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This is a version (aka dlmalloc) of malloc/free/realloc written by
Doug Lea and released to the public domain. Use, modify, and
redistribute this code without permission or acknowledgment in any
way you wish. Send questions, comments, complaints, performance
data, etc to
* VERSION 2.7.2 Sat Aug 17 09:07:30 2002 Doug Lea (dl at gee)
Note: There may be an updated version of this malloc obtainable at
Check before installing!
* Quickstart
This library is all in one file to simplify the most common usage:
ftp it, compile it (-O), and link it into another program. All
of the compile-time options default to reasonable values for use on
most unix platforms. Compile -DWIN32 for reasonable defaults on windows.
You might later want to step through various compile-time and dynamic
tuning options.
For convenience, an include file for code using this malloc is at:
You don't really need this .h file unless you call functions not
defined in your system include files. The .h file contains only the
excerpts from this file needed for using this malloc on ANSI C/C++
systems, so long as you haven't changed compile-time options about
naming and tuning parameters. If you do, then you can create your
own malloc.h that does include all settings by cutting at the point
indicated below.
* 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For a longer but slightly out of date high-level description, see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_t* p);
realloc(Void_t* 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_t* chunks[]);
independent_comalloc(size_t n_elements, size_t sizes[], Void_t* chunks[]);
pvalloc(size_t n);
cfree(Void_t* p);
malloc_trim(size_t pad);
malloc_usable_size(Void_t* 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: NOT thread-safe unless USE_MALLOC_LOCK defined
When USE_MALLOC_LOCK is defined, wrappers are created to
surround every public call with either a pthread mutex or
a win32 spinlock (depending on WIN32). This is not
especially fast, and can be a major bottleneck.
It is designed only to provide minimal protection
in concurrent environments, and to provide a basis for
extensions. If you are using malloc in a concurrent program,
you would be far better off obtaining ptmalloc, which is
derived from a version of this malloc, and is well-tuned for
concurrent programs. (See Note that
even when USE_MALLOC_LOCK is defined, you can guarantee
full thread-safety only if no threads acquire memory through
direct calls to MORECORE or other system-level allocators.
Compliance: I believe it is compliant with the 1997 Single Unix Specification
(See 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. It is also reported to work on WIN32 platforms.
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:
__STD_C derived from C compiler defines
WIN32 NOT defined
USE_MEMCPY 1 if HAVE_MEMCPY is defined
HAVE_MMAP defined as 1
HAVE_MREMAP 0 unless linux defined
malloc_getpagesize derived from system #includes, or 4096 if not
LACKS_UNISTD_H NOT defined unless WIN32
LACKS_SYS_PARAM_H NOT defined unless WIN32
LACKS_SYS_MMAN_H NOT defined unless WIN32
Changing default word sizes:
PTR_UINT unsigned long
CHUNK_SIZE_T unsigned long
Configuration and functionality options:
DEBUG NOT defined
MALLOC_FAILURE_ACTION errno = ENOMEM, if __STD_C defined, else no-op
Options for customizing MORECORE:
Tuning options that are also dynamically changeable via mallopt:
There are several other #defined constants and macros that you
probably don't want to touch unless you are extending or adapting malloc.
WIN32 sets up defaults for MS environment and compilers.
Otherwise defaults are for unix.
/* #define WIN32 */
#ifdef WIN32
# define WIN32_LEAN_AND_MEAN
# include <windows.h>
/* Win32 doesn't supply or need the following headers */
/* Use the supplied emulation of sbrk */
# define MORECORE sbrk
# define MORECORE_FAILURE ((void*)(-1))
/* Use the supplied emulation of mmap and munmap */
# define HAVE_MMAP 1
# define MUNMAP_FAILURE (-1)
# define MMAP_CLEARS 1
/* These values don't really matter in windows mmap emulation */
# define MAP_PRIVATE 1
# define MAP_ANONYMOUS 2
# define PROT_READ 1
# define PROT_WRITE 2
/* Emulation functions defined at the end of this file */
/* If USE_MALLOC_LOCK, use supplied critical-section-based lock functions */
static int slwait(int *sl);
static int slrelease(int *sl);
# endif
static long getpagesize(void);
static long getregionsize(void);
static void *sbrk(long size);
static void *mmap(void *ptr, long size, long prot, long type,
long handle, long arg);
static long munmap(void *ptr, long size);
static void vminfo (unsigned long*free, unsigned long*reserved,
unsigned long*committed);
static int cpuinfo (int whole, unsigned long*kernel, unsigned long*user);
__STD_C should be nonzero if using ANSI-standard C compiler, a C++
compiler, or a C compiler sufficiently close to ANSI to get away
with it.
#ifndef __STD_C
# if defined(__STDC__) || defined(_cplusplus)
# define __STD_C 1
# else
# define __STD_C 0
# endif
#endif /*__STD_C*/
Void_t* is the pointer type that malloc should say it returns
#ifndef Void_t
# if (__STD_C || defined(WIN32))
# define Void_t void
# else
# define Void_t char
# endif
#endif /*Void_t*/
#if __STD_C
# include <stddef.h> /* for size_t */
# include <sys/types.h>
#ifdef __cplusplus
extern "C" {
/* define LACKS_UNISTD_H if your system does not have a <unistd.h>. */
/* #define LACKS_UNISTD_H */
# include <unistd.h>
/* define LACKS_SYS_PARAM_H if your system does not have a <sys/param.h>. */
/* #define LACKS_SYS_PARAM_H */
#include <stdio.h> /* needed for malloc_stats */
#include <errno.h> /* needed for optional MALLOC_FAILURE_ACTION */
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 -DDEBUG, 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 DEBUG set will
attempt to check every non-mmapped allocated and free chunk in the
course of computing the summaries. (By nature, mmapped regions
cannot be checked very much automatically.)
Setting 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 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.
# include <assert.h>
# define assert(x) ((void)0)
The unsigned integer type used for comparing any two chunk sizes.
