(memory_sbrk_less, 2, new_brk, extra);
if (new_brk != (char *) MORECORE_FAILURE) {
released = (long) (current_brk - new_brk);
if (released != 0) {
/* Success. Adjust top. */
av->system_mem -= released;
set_head(av->top, (top_size - released) | PREV_INUSE);
return 1;
return 0;
static void internal_function munmap_chunk(mchunkptr p) {
INTERNAL_SIZE_T size = chunksize(p);
/* Do nothing if the chunk is a faked mmapped chunk in the dumped
main arena. We never free this memory. */
uintptr_t block = (uintptr_t) p - prev_size(p);
size_t total_size = prev_size(p) + size;
/* Unfortunately we have to do the compilers job by hand here. Normally
we would test BLOCK and TOTAL-SIZE separately for compliance with the
page size. But gcc does not recognize the optimization possibility
(in the moment at least) so we combine the two values into one before
the bit test. */
if (__builtin_expect(((block | total_size) & (GLRO(dl_pagesize) - 1)) != 0,
0)) {
malloc_printerr(check_action, "munmap_chunk(): invalid pointer",
chunk2mem(p), NULL);
atomic_add(&mp_.mmapped_mem, -total_size);
/* If munmap failed the process virtual memory address space is in a
bad shape. Just leave the block hanging around, the process will
terminate shortly anyway since not much can be done. */
__munmap((char *) block, total_size);
static mchunkptr internal_function mremap_chunk(mchunkptr p, size_t new_size) {
size_t pagesize = GLRO(dl_pagesize);
INTERNAL_SIZE_T offset = prev_size(p);
INTERNAL_SIZE_T size = chunksize(p);
char * cp;
assert(((size + offset) & (GLRO(dl_pagesize) - 1)) == 0);
/* Note the extra SIZE_SZ overhead as in mmap_chunk(). */
new_size = ALIGN_UP(new_size + offset + SIZE_SZ, pagesize);
/* No need to remap if the number of pages does not change. */
if (size + offset == new_size) return p;
cp = (char *) __mremap((char *) p - offset, size + offset, new_size,
if (cp == MAP_FAILED) return 0;
p = (mchunkptr)(cp + offset);
assert(prev_size(p) == offset);
set_head(p, (new_size - offset) | IS_MMAPPED);
new = atomic_exchange_and_add(&mp_.mmapped_mem, new_size - size - offset) +
new_size - size - offset;
atomic_max(&mp_.max_mmapped_mem, new);
return p;
#endif /* HAVE_MREMAP */
/*------------------------ Public wrappers. --------------------------------*/
void *__libc_malloc(size_t bytes) {
mstate ar_ptr;
void * victim;
void *(*hook)(size_t, const void *) = atomic_forced_read(__malloc_hook);
if (__builtin_expect(hook != NULL, 0))
return (*hook)(bytes, RETURN_ADDRESS(0));
arena_get(ar_ptr, bytes);
victim = _int_malloc(ar_ptr, bytes);
/* Retry with another arena only if we were able to find a usable arena
before. */
if (!victim && ar_ptr != NULL) {
LIBC_PROBE(memory_malloc_retry, 1, bytes);
ar_ptr = arena_get_retry(ar_ptr, bytes);
victim = _int_malloc(ar_ptr, bytes);
if (ar_ptr != NULL) __libc_lock_unlock(ar_ptr->mutex);
assert(!victim || chunk_is_mmapped(mem2chunk(victim)) ||
ar_ptr == arena_for_chunk(mem2chunk(victim)));
return victim;
void __libc_free(void *mem) {
mstate ar_ptr;
mchunkptr p; /* chunk corresponding to mem */
void (*hook)(void *, const void *) = atomic_forced_read(__free_hook);
if (__builtin_expect(hook != NULL, 0)) {
(*hook)(mem, RETURN_ADDRESS(0));
if (mem == 0) /* free(0) has no effect */
p = mem2chunk(mem);
if (chunk_is_mmapped(p)) /* release mmapped memory. */
/* See if the dynamic brk/mmap threshold needs adjusting.
Dumped fake mmapped chunks do not affect the threshold. */
if (!mp_.no_dyn_threshold && chunksize_nomask(p) > mp_.mmap_threshold &&
chunksize_nomask(p) <= DEFAULT_MMAP_THRESHOLD_MAX &&
mp_.mmap_threshold = chunksize(p);
mp_.trim_threshold = 2 * mp_.mmap_threshold;
LIBC_PROBE(memory_mallopt_free_dyn_thresholds, 2,
mp_.mmap_threshold, mp_.trim_threshold);
ar_ptr = arena_for_chunk(p);
_int_free(ar_ptr, p, 0);
void *__libc_realloc(void *oldmem, size_t bytes) {
mstate ar_ptr;
INTERNAL_SIZE_T nb; /* padded request size */
void *newp; /* chunk to return */
void *(*hook)(void *, size_t, const void *) =
if (__builtin_expect(hook != NULL, 0))
return (*hook)(oldmem, bytes, RETURN_ADDRESS(0));
if (bytes == 0 && oldmem != NULL) {
return 0;
/* realloc of null is supposed to be same as malloc */
if (oldmem == 0) return __libc_malloc(bytes);
/* chunk corresponding to oldmem */
const mchunkptr oldp = mem2chunk(oldmem);
/* its size */
const INTERNAL_SIZE_T oldsize = chunksize(oldp);
if (chunk_is_mmapped(oldp))
ar_ptr = NULL;
ar_ptr = arena_for_chunk(oldp);
/* Little security check which won't hurt performance: the allocator
never wrapps around at the end of the address space. Therefore
we can exclude some size values which might appear here by
accident or by "design" from some intruder. We need to bypass
this check for dumped fake mmap chunks from the old main arena
because the new malloc may provide additional alignment. */
if ((__builtin_expect((uintptr_t) oldp > (uintptr_t) -oldsize, 0) ||
__builtin_expect(misaligned_chunk(oldp), 0)) &&
malloc_printerr(check_action, "realloc(): invalid pointer", oldmem,
return NULL;
checked_request2size(bytes, nb);
if (chunk_is_mmapped(oldp)) {
/* If this is a faked mmapped chunk from the dumped main arena,
always make a copy (and do not free the old chunk). */
/* Must alloc, copy, free. */
void *newmem = __libc_malloc(bytes);
if (newmem == 0) return NULL;
/* Copy as many bytes as are available from the old chunk
and fit into the new size. NB: The overhead for faked
mmapped chunks is only SIZE_SZ, not 2 * SIZE_SZ as for
regular mmapped chunks. */
if (bytes > oldsize - SIZE_SZ) bytes = oldsize - SIZE_SZ;
memcpy(newmem, oldmem, bytes);
return newmem;
void *newmem;
newp = mremap_chunk(oldp, nb);
if (newp) return chunk2mem(newp);
/* Note the extra SIZE_SZ overhead. */
if (oldsize - SIZE_SZ >= nb) return oldmem; /* do nothing */
/* Must alloc, copy, free. */
newmem = __libc_malloc(bytes);
if (newmem == 0) return 0; /* propagate failure */
memcpy(newmem, oldmem, oldsize - 2 * SIZE_SZ);
return newmem;
newp = _int_realloc(ar_ptr, oldp, oldsize, nb);
assert(!newp || chunk_is_mmapped(mem2chunk(newp)) ||
ar_ptr == arena_for_chunk(mem2chunk(newp)));
if (newp == NULL) {
/* Try harder to allocate memory in other arenas. */
LIBC_PROBE(memory_realloc_retry, 2, bytes, oldmem);
newp = __libc_malloc(bytes);
if (newp != NULL) {
memcpy(newp, oldmem, oldsize - SIZE_SZ);
_int_free(ar_ptr, oldp, 0);
return newp;
void *__libc_memalign(size_t alignment, size_t bytes) {
void *address = RETURN_ADDRESS(0);
return _mid_memalign(alignment, bytes, address);
static void *_mid_memalign(size_t alignment, size_t bytes, void *address) {
mstate ar_ptr;
void * p;
void *(*hook)(size_t, size_t, const void *) =
if (__builtin_expect(hook != NULL, 0))
return (*hook)(alignment, bytes, address);
/* If we need less alignment than we give anyway, just relay to malloc. */
if (alignment <= MALLOC_ALIGNMENT) return __libc_malloc(bytes);
/* Otherwise, ensure that it is at least a minimum chunk size */