This should be at least as wide as size_t, but should not be signed.
#ifndef CHUNK_SIZE_T
# define CHUNK_SIZE_T unsigned long
The unsigned integer type used to hold addresses when they are
manipulated as integers. Except that it is not defined on all
systems, intptr_t would suffice.
#ifndef PTR_UINT
# define PTR_UINT unsigned long
INTERNAL_SIZE_T is the word-size used for internal bookkeeping
of chunk sizes.
The default version is the same as size_t.
While not strictly necessary, it is best to define this as an
unsigned type, even if size_t is a signed type. This may avoid some
artificial size limitations on some systems.
On a 64-bit machine, you may be able to reduce malloc overhead by
defining INTERNAL_SIZE_T to be a 32 bit `unsigned int' at the
expense of not being able to handle more than 2^32 of malloced
space. If this limitation is acceptable, you are encouraged to set
this unless you are on a platform requiring 16byte alignments. In
this case the alignment requirements turn out to negate any
potential advantages of decreasing size_t word size.
Implementors: Beware of the possible combinations of:
- INTERNAL_SIZE_T might be signed or unsigned, might be 32 or 64 bits,
and might be the same width as int or as long
- size_t might have different width and signedness as INTERNAL_SIZE_T
- int and long might be 32 or 64 bits, and might be the same width
To deal with this, most comparisons and difference computations
among INTERNAL_SIZE_Ts should cast them to CHUNK_SIZE_T, being
aware of the fact that casting an unsigned int to a wider long does
not sign-extend. (This also makes checking for negative numbers
awkward.) Some of these casts result in harmless compiler warnings
on some systems.
# define INTERNAL_SIZE_T size_t
/* The corresponding word size */
#define SIZE_SZ (sizeof (INTERNAL_SIZE_T))
MALLOC_ALIGNMENT is the minimum alignment for malloc'ed chunks.
It must be a power of two at least 2 * SIZE_SZ, even on machines
for which smaller alignments would suffice. It may be defined as
larger than this though. Note however that code and data structures
are optimized for the case of 8-byte alignment.
/* The corresponding bit mask value */
REALLOC_ZERO_BYTES_FREES should be set if a call to
realloc with zero bytes should be the same as a call to free.
Some people think it should. 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
# define TRIM_FASTBINS 0
USE_DL_PREFIX will prefix all public routines with the string 'dl'.
This is necessary when you only want to use this malloc in one part
of a program, using your regular system malloc elsewhere.
/* #define USE_DL_PREFIX */
USE_MALLOC_LOCK causes wrapper functions to surround each
callable routine with pthread mutex lock/unlock.
/* #define USE_MALLOC_LOCK */
If USE_PUBLIC_MALLOC_WRAPPERS is defined, every public routine is
actually a wrapper function that first calls MALLOC_PREACTION, then
calls the internal routine, and follows it with
MALLOC_POSTACTION. This is needed for locking, but you can also use
this, without USE_MALLOC_LOCK, for purposes of interception,
instrumentation, etc. It is a sad fact that using wrappers often
noticeably degrades performance of malloc-intensive programs.
Two-phase name translation.
All of the actual routines are given mangled names.
When wrappers are used, they become the public callable versions.
When DL_PREFIX is used, the callable names are prefixed.
# define cALLOc public_cALLOc
# define fREe public_fREe
# define cFREe public_cFREe
# define mALLOc public_mALLOc
# define mEMALIGn public_mEMALIGn
# define rEALLOc public_rEALLOc
# define vALLOc public_vALLOc
# define pVALLOc public_pVALLOc
# define mALLINFo public_mALLINFo
# define mALLOPt public_mALLOPt
# define mTRIm public_mTRIm
# define mSTATs public_mSTATs
# define mUSABLe public_mUSABLe
# define iCALLOc public_iCALLOc
# define iCOMALLOc public_iCOMALLOc
# define public_cALLOc dlcalloc
# define public_fREe dlfree
# define public_cFREe dlcfree
# define public_mALLOc dlmalloc
# define public_mEMALIGn dlmemalign
# define public_rEALLOc dlrealloc
# define public_vALLOc dlvalloc
# define public_pVALLOc dlpvalloc
# define public_mALLINFo dlmallinfo
# define public_mALLOPt dlmallopt
# define public_mTRIm dlmalloc_trim
# define public_mSTATs dlmalloc_stats
# define public_mUSABLe dlmalloc_usable_size
# define public_iCALLOc dlindependent_calloc
# define public_iCOMALLOc dlindependent_comalloc
#else /* USE_DL_PREFIX */
# define public_cALLOc calloc
# define public_fREe free
# define public_cFREe cfree
# define public_mALLOc malloc
# define public_mEMALIGn memalign
# define public_rEALLOc realloc
# define public_vALLOc valloc
# define public_pVALLOc pvalloc
# define public_mALLINFo mallinfo
# define public_mALLOPt mallopt
# define public_mTRIm malloc_trim
# define public_mSTATs malloc_stats
# define public_mUSABLe malloc_usable_size
# define public_iCALLOc independent_calloc
# define public_iCOMALLOc independent_comalloc
#endif /* USE_DL_PREFIX */
HAVE_MEMCPY should be defined if you are not otherwise using
ANSI STD C, but still have memcpy and memset in your C library
and want to use them in calloc and realloc. Otherwise simple
macro versions are defined below.
USE_MEMCPY should be defined as 1 if you actually want to
have memset and memcpy called. People report that the macro
versions are faster than libc versions on some systems.
Even if USE_MEMCPY is set to 1, loops to copy/clear small chunks
(of <= 36 bytes) are manually unrolled in realloc and calloc.
#ifndef USE_MEMCPY
# define USE_MEMCPY 1
# else
# define USE_MEMCPY 0
# endif
#if (__STD_C || defined(HAVE_MEMCPY))
# ifdef WIN32
/* On Win32 memset and memcpy are already declared in windows.h */
# else
# if __STD_C
void* memset(void*, int, size_t);
void* memcpy(void*, const void*, size_t);
# else
Void_t* memset();
Void_t* memcpy();
# endif
# endif
MALLOC_FAILURE_ACTION is the action to take before "return 0" when
malloc fails to be able to return memory, either because memory is
exhausted or because of illegal arguments.
By default, sets errno if running on STD_C platform, else does nothing.
# if __STD_C
errno = ENOMEM;
# else
# endif
MORECORE-related declarations. By default, rely on sbrk
# if !defined(__FreeBSD__) && !defined(__OpenBSD__) && \
!defined(__NetBSD__) && !defined(__GNUC__)
# if __STD_C
extern Void_t* sbrk(ptrdiff_t);
# else
extern Void_t* sbrk();
# endif
# endif
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.
# define MORECORE_FAILURE (-1)
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.
Define HAVE_MMAP as true to optionally make malloc() use mmap() to
allocate very large blocks. These will be returned to the
operating system immediately after a free(). Also, if mmap
is available, it is used as a backup strategy in cases where
MORECORE fails to provide space from system.
This malloc is best tuned to work with mmap for large requests.
If you do not have mmap, operations involving very large chunks (1MB
or so) may be slower than you'd like.
#ifndef HAVE_MMAP
# define HAVE_MMAP 1
Standard unix mmap using /dev/zero clears memory so calloc doesn't
need to.
# ifndef MMAP_CLEARS
# define MMAP_CLEARS 1
# endif
#else /* no mmap */
# ifndef MMAP_CLEARS
# define MMAP_CLEARS 0
# endif
MMAP_AS_MORECORE_SIZE is the minimum mmap size argument to use if
sbrk fails, and mmap is used as a backup (which is done only if
HAVE_MMAP). 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. This is currently only possible on Linux with
kernel versions newer than 1.3.77.
# ifdef linux
# define HAVE_MREMAP 1
# else
# define HAVE_MREMAP 0
# endif
#endif /* HAVE_MMAP */
The system page size. To the extent possible, this malloc manages
memory from the system in page-size units. Note that this value is
cached during initialization into a field of malloc_state. So even
if malloc_getpagesize is a function, it is only called once.
The following mechanics for getpagesize were adapted from bsd/gnu
getpagesize.h. If none of the system-probes here apply, a value of
4096 is used, which should be OK: If they don't apply, then using
the actual value probably doesn't impact performance.
#ifndef malloc_getpagesize
# include <unistd.h>
# endif
# ifdef _SC_PAGESIZE /* some SVR4 systems omit an underscore */
# ifndef _SC_PAGE_SIZE
# endif
# endif
# ifdef _SC_PAGE_SIZE
# define malloc_getpagesize sysconf(_SC_PAGE_SIZE)
# else
# if defined(BSD) || defined(DGUX) || defined(HAVE_GETPAGESIZE)
extern size_t getpagesize();
# define malloc_getpagesize getpagesize()
# else
# ifdef WIN32 /* use supplied emulation of getpagesize */
# define malloc_getpagesize getpagesize()
# else
# include <sys/param.h>
# endif
# define malloc_getpagesize EXEC_PAGESIZE
# else
# ifdef NBPG
# ifndef CLSIZE
# define malloc_getpagesize NBPG
# else
# define malloc_getpagesize (NBPG * CLSIZE)
# endif
# else
# ifdef NBPC
# define malloc_getpagesize NBPC
# else
# ifdef PAGESIZE
# define malloc_getpagesize PAGESIZE
# else /* just guess */
# define malloc_getpagesize (4096)
# endif
# endif
# endif
# endif
# endif
# endif
# endif
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 instead filled by mallinfo() with
other numbers that might be of interest.
HAVE_USR_INCLUDE_MALLOC_H should be set if you have a
/usr/include/malloc.h file that includes a declaration of struct
mallinfo. If so, it is included; else an SVID2/XPG2 compliant
version is declared below. These must be precisely the same for
mallinfo() to work. The original SVID version of this struct,
defined on most systems with mallinfo, declares all fields as
ints. But some others define as unsigned long. If your system
defines the fields using a type of different width than listed here,
you must #include your system version and #define
# include "/usr/include/malloc.h"
/* SVID2/XPG mallinfo structure */
struct mallinfo {
int arena; /* non-mmapped space allocated from system */
int ordblks; /* number of free chunks */
int smblks; /* number of fastbin blocks */
int hblks; /* number of mmapped regions */
int hblkhd; /* space in mmapped regions */
int usmblks; /* maximum total allocated space */
int fsmblks; /* space available in freed fastbin blocks */
int uordblks; /* total allocated space */
int fordblks; /* total free space */
int keepcost; /* top-most, releasable (via malloc_trim) space */
SVID/XPG defines four standard parameter 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 other
options in mallopt described below.
/* ---------- 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.
#if __STD_C
Void_t* public_mALLOc(size_t);
Void_t* public_mALLOc();
free(Void_t* 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.
#if __STD_C
void public_fREe(Void_t*);
void public_fREe();
calloc(size_t n_elements, size_t element_size);
Returns a pointer to n_elements * element_size bytes, with all locations
set to zero.
#if __STD_C
Void_t* public_cALLOc(size_t, size_t);
Void_t* public_cALLOc();
realloc(Void_t* 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.
#if __STD_C
Void_t* public_rEALLOc(Void_t*, size_t);
Void_t* public_rEALLOc();
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.
#if __STD_C
Void_t* public_mEMALIGn(size_t, size_t);
Void_t* public_mEMALIGn();
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.
#if __STD_C
Void_t* public_vALLOc(size_t);
Void_t* public_vALLOc();
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 -1 256*1024 any (-1U disables trimming)
M_TOP_PAD -2 0 any
M_MMAP_THRESHOLD -3 256*1024 any (or 0 if no MMAP support)
M_MMAP_MAX -4 65536 any (0 disables use of mmap)
#if __STD_C
int public_mALLOPt(int, int);
int public_mALLOPt();
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, reused, or consolidated)
hblks: current number of mmapped regions
hblkhd: total bytes held in mmapped regions
usmblks: the maximum total allocated space. This will be greater
than current total if trimming has occurred.