if (alignment < MINSIZE) alignment = MINSIZE;
/* If the alignment is greater than SIZE_MAX / 2 + 1 it cannot be a
power of 2 and will cause overflow in the check below. */
if (alignment > SIZE_MAX / 2 + 1) {
return 0;
/* Check for overflow. */
if (bytes > SIZE_MAX - alignment - MINSIZE) {
return 0;
/* Make sure alignment is power of 2. */
if (!powerof2(alignment)) {
size_t a = MALLOC_ALIGNMENT * 2;
while (a < alignment)
a <<= 1;
alignment = a;
arena_get(ar_ptr, bytes + alignment + MINSIZE);
p = _int_memalign(ar_ptr, alignment, bytes);
if (!p && ar_ptr != NULL) {
LIBC_PROBE(memory_memalign_retry, 2, bytes, alignment);
ar_ptr = arena_get_retry(ar_ptr, bytes);
p = _int_memalign(ar_ptr, alignment, bytes);
if (ar_ptr != NULL) __libc_lock_unlock(ar_ptr->mutex);
assert(!p || chunk_is_mmapped(mem2chunk(p)) ||
ar_ptr == arena_for_chunk(mem2chunk(p)));
return p;
/* For ISO C11. */
weak_alias(__libc_memalign, aligned_alloc) libc_hidden_def(__libc_memalign)
void *__libc_valloc(size_t bytes) {
if (__malloc_initialized < 0) ptmalloc_init();
void * address = RETURN_ADDRESS(0);
size_t pagesize = GLRO(dl_pagesize);
return _mid_memalign(pagesize, bytes, address);
void *__libc_pvalloc(size_t bytes) {
if (__malloc_initialized < 0) ptmalloc_init();
void * address = RETURN_ADDRESS(0);
size_t pagesize = GLRO(dl_pagesize);
size_t rounded_bytes = ALIGN_UP(bytes, pagesize);
/* Check for overflow. */
if (bytes > SIZE_MAX - 2 * pagesize - MINSIZE) {
return 0;
return _mid_memalign(pagesize, rounded_bytes, address);
void *__libc_calloc(size_t n, size_t elem_size) {
mstate av;
mchunkptr oldtop, p;
INTERNAL_SIZE_T bytes, sz, csz, oldtopsize;
void * mem;
unsigned long clearsize;
unsigned long nclears;
/* size_t is unsigned so the behavior on overflow is defined. */
bytes = n * elem_size;
(((INTERNAL_SIZE_T) 1) << (8 * sizeof(INTERNAL_SIZE_T) / 2))
if (__builtin_expect((n | elem_size) >= HALF_INTERNAL_SIZE_T, 0)) {
if (elem_size != 0 && bytes / elem_size != n) {
return 0;
void *(*hook)(size_t, const void *) = atomic_forced_read(__malloc_hook);
if (__builtin_expect(hook != NULL, 0)) {
sz = bytes;
mem = (*hook)(sz, RETURN_ADDRESS(0));
if (mem == 0) return 0;
return memset(mem, 0, sz);
sz = bytes;
arena_get(av, sz);
if (av) {
/* Check if we hand out the top chunk, in which case there may be no
need to clear. */
oldtop = top(av);
oldtopsize = chunksize(top(av));
/* Only newly allocated memory is guaranteed to be cleared. */
if (av == &main_arena &&
oldtopsize < mp_.sbrk_base + av->max_system_mem - (char *) oldtop)
oldtopsize = (mp_.sbrk_base + av->max_system_mem - (char *) oldtop);
if (av != &main_arena) {
heap_info *heap = heap_for_ptr(oldtop);
if (oldtopsize <
(char *) heap + heap->mprotect_size - (char *) oldtop)
oldtopsize =
(char *) heap + heap->mprotect_size - (char *) oldtop;
} else {
/* No usable arenas. */
oldtop = 0;
oldtopsize = 0;
mem = _int_malloc(av, sz);
assert(!mem || chunk_is_mmapped(mem2chunk(mem)) ||
av == arena_for_chunk(mem2chunk(mem)));
if (mem == 0 && av != NULL) {
LIBC_PROBE(memory_calloc_retry, 1, sz);
av = arena_get_retry(av, sz);
mem = _int_malloc(av, sz);
if (av != NULL) __libc_lock_unlock(av->mutex);
/* Allocation failed even after a retry. */
if (mem == 0) return 0;
p = mem2chunk(mem);
/* Two optional cases in which clearing not necessary */
if (chunk_is_mmapped(p)) {
if (__builtin_expect(perturb_byte, 0)) return memset(mem, 0, sz);
return mem;
csz = chunksize(p);
if (perturb_byte == 0 && (p == oldtop && csz > oldtopsize)) {