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.
#if __STD_C
struct mallinfo public_mALLINFo(void);
struct mallinfo public_mALLINFo();
independent_calloc(size_t n_elements, size_t element_size, Void_t* chunks[]);
independent_calloc is similar to calloc, but instead of returning a
single cleared space, it returns an array of pointers to n_elements
independent elements that can hold contents of size elem_size, each
of which starts out cleared, and can be independently freed,
realloc'ed etc. The elements are guaranteed to be adjacently
allocated (this is not guaranteed to occur with multiple callocs or
mallocs), which may also improve cache locality in some
The "chunks" argument is optional (i.e., may be null, which is
probably the most typical usage). If it is null, the returned array
is itself dynamically allocated and should also be freed when it is
no longer needed. Otherwise, the chunks array must be of at least
n_elements in length. It is filled in with the pointers to the
In either case, independent_calloc returns this pointer array, or
null if the allocation failed. If n_elements is zero and "chunks"
is null, it returns a chunk representing an array with zero elements
(which should be freed if not wanted).
Each element must be individually freed when it is no longer
needed. If you'd like to instead be able to free all at once, you
should instead use regular calloc and assign pointers into this
space to represent elements. (In this case though, you cannot
independently free elements.)
independent_calloc simplifies and speeds up implementations of many
kinds of pools. It may also be useful when constructing large data
structures that initially have a fixed number of fixed-sized nodes,
but the number is not known at compile time, and some of the nodes
may later need to be freed. For example:
struct Node { int item; struct Node* next; };
struct Node* build_list() {
struct Node** pool;
int n = read_number_of_nodes_needed();
if (n <= 0) return 0;
pool = (struct Node**)(independent_calloc(n, sizeof (struct Node), 0);
if (pool == 0) die();
/ / organize into a linked list...
struct Node* first = pool[0];
for (i = 0; i < n-1; ++i)
pool[i]->next = pool[i+1];
free(pool); / / Can now free the array (or not, if it is needed later)
return first;
#if __STD_C
Void_t** public_iCALLOc(size_t, size_t, Void_t**);
Void_t** public_iCALLOc();
independent_comalloc(size_t n_elements, size_t sizes[], Void_t* chunks[]);
independent_comalloc allocates, all at once, a set of n_elements
chunks with sizes indicated in the "sizes" array. It returns
an array of pointers to these elements, each of which can be
independently freed, realloc'ed etc. The elements are guaranteed to
be adjacently allocated (this is not guaranteed to occur with
multiple callocs or mallocs), which may also improve cache locality
in some applications.
The "chunks" argument is optional (i.e., may be null). If it is null
the returned array is itself dynamically allocated and should also
be freed when it is no longer needed. Otherwise, the chunks array
must be of at least n_elements in length. It is filled in with the
pointers to the chunks.
In either case, independent_comalloc returns this pointer array, or
null if the allocation failed. If n_elements is zero and chunks is
null, it returns a chunk representing an array with zero elements
(which should be freed if not wanted).
Each element must be individually freed when it is no longer
needed. If you'd like to instead be able to free all at once, you
should instead use a single regular malloc, and assign pointers at
particular offsets in the aggregate space. (In this case though, you
cannot independently free elements.)
independent_comallac differs from independent_calloc in that each
element may have a different size, and also that it does not
automatically clear elements.
independent_comalloc can be used to speed up allocation in cases
where several structs or objects must always be allocated at the
same time. For example:
struct Head { ... }
struct Foot { ... }
void send_message(char* msg) {
int msglen = strlen(msg);
size_t sizes[3] = { sizeof (struct Head), msglen, sizeof (struct Foot) };
void* chunks[3];
if (independent_comalloc(3, sizes, chunks) == 0)
struct Head* head = (struct Head*)(chunks[0]);
char* body = (char*)(chunks[1]);
struct Foot* foot = (struct Foot*)(chunks[2]);
/ / ...
In general though, independent_comalloc is worth using only for
larger values of n_elements. For small values, you probably won't
detect enough difference from series of malloc calls to bother.
Overuse of independent_comalloc can increase overall memory usage,
since it cannot reuse existing noncontiguous small chunks that
might be available for some of the elements.
#if __STD_C
Void_t** public_iCOMALLOc(size_t, size_t*, Void_t**);
Void_t** public_iCOMALLOc();
pvalloc(size_t n);
Equivalent to valloc(minimum-page-that-holds(n)), that is,
round up n to nearest pagesize.
#if __STD_C
Void_t* public_pVALLOc(size_t);
Void_t* public_pVALLOc();
cfree(Void_t* p);
Equivalent to free(p).
cfree is needed/defined on some systems that pair it with calloc,
for odd historical reasons (such as: cfree is used in example
code in the first edition of K&R).
#if __STD_C
void public_cFREe(Void_t*);
void public_cFREe();
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
rreturn 0.
#if __STD_C
int public_mTRIm(size_t);
int public_mTRIm();
malloc_usable_size(Void_t* 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);
#if __STD_C
size_t public_mUSABLe(Void_t*);
size_t public_mUSABLe();
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.
#if __STD_C
void public_mSTATs();
void public_mSTATs();
/* 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 16 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
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 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 (256 * 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)
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
# define DEFAULT_MMAP_THRESHOLD (256 * 1024)
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. If
HAVE_MMAP is not set, the default value is 0, and attempts to set it
to non-zero values in mallopt will fail.
#define M_MMAP_MAX -4
# define DEFAULT_MMAP_MAX (65536)
# else
# define DEFAULT_MMAP_MAX (0)
# endif
#ifdef __cplusplus
}; /* end of extern "C" */
To make a fully customizable malloc.h header file, cut everything
above this line, put into file malloc.h, edit to suit, and #include it
on the next line, as well as in programs that use this malloc.
/* #include "malloc.h" */
/* --------------------- public wrappers ---------------------- */
/* Declare all routines as internal */
# if __STD_C
static Void_t* mALLOc(size_t);
static void fREe(Void_t*);
static Void_t* rEALLOc(Void_t*, size_t);
static Void_t* mEMALIGn(size_t, size_t);
static Void_t* vALLOc(size_t);
static Void_t* pVALLOc(size_t);
static Void_t* cALLOc(size_t, size_t);
static Void_t** iCALLOc(size_t, size_t, Void_t**);
static Void_t** iCOMALLOc(size_t, size_t*, Void_t**);
static void cFREe(Void_t*);
static int mTRIm(size_t);
static size_t mUSABLe(Void_t*);
static void mSTATs();
static int mALLOPt(int, int);
static struct mallinfo mALLINFo(void);
# else
static Void_t* mALLOc();
static void fREe();
static Void_t* rEALLOc();
static Void_t* mEMALIGn();
static Void_t* vALLOc();
static Void_t* pVALLOc();
static Void_t* cALLOc();
static Void_t** iCALLOc();
static Void_t** iCOMALLOc();
static void cFREe();
static int mTRIm();
static size_t mUSABLe();
static void mSTATs();
static int mALLOPt();
static struct mallinfo mALLINFo();
# endif
defined to return 0 on success, and nonzero on failure.
The return value of MALLOC_POSTACTION is currently ignored
in wrapper functions since there is no reasonable default
action to take on failure.
# ifdef WIN32
static int mALLOC_MUTEx;
# define MALLOC_POSTACTION slrelease(&mALLOC_MUTEx)
# else
# include <pthread.h>
static pthread_mutex_t mALLOC_MUTEx = PTHREAD_MUTEX_INITIALIZER;
# define MALLOC_PREACTION pthread_mutex_lock(&mALLOC_MUTEx)
# define MALLOC_POSTACTION pthread_mutex_unlock(&mALLOC_MUTEx)
# endif /* USE_MALLOC_LOCK */
# else
/* Substitute anything you like for these */
# endif
Void_t* public_mALLOc(size_t bytes) {
Void_t* m;
return 0;
m = mALLOc(bytes);
return m;
void public_fREe(Void_t* m) {
Void_t* public_rEALLOc(Void_t* m, size_t bytes) {
return 0;
m = rEALLOc(m, bytes);
return m;
Void_t* public_mEMALIGn(size_t alignment, size_t bytes) {
Void_t* m;
return 0;
m = mEMALIGn(alignment, bytes);
return m;
Void_t* public_vALLOc(size_t bytes) {
Void_t* m;
return 0;
m = vALLOc(bytes);
return m;
Void_t* public_pVALLOc(size_t bytes) {
Void_t* m;
return 0;
m = pVALLOc(bytes);
return m;
Void_t* public_cALLOc(size_t n, size_t elem_size) {
Void_t* m;
return 0;
m = cALLOc(n, elem_size);
return m;
Void_t** public_iCALLOc(size_t n, size_t elem_size, Void_t** chunks) {
Void_t** m;
return 0;
m = iCALLOc(n, elem_size, chunks);
return m;
Void_t** public_iCOMALLOc(size_t n, size_t sizes[], Void_t** chunks) {
Void_t** m;
return 0;
m = iCOMALLOc(n, sizes, chunks);
return m;
void public_cFREe(Void_t* m) {
int public_mTRIm(size_t s) {
int result;
return 0;
result = mTRIm(s);
return result;
size_t public_mUSABLe(Void_t* m) {
size_t result;
return 0;
result = mUSABLe(m);
return result;
void public_mSTATs() {
struct mallinfo public_mALLINFo() {
struct mallinfo m;
struct mallinfo nm = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
return nm;
m = mALLINFo();
return m;
int public_mALLOPt(int p, int v) {
int result;
return 0;
result = mALLOPt(p, v);
return result;
/* ------------- Optional versions of memcopy ---------------- */
Note: memcpy is ONLY invoked with non-overlapping regions,
so the (usually slower) memmove is not needed.
# define MALLOC_COPY(dest, src, nbytes) memcpy((dest), (src), (nbytes))
# define MALLOC_ZERO(dest, nbytes) memset((dest), 0, (nbytes))
#else /* !USE_MEMCPY */
/* Use Duff's device for good zeroing/copying performance. */
# define MALLOC_ZERO(charp, nbytes) \
do { \
CHUNK_SIZE_T mctmp = (nbytes)/sizeof (INTERNAL_SIZE_T); \
long mcn; \
if (mctmp < 8) mcn = 0; else { mcn = (mctmp-1)/8; mctmp %= 8; } \
switch (mctmp) { \
case 0: for (;;) { *mzp++ = 0; \
case 7: *mzp++ = 0; \
case 6: *mzp++ = 0; \
case 5: *mzp++ = 0; \
case 4: *mzp++ = 0; \
case 3: *mzp++ = 0; \
case 2: *mzp++ = 0; \
case 1: *mzp++ = 0; if (mcn <= 0) break; --mcn; } \
} \
} while (0)
# define MALLOC_COPY(dest,src,nbytes) \
do { \
INTERNAL_SIZE_T* mcsrc = (INTERNAL_SIZE_T*) (src); \
INTERNAL_SIZE_T* mcdst = (INTERNAL_SIZE_T*) (dest); \
CHUNK_SIZE_T mctmp = (nbytes)/sizeof (INTERNAL_SIZE_T); \
long mcn; \
if (mctmp < 8) mcn = 0; else { mcn = (mctmp-1)/8; mctmp %= 8; } \
switch (mctmp) { \
case 0: for (;;) { *mcdst++ = *mcsrc++; \
case 7: *mcdst++ = *mcsrc++; \
case 6: *mcdst++ = *mcsrc++; \
case 5: *mcdst++ = *mcsrc++; \
case 4: *mcdst++ = *mcsrc++; \
case 3: *mcdst++ = *mcsrc++; \
case 2: *mcdst++ = *mcsrc++; \
case 1: *mcdst++ = *mcsrc++; if (mcn <= 0) break; --mcn; } \
} \
} while (0)
/* ------------------ MMAP support ------------------ */
# ifndef LACKS_FCNTL_H
# include <fcntl.h>
# endif
# include <sys/mman.h>
# endif
# if !defined(MAP_ANONYMOUS) && defined(MAP_ANON)
# endif
Nearly all versions of mmap support MAP_ANONYMOUS,
so the following is unlikely to be needed, but is
supplied just in case.