/* clear only the bytes from non-freshly-sbrked memory */
csz = oldtopsize;
/* Unroll clear of <= 36 bytes (72 if 8byte sizes). We know that
contents have an odd number of INTERNAL_SIZE_T-sized words;
minimally 3. */
d = (INTERNAL_SIZE_T *) mem;
clearsize = csz - SIZE_SZ;
nclears = clearsize / sizeof(INTERNAL_SIZE_T);
assert(nclears >= 3);
if (nclears > 9)
return memset(d, 0, clearsize);
else {
*(d + 0) = 0;
*(d + 1) = 0;
*(d + 2) = 0;
if (nclears > 4) {
*(d + 3) = 0;
*(d + 4) = 0;
if (nclears > 6) {
*(d + 5) = 0;
*(d + 6) = 0;
if (nclears > 8) {
*(d + 7) = 0;
*(d + 8) = 0;
return mem;
------------------------------ malloc ------------------------------
static void *_int_malloc(mstate av, size_t bytes) {
INTERNAL_SIZE_T nb; /* normalized request size */
unsigned int idx; /* associated bin index */
mbinptr bin; /* associated bin */
mchunkptr victim; /* inspected/selected chunk */
INTERNAL_SIZE_T size; /* its size */
int victim_index; /* its bin index */
mchunkptr remainder; /* remainder from a split */
unsigned long remainder_size; /* its size */
unsigned int block; /* bit map traverser */
unsigned int bit; /* bit map traverser */
unsigned int map; /* current word of binmap */
mchunkptr fwd; /* misc temp for linking */
mchunkptr bck; /* misc temp for linking */
const char *errstr = NULL;
Convert request size to internal form by adding SIZE_SZ bytes
overhead plus possibly more to obtain necessary alignment and/or
to obtain a size of at least MINSIZE, the smallest allocatable
size. Also, checked_request2size traps (returning 0) request sizes
that are so large that they wrap around zero when padded and
checked_request2size(bytes, nb);
/* There are no usable arenas. Fall back to sysmalloc to get a chunk from
mmap. */
if (__glibc_unlikely(av == NULL)) {
void *p = sysmalloc(nb, av);
if (p != NULL) alloc_perturb(p, bytes);
return p;
If the size qualifies as a fastbin, first check corresponding bin.
This code is safe to execute even if av is not yet initialized, so we
can try it without checking, which saves some time on this fast path.
if ((unsigned long) (nb) <= (unsigned long) (get_max_fast())) {
idx = fastbin_index(nb);
mfastbinptr *fb = &fastbin(av, idx);
mchunkptr pp = *fb;
do {
victim = pp;
if (victim == NULL) break;
} while ((pp = catomic_compare_and_exchange_val_acq(fb, victim->fd,
victim)) != victim);
if (victim != 0) {
if (__builtin_expect(fastbin_index(chunksize(victim)) != idx, 0)) {
errstr = "malloc(): memory corruption (fast)";
malloc_printerr(check_action, errstr, chunk2mem(victim), av);
return NULL;
check_remalloced_chunk(av, victim, nb);
void *p = chunk2mem(victim);
alloc_perturb(p, bytes);
return p;
If a small request, check regular bin. Since these "smallbins"
hold one size each, no searching within bins is necessary.
(For a large request, we need to wait until unsorted chunks are
processed to find best fit. But for small ones, fits are exact
anyway, so we can check now, which is faster.)
if (in_smallbin_range(nb)) {
idx = smallbin_index(nb);
bin = bin_at(av, idx);
if ((victim = last(bin)) != bin) {
if (victim == 0) /* initialization check */
else {
bck = victim->bk;
if (__glibc_unlikely(bck->fd != victim)) {
errstr = "malloc(): smallbin double linked list corrupted";
goto errout;
set_inuse_bit_at_offset(victim, nb);
bin->bk = bck;
bck->fd = bin;
if (av != &main_arena) set_non_main_arena(victim);
check_malloced_chunk(av, victim, nb);
void *p = chunk2mem(victim);
alloc_perturb(p, bytes);
return p;
If this is a large request, consolidate fastbins before continuing.