static int dev_zero_fd = -1; /* Cached file descriptor for /dev/zero. */
# define MMAP(addr, size, prot, flags) ((dev_zero_fd < 0) ? \
(dev_zero_fd = open("/dev/zero", O_RDWR), \
mmap((addr), (size), (prot), (flags), dev_zero_fd, 0)) : \
mmap((addr), (size), (prot), (flags), dev_zero_fd, 0))
# else
# define MMAP(addr, size, prot, flags) \
(mmap((addr), (size), (prot), (flags)|MAP_ANONYMOUS, -1, 0))
# endif
#endif /* HAVE_MMAP */
----------------------- 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 prev_size; /* Size of previous chunk (if free). */
INTERNAL_SIZE_T size; /* Size in bytes, including overhead. */
struct malloc_chunk* fd; /* double links -- used only if free. */
struct malloc_chunk* bk;
typedef struct malloc_chunk* mchunkptr;
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 allocated | |
| Size of chunk, in bytes |P|
mem-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| User data starts here... .
. .
. (malloc_usable_space() bytes) .
. |
nextchunk-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Size of chunk |
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 |
`head:' | Size of chunk, in bytes |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 |
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.
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 two 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 (IS_MMAPPED) set in their size fields. Because they are
allocated one-by-one, each must contain its own trailing size field.
---------- Size and alignment checks and conversions ----------
/* conversion from malloc headers to user pointers, and back */
#define chunk2mem(p) ((Void_t*)((char*)(p) + 2*SIZE_SZ))
#define mem2chunk(mem) ((mchunkptr)((char*)(mem) - 2*SIZE_SZ))
/* The smallest possible chunk */
#define MIN_CHUNK_SIZE (sizeof (struct malloc_chunk))
/* The smallest size we can malloc is an aligned minimal chunk */
#define MINSIZE \
/* Check if m has acceptable alignment */
#define aligned_OK(m) (((PTR_UINT)((m)) & (MALLOC_ALIGN_MASK)) == 0)
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 sero.
#define REQUEST_OUT_OF_RANGE(req) \
((CHUNK_SIZE_T)(req) >= \
/* 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)) { \
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)->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)->size & IS_MMAPPED)
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) ((p)->size & ~(SIZE_BITS))
/* Ptr to next physical malloc_chunk. */
#define next_chunk(p) ((mchunkptr)(((char*)(p)) + ((p)->size & ~PREV_INUSE)))
/* Ptr to previous physical malloc_chunk */
#define prev_chunk(p) ((mchunkptr)(((char*)(p)) - ((p)->prev_size)))
/* 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))+((p)->size & ~PREV_INUSE)))->size) & PREV_INUSE)
/* set/clear chunk as being inuse without otherwise disturbing */
#define set_inuse(p)\
((mchunkptr)(((char*)(p)) + ((p)->size & ~PREV_INUSE)))->size |= PREV_INUSE
#define clear_inuse(p)\
((mchunkptr)(((char*)(p)) + ((p)->size & ~PREV_INUSE)))->size &= ~(PREV_INUSE)
/* check/set/clear inuse bits in known places */
#define inuse_bit_at_offset(p, s)\
(((mchunkptr)(((char*)(p)) + (s)))->size & PREV_INUSE)
#define set_inuse_bit_at_offset(p, s)\
(((mchunkptr)(((char*)(p)) + (s)))->size |= PREV_INUSE)
#define clear_inuse_bit_at_offset(p, s)\
(((mchunkptr)(((char*)(p)) + (s)))->size &= ~(PREV_INUSE))
/* Set size at head, without disturbing its use bit */
#define set_head_size(p, s) ((p)->size = (((p)->size & PREV_INUSE) | (s)))
/* Set size/use field */
#define set_head(p, s) ((p)->size = (s))
/* Set size at footer (only when chunk is not in use) */
#define set_foot(p, s) (((mchunkptr)((char*)(p) + (s)))->prev_size = (s))
-------------------- 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 HAVE_MMAP is true, but 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 preceded 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]) - (SIZE_SZ<<1)))
/* 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(P, BK, FD) { \
(FD) = (P)->fd; \
(BK) = (P)->bk; \
(FD)->bk = (BK); \
(BK)->fd = (FD); \
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
The bins top out around 1MB because we expect to service large
requests via mmap.
#define NBINS 96
#define NSMALLBINS 32
#define MIN_LARGE_SIZE 256
#define in_smallbin_range(sz) \
#define smallbin_index(sz) (((unsigned)(sz)) >> 3)
Compute index for size. We expect this to be inlined when
compiled with optimization, else not, which works out well.
static int largebin_index(unsigned int sz) {
unsigned int x = sz >> SMALLBIN_WIDTH;
unsigned int m; /* bit position of highest set bit of m */
if (x >= 0x10000) return NBINS-1;
/* On intel, use BSRL instruction to find highest bit */
#if defined(__GNUC__) && defined(i386)
__asm__("bsrl %1,%0\n\t"
: "=r" (m)
: "g" (x));
Based on branch-free nlz algorithm in chapter 5 of Henry
S. Warren Jr's book "Hacker's Delight".
unsigned int n = ((x - 0x100) >> 16) & 8;
x <<= n;
m = ((x - 0x1000) >> 16) & 4;
n += m;
x <<= m;
m = ((x - 0x4000) >> 16) & 2;
n += m;
x = (x << m) >> 14;
m = 13 - n + (x & ~(x>>1));
/* Use next 2 bits to create finer-granularity bins */
return NSMALLBINS + (m << 2) + ((sz >> (m + 6)) & 3);
#define bin_index(sz) \
((in_smallbin_range(sz)) ? smallbin_index(sz) : largebin_index(sz))
FIRST_SORTED_BIN_SIZE is the chunk size corresponding to the
first bin that is maintained in sorted order. This must
be the smallest size corresponding to a given bin.