While it might look excessive to kill all fastbins before
even seeing if there is space available, this avoids
fragmentation problems normally associated with fastbins.
Also, in practice, programs tend to have runs of either small or
large requests, but less often mixtures, so consolidation is not
invoked all that often in most programs. And the programs that
it is called frequently in otherwise tend to fragment.
else {
idx = largebin_index(nb);
if (have_fastchunks(av)) malloc_consolidate(av);
Process recently freed or remaindered chunks, taking one only if
it is exact fit, or, if this a small request, the chunk is remainder from
the most recent non-exact fit. Place other traversed chunks in
bins. Note that this step is the only place in any routine where
chunks are placed in bins.
The outer loop here is needed because we might not realize until
near the end of malloc that we should have consolidated, so must
do so and retry. This happens at most once, and only when we would
otherwise need to expand memory to service a "small" request.
for (;;) {
int iters = 0;
// walk from the unsorted head to end to find one chunk
// First In First Out
while ((victim = unsorted_chunks(av)->bk) != unsorted_chunks(av)) {
bck = victim->bk;
if (__builtin_expect(chunksize_nomask(victim) <= 2 * SIZE_SZ, 0) ||
__builtin_expect(chunksize_nomask(victim) > av->system_mem, 0))
malloc_printerr(check_action, "malloc(): memory corruption",
chunk2mem(victim), av);
size = chunksize(victim);
If a small request, try to use last remainder if it is the
only chunk in unsorted bin. This helps promote locality for
runs of consecutive small requests. This is the only
exception to best-fit, and applies only when there is
no exact fit for a small chunk.
if (in_smallbin_range(nb) && bck == unsorted_chunks(av) &&
victim == av->last_remainder &&
(unsigned long) (size) > (unsigned long) (nb + MINSIZE)) {
/* split and reattach remainder */
remainder_size = size - nb;
remainder = chunk_at_offset(victim, nb);
unsorted_chunks(av)->bk = unsorted_chunks(av)->fd = remainder;
av->last_remainder = remainder;
remainder->bk = remainder->fd = unsorted_chunks(av);
if (!in_smallbin_range(remainder_size)) {
remainder->fd_nextsize = NULL;
remainder->bk_nextsize = NULL;
set_head(victim, nb | PREV_INUSE |
(av != &main_arena ? NON_MAIN_ARENA : 0));
set_head(remainder, remainder_size | PREV_INUSE);
set_foot(remainder, remainder_size);
check_malloced_chunk(av, victim, nb);
void *p = chunk2mem(victim);
alloc_perturb(p, bytes);
return p;
/* remove from unsorted list */
unsorted_chunks(av)->bk = bck;
bck->fd = unsorted_chunks(av);
/* Take now instead of binning if exact fit */
if (size == nb) {
set_inuse_bit_at_offset(victim, size);
if (av != &main_arena) set_non_main_arena(victim);
check_malloced_chunk(av, victim, nb);
void *p = chunk2mem(victim);
alloc_perturb(p, bytes);
return p;
/* place chunk in bin */
if (in_smallbin_range(size)) {
victim_index = smallbin_index(size);
bck = bin_at(av, victim_index);
fwd = bck->fd;
} else {
victim_index = largebin_index(size);
bck = bin_at(av, victim_index);
fwd = bck->fd;
/* maintain large bins in sorted order */
if (fwd != bck) {
/* Or with inuse bit to speed comparisons */
size |= PREV_INUSE;
/* if smaller than smallest, bypass loop below */
if ((unsigned long) (size) <
(unsigned long) chunksize_nomask(bck->bk)) {
fwd = bck;
bck = bck->bk;
victim->fd_nextsize = fwd->fd;
victim->bk_nextsize = fwd->fd->bk_nextsize;
fwd->fd->bk_nextsize =
victim->bk_nextsize->fd_nextsize = victim;
} else {
while ((unsigned long) size < chunksize_nomask(fwd)) {
fwd = fwd->fd_nextsize;
if ((unsigned long) size ==
(unsigned long) chunksize_nomask(fwd))
/* Always insert in the second position. */
fwd = fwd->fd;
else {
victim->fd_nextsize = fwd;
victim->bk_nextsize = fwd->bk_nextsize;
fwd->bk_nextsize = victim;
victim->bk_nextsize->fd_nextsize = victim;
bck = fwd->bk;
} else
victim->fd_nextsize = victim->bk_nextsize = victim;
mark_bin(av, victim_index);
victim->bk = bck;
victim->fd = fwd;
fwd->bk = victim;
bck->fd = victim;
#define MAX_ITERS 10000
if (++iters >= MAX_ITERS) break;
If a large request, scan through the chunks of current bin in
sorted order to find smallest that fits. Use the skip list for this.
if (!in_smallbin_range(nb)) {
bin = bin_at(av, idx);
/* skip scan if empty or largest chunk is too small */
if ((victim = first(bin)) != bin &&
(unsigned long) chunksize_nomask(victim) >=
(unsigned long) (nb)) {
victim = victim->bk_nextsize;
while (((unsigned long) (size = chunksize(victim)) <
(unsigned long) (nb)))
victim = victim->bk_nextsize;
/* Avoid removing the first entry for a size so that the skip
list does not have to be rerouted. */
if (victim != last(bin) &&
chunksize_nomask(victim) == chunksize_nomask(victim->fd))
victim = victim->fd;
remainder_size = size - nb;
unlink(av, victim, bck, fwd);
/* Exhaust */
if (remainder_size < MINSIZE) {
set_inuse_bit_at_offset(victim, size);
if (av != &main_arena) set_non_main_arena(victim);
/* Split */
else {
remainder = chunk_at_offset(victim, nb);
/* We cannot assume the unsorted list is empty and therefore
have to perform a complete insert here. */
bck = unsorted_chunks(av);
fwd = bck->fd;
if (__glibc_unlikely(fwd->bk != bck)) {
errstr = "malloc(): corrupted unsorted chunks";
goto errout;
remainder->bk = bck;
remainder->fd = fwd;
bck->fd = remainder;
fwd->bk = remainder;
if (!in_smallbin_range(remainder_size)) {
remainder->fd_nextsize = NULL;
remainder->bk_nextsize = NULL;
(av != &main_arena ? NON_MAIN_ARENA : 0));
set_head(remainder, remainder_size | PREV_INUSE);
set_foot(remainder, remainder_size);
check_malloced_chunk(av, victim, nb);
void *p = chunk2mem(victim);
alloc_perturb(p, bytes);
return p;
Search for a chunk by scanning bins, starting with next largest
bin. This search is strictly by best-fit; i.e., the smallest
(with ties going to approximately the least recently used) chunk
that fits is selected.
The bitmap avoids needing to check that most blocks are nonempty.
The particular case of skipping all bins during warm-up phases
when no chunks have been returned yet is faster than it might look.
bin = bin_at(av, idx);
block = idx2block(idx);
map = av->binmap[ block ];
bit = idx2bit(idx);
for (;;) {
/* Skip rest of block if there are no more set bits in this block.
if (bit > map || bit == 0) {
do {
if (++block >= BINMAPSIZE) /* out of bins */
goto use_top;
} while ((map = av->binmap[ block ]) == 0);
bin = bin_at(av, (block << BINMAPSHIFT));
bit = 1;
/* Advance to bin with set bit. There must be one. */
while ((bit & map) == 0) {
bin = next_bin(bin);
bit <<= 1;
assert(bit != 0);
/* Inspect the bin. It is likely to be non-empty */
victim = last(bin);
/* If a false alarm (empty bin), clear the bit. */
if (victim == bin) {
av->binmap[ block ] = map &= ~bit; /* Write through */
bin = next_bin(bin);
bit <<= 1;
else {
size = chunksize(victim);
/* We know the first chunk in this bin is big enough to use. */
assert((unsigned long) (size) >= (unsigned long) (nb));
remainder_size = size - nb;
/* unlink */
unlink(av, victim, bck, fwd);
/* Exhaust */
if (remainder_size < MINSIZE) {
set_inuse_bit_at_offset(victim, size);
if (av != &main_arena) set_non_main_arena(victim);
/* Split */
else {
remainder = chunk_at_offset(victim, nb);
/* We cannot assume the unsorted list is empty and therefore
have to perform a complete insert here. */
bck = unsorted_chunks(av);
fwd = bck->fd;
if (__glibc_unlikely(fwd->bk != bck)) {
errstr = "malloc(): corrupted unsorted chunks 2";
goto errout;
remainder->bk = bck;
remainder->fd = fwd;
bck->fd = remainder;
fwd->bk = remainder;
/* advertise as last remainder */
if (in_smallbin_range(nb)) av->last_remainder = remainder;
if (!in_smallbin_range(remainder_size)) {
remainder->fd_nextsize = NULL;
remainder->bk_nextsize = NULL;
(av != &main_arena ? NON_MAIN_ARENA : 0));
set_head(remainder, remainder_size | PREV_INUSE);
set_foot(remainder, remainder_size);
check_malloced_chunk(av, victim, nb);
void *p = chunk2mem(victim);
alloc_perturb(p, bytes);
return p;
If large enough, split off the chunk bordering the end of memory
(held in av->top). Note that this is in accord with the best-fit
search rule. In effect, av->top is treated as larger (and thus
less well fitting) than any other available chunk since it can
be extended to be as large as necessary (up to system
We require that av->top always exists (i.e., has size >=
MINSIZE) after initialization, so if it would otherwise be
exhausted by current request, it is replenished. (The main
reason for ensuring it exists is that we may need MINSIZE space
to put in fenceposts in sysmalloc.)