Normally, this should be MIN_LARGE_SIZE. But you can weaken
best fit guarantees to sometimes speed up malloc by increasing value.
Doing this means that malloc may choose a chunk that is
non-best-fitting by up to the width of the bin.
Some useful cutoff values:
512 - all bins sorted
2560 - leaves bins <= 64 bytes wide unsorted
12288 - leaves bins <= 512 bytes wide unsorted
65536 - leaves bins <= 4096 bytes wide unsorted
262144 - leaves bins <= 32768 bytes wide unsorted
-1 - no bins sorted (not recommended!)
/* #define FIRST_SORTED_BIN_SIZE 65536 */
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 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 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;
/* offset 2 to use otherwise unindexable first 2 bins */
#define fastbin_index(sz) ((((unsigned int)(sz)) >> 3) - 2)
/* The maximum fastbin request size we support */
#define MAX_FAST_SIZE 80
#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 loarge chunks even
if trimming is not used.
((unsigned long)(DEFAULT_TRIM_THRESHOLD) >> 1)
Since the lowest 2 bits in max_fast don't matter in size comparisons,
they are used as flags.
ANYCHUNKS_BIT held in max_fast indicates that there may be any
freed chunks at all. It is set true when entering a chunk into any
#define ANYCHUNKS_BIT (1U)
#define have_anychunks(M) (((M)->max_fast & ANYCHUNKS_BIT))
#define set_anychunks(M) ((M)->max_fast |= ANYCHUNKS_BIT)
#define clear_anychunks(M) ((M)->max_fast &= ~ANYCHUNKS_BIT)
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.
#define have_fastchunks(M) (((M)->max_fast & FASTCHUNKS_BIT))
#define set_fastchunks(M) ((M)->max_fast |= (FASTCHUNKS_BIT|ANYCHUNKS_BIT))
#define clear_fastchunks(M) ((M)->max_fast &= ~(FASTCHUNKS_BIT))
Set value of max_fast.
Use impossibly small value if 0.
#define set_max_fast(M, s) \
(M)->max_fast = (((s) == 0)? SMALLBIN_WIDTH: request2size(s)) | \
#define get_max_fast(M) \
morecore_properties is a status word holding dynamically discovered
or controlled properties of the morecore function
#define contiguous(M) \
(((M)->morecore_properties & MORECORE_CONTIGUOUS_BIT))
#define noncontiguous(M) \
(((M)->morecore_properties & MORECORE_CONTIGUOUS_BIT) == 0)
#define set_contiguous(M) \
((M)->morecore_properties |= MORECORE_CONTIGUOUS_BIT)
#define set_noncontiguous(M) \
((M)->morecore_properties &= ~MORECORE_CONTIGUOUS_BIT)
----------- Internal state representation and initialization -----------
struct malloc_state {
/* The maximum chunk size to be eligible for fastbin */
INTERNAL_SIZE_T max_fast; /* low 2 bits used as flags */
/* Fastbins */
mfastbinptr fastbins[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];
/* Bitmap of bins. Trailing zero map handles cases of largest binned size */
unsigned int binmap[BINMAPSIZE+1];
/* Tunable parameters */
CHUNK_SIZE_T trim_threshold;
INTERNAL_SIZE_T mmap_threshold;
/* Memory map support */
int n_mmaps;
int n_mmaps_max;
int max_n_mmaps;
/* Cache malloc_getpagesize */
unsigned int pagesize;
/* Track properties of MORECORE */
unsigned int morecore_properties;
/* Statistics */
INTERNAL_SIZE_T mmapped_mem;
INTERNAL_SIZE_T sbrked_mem;
INTERNAL_SIZE_T max_sbrked_mem;
INTERNAL_SIZE_T max_mmapped_mem;
INTERNAL_SIZE_T max_total_mem;
typedef struct malloc_state *mstate;
There is exactly one instance of this struct in this malloc.
If you are adapting this malloc in a way that does NOT use a static
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 av_; /* never directly referenced */
All uses of av_ are via get_malloc_state().
At most one "call" to get_malloc_state is made per invocation of
the public versions of malloc and free, but other routines
that in turn invoke malloc and/or free may call more then once.
Also, it is called in check* routines if DEBUG is set.
#define get_malloc_state() (&(av_))
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.)
#if __STD_C
static void malloc_init_state(mstate av)
static void malloc_init_state(av) 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;
av->top_pad = DEFAULT_TOP_PAD;
av->n_mmaps_max = DEFAULT_MMAP_MAX;
av->mmap_threshold = DEFAULT_MMAP_THRESHOLD;
av->trim_threshold = DEFAULT_TRIM_THRESHOLD;
set_max_fast(av, DEFAULT_MXFAST);
av->top = initial_top(av);
av->pagesize = malloc_getpagesize;
Other internal utilities operating on mstates
#if __STD_C
static Void_t* sYSMALLOc(INTERNAL_SIZE_T, mstate);
static int sYSTRIm(size_t, mstate);
static void malloc_consolidate(mstate);
static Void_t** iALLOc(size_t, size_t*, int, Void_t**);
static Void_t* sYSMALLOc();
static int sYSTRIm();
static void malloc_consolidate();
static Void_t** iALLOc();
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!)