victim = av->top;
size = chunksize(victim);
if ((unsigned long) (size) >= (unsigned long) (nb + MINSIZE)) {
remainder_size = size - nb;
remainder = chunk_at_offset(victim, nb);
av->top = remainder;
set_head(victim, nb | PREV_INUSE |
(av != &main_arena ? NON_MAIN_ARENA : 0));
set_head(remainder, remainder_size | PREV_INUSE);
check_malloced_chunk(av, victim, nb);
void *p = chunk2mem(victim);
alloc_perturb(p, bytes);
return p;
/* When we are using atomic ops to free fast chunks we can get
here for all block sizes. */
else if (have_fastchunks(av)) {
/* restore original bin index */
if (in_smallbin_range(nb))
idx = smallbin_index(nb);
idx = largebin_index(nb);
Otherwise, relay to handle system-dependent cases
else {
void *p = sysmalloc(nb, av);
if (p != NULL) alloc_perturb(p, bytes);
return p;
------------------------------ free ------------------------------
static void _int_free(mstate av, mchunkptr p, int have_lock) {
INTERNAL_SIZE_T size; /* its size */
mfastbinptr * fb; /* associated fastbin */
mchunkptr nextchunk; /* next contiguous chunk */
INTERNAL_SIZE_T nextsize; /* its size */
int nextinuse; /* true if nextchunk is used */
INTERNAL_SIZE_T prevsize; /* size of previous contiguous chunk */
mchunkptr bck; /* misc temp for linking */
mchunkptr fwd; /* misc temp for linking */
const char *errstr = NULL;
int locked = 0;
size = chunksize(p);
/* Little security check which won't hurt performance: the
allocator never wrapps around at the end of the address space.
Therefore we can exclude some size values which might appear
here by accident or by "design" from some intruder. */
if (__builtin_expect((uintptr_t) p > (uintptr_t) -size, 0) ||
__builtin_expect(misaligned_chunk(p), 0)) {
errstr = "free(): invalid pointer";
if (!have_lock && locked) __libc_lock_unlock(av->mutex);
malloc_printerr(check_action, errstr, chunk2mem(p), av);
/* We know that each chunk is at least MINSIZE bytes in size or a
multiple of MALLOC_ALIGNMENT. */
if (__glibc_unlikely(size < MINSIZE || !aligned_OK(size))) {
errstr = "free(): invalid size";
goto errout;
check_inuse_chunk(av, p);
If eligible, place chunk on a fastbin so it can be found
and used quickly in malloc.
if ((unsigned long) (size) <= (unsigned long) (get_max_fast())
If TRIM_FASTBINS set, don't place chunks
bordering top into fastbins
&& (chunk_at_offset(p, size) != av->top)
) {
if (__builtin_expect(
chunksize_nomask(chunk_at_offset(p, size)) <= 2 * SIZE_SZ, 0) ||
chunksize(chunk_at_offset(p, size)) >= av->system_mem, 0)) {
/* We might not have a lock at this point and concurrent
of system_mem might have let to a false positive. Redo the test
after getting the lock. */
if (have_lock || ({
assert(locked == 0);
locked = 1;
chunksize_nomask(chunk_at_offset(p, size)) <= 2 * SIZE_SZ ||
chunksize(chunk_at_offset(p, size)) >= av->system_mem;
})) {
errstr = "free(): invalid next size (fast)";
goto errout;
if (!have_lock) {
locked = 0;
free_perturb(chunk2mem(p), size - 2 * SIZE_SZ);
unsigned int idx = fastbin_index(size);
fb = &fastbin(av, idx);
/* Atomically link P to its fastbin: P->FD = *FB; *FB = P; */
mchunkptr old = *fb, old2;
unsigned int old_idx = ~0u;
do {
/* Check that the top of the bin is not the record we are going to
(i.e., double free). */
if (__builtin_expect(old == p, 0)) {
errstr = "double free or corruption (fasttop)";
goto errout;
/* Check that size of fastbin chunk at the top is the same as
size of the chunk that we are adding. We can dereference OLD
only if we have the lock, otherwise it might have already been
deallocated. See use of OLD_IDX below for the actual check. */
if (have_lock && old != NULL)
old_idx = fastbin_index(chunksize(old));
p->fd = old2 = old;
} while ((old = catomic_compare_and_exchange_val_rel(fb, p, old2)) !=
if (have_lock && old != NULL && __builtin_expect(old_idx != idx, 0)) {
errstr = "invalid fastbin entry (free)";
goto errout;
Consolidate other non-mmapped chunks as they arrive.
else if (!chunk_is_mmapped(p)) {
if (!have_lock) {
locked = 1;
nextchunk = chunk_at_offset(p, size);
/* Lightweight tests: check whether the block is already the
top block. */
if (__glibc_unlikely(p == av->top)) {
errstr = "double free or corruption (top)";
goto errout;
/* Or whether the next chunk is beyond the boundaries of the arena. */
if (__builtin_expect(contiguous(av) &&
(char *) nextchunk >=
((char *) av->top + chunksize(av->top)),
0)) {
errstr = "double free or corruption (out)";
goto errout;
/* Or whether the block is actually not marked used. */
if (__glibc_unlikely(!prev_inuse(nextchunk))) {
errstr = "double free or corruption (!prev)";
goto errout;
nextsize = chunksize(nextchunk);
if (__builtin_expect(chunksize_nomask(nextchunk) <= 2 * SIZE_SZ, 0) ||
__builtin_expect(nextsize >= av->system_mem, 0)) {
errstr = "free(): invalid next size (normal)";
goto errout;
free_perturb(chunk2mem(p), size - 2 * SIZE_SZ);
/* consolidate backward */
if (!prev_inuse(p)) {
prevsize = prev_size(p);
size += prevsize;
p = chunk_at_offset(p, -((long) prevsize));
unlink(av, p, bck, fwd);
if (nextchunk != av->top) {
/* get and clear inuse bit */
nextinuse = inuse_bit_at_offset(nextchunk, nextsize);
/* consolidate forward */
if (!nextinuse) {
unlink(av, nextchunk, bck, fwd);
size += nextsize;
} else
clear_inuse_bit_at_offset(nextchunk, 0);
Place the chunk in unsorted chunk list. Chunks are
not placed into regular bins until after they have
been given one chance to be used in malloc.
// insert the unsorted chunk at the end of the unsorted bin
// First In First Out
bck = unsorted_chunks(av);
fwd = bck->fd;
if (__glibc_unlikely(fwd->bk != bck)) {
errstr = "free(): corrupted unsorted chunks";
goto errout;
p->fd = fwd;
p->bk = bck;
if (!in_smallbin_range(size)) {
p->fd_nextsize = NULL;
p->bk_nextsize = NULL;
bck->fd = p;
fwd->bk = p;
set_head(p, size | PREV_INUSE);
set_foot(p, size);
check_free_chunk(av, p);
If the chunk borders the current high end of memory,
consolidate into top
else {
size += nextsize;
set_head(p, size | PREV_INUSE);
av->top = p;
check_chunk(av, p);
If freeing a large space, consolidate possibly-surrounding
chunks. Then, if the total unused topmost memory exceeds trim
threshold, ask malloc_trim to reduce top.
Unless max_fast is 0, we don't know if there are fastbins
bordering top, so we cannot tell for sure whether threshold
has been reached unless fastbins are consolidated. But we
don't want to consolidate on each free. As a compromise,
consolidation is performed if FASTBIN_CONSOLIDATION_THRESHOLD
is reached.
if ((unsigned long) (size) >= FASTBIN_CONSOLIDATION_THRESHOLD) {