#if ! DEBUG
# define check_chunk(P)
# define check_free_chunk(P)
# define check_inuse_chunk(P)
# define check_remalloced_chunk(P,N)
# define check_malloced_chunk(P,N)
# define check_malloc_state()
# define check_chunk(P) do_check_chunk((P))
# define check_free_chunk(P) do_check_free_chunk((P))
# define check_inuse_chunk(P) do_check_inuse_chunk((P))
# define check_remalloced_chunk(P,N) do_check_remalloced_chunk((P),(N))
# define check_malloced_chunk(P,N) do_check_malloced_chunk((P),(N))
# define check_malloc_state() do_check_malloc_state()
Properties of all chunks
# if __STD_C
static void do_check_chunk(mchunkptr p)
# else
static void do_check_chunk(p) mchunkptr p;
# endif
mstate av = get_malloc_state();
CHUNK_SIZE_T 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->sbrked_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((CHUNK_SIZE_T)(sz) >= MINSIZE);
/* top predecessor always marked inuse */
else {
/* 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(((p->prev_size + sz) & (av->pagesize-1)) == 0);
/* mem is aligned */
# else
/* force an appropriate assert violation if debug set */
# endif
Properties of free chunks
# if __STD_C
static void do_check_free_chunk(mchunkptr p)
# else
static void do_check_free_chunk(p) mchunkptr p;
# endif
mstate av = get_malloc_state();
mchunkptr next = chunk_at_offset(p, sz);
/* Chunk must claim to be free ... */
assert (!chunk_is_mmapped(p));
/* Unless a special marker, must have OK fields */
if ((CHUNK_SIZE_T)(sz) >= MINSIZE)
assert((sz & MALLOC_ALIGN_MASK) == 0);
/* ... matching footer field */
assert(next->prev_size == 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
# if __STD_C
static void do_check_inuse_chunk(mchunkptr p)
# else
static void do_check_inuse_chunk(p) mchunkptr p;
# endif
mstate av = get_malloc_state();
mchunkptr next;
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);
if (next == av->top) {
assert(chunksize(next) >= MINSIZE);
else if (!inuse(next))
Properties of chunks recycled from fastbins
# if __STD_C
static void do_check_remalloced_chunk(mchunkptr p, INTERNAL_SIZE_T s)
# else
static void do_check_remalloced_chunk(p, s) mchunkptr p; INTERNAL_SIZE_T s;
# endif
/* Legal size ... */
assert((sz & MALLOC_ALIGN_MASK) == 0);
assert((CHUNK_SIZE_T)(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
# if __STD_C
static void do_check_malloced_chunk(mchunkptr p, INTERNAL_SIZE_T s)
# else
static void do_check_malloced_chunk(p, s) mchunkptr p; INTERNAL_SIZE_T s;
# endif
/* same as recycled case ... */
do_check_remalloced_chunk(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 = get_malloc_state();
int i;
mchunkptr p;
mchunkptr q;
mbinptr b;
unsigned int binbit;
int empty;
unsigned int idx;
CHUNK_SIZE_T 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))
/* pagesize is a power of 2 */
assert((av->pagesize & (av->pagesize-1)) == 0);
/* properties of fastbins */
/* max_fast is in allowed range */
assert(get_max_fast(av) <= request2size(MAX_FAST_SIZE));
max_fast_bin = fastbin_index(av->max_fast);
for (i = 0; i < NFASTBINS; ++i) {
p = av->fastbins[i];
/* all bins past max_fast are empty */
if (i > max_fast_bin)
assert(p == 0);
while (p != 0) {
/* each chunk claims to be inuse */
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) {
binbit = get_binmap(av,i);
empty = last(b) == b;
if (!binbit)
else if (!empty)
for (p = last(b); p != b; p = p->bk) {
/* each chunk claims to be free */
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 ||
(CHUNK_SIZE_T)chunksize(p->bk) >=
/* chunk is followed by a legal chain of inuse chunks */
for (q = next_chunk(p);
(q != av->top && inuse(q) &&
(CHUNK_SIZE_T)(chunksize(q)) >= MINSIZE);
q = next_chunk(q))
/* top chunk is OK */
/* sanity checks for statistics */
assert(total <= (CHUNK_SIZE_T)(av->max_total_mem));
assert(av->n_mmaps >= 0);
assert(av->n_mmaps <= av->max_n_mmaps);
assert((CHUNK_SIZE_T)(av->sbrked_mem) <=
assert((CHUNK_SIZE_T)(av->mmapped_mem) <=
assert((CHUNK_SIZE_T)(av->max_total_mem) >=
(CHUNK_SIZE_T)(av->mmapped_mem) + (CHUNK_SIZE_T)(av->sbrked_mem));
/* ----------- 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.
#if __STD_C
static Void_t* sYSMALLOc(INTERNAL_SIZE_T nb, mstate av)
static Void_t* sYSMALLOc(nb, av) 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 */
CHUNK_SIZE_T remainder_size; /* its size */
CHUNK_SIZE_T sum; /* for updating stats */
size_t pagemask = av->pagesize - 1;
If there is space available in fastbins, consolidate and retry
malloc from scratch rather than getting memory from system. This
can occur only if nb is in smallbin range so we didn't consolidate
upon entry to malloc. It is much easier to handle this case here
than in malloc proper.
if (have_fastchunks(av)) {
return mALLOc(nb - MALLOC_ALIGN_MASK);
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 ((CHUNK_SIZE_T)(nb) >= (CHUNK_SIZE_T)(av->mmap_threshold) &&
(av->n_mmaps < av->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.
size = (nb + SIZE_SZ + MALLOC_ALIGN_MASK + pagemask) & ~pagemask;
/* Don't try if size wraps around 0 */
if ((CHUNK_SIZE_T)(size) > (CHUNK_SIZE_T)(nb)) {
mm = (char*)(MMAP(0, size, PROT_READ|PROT_WRITE, MAP_PRIVATE));
if (mm != (char*)(MORECORE_FAILURE)) {
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().
front_misalign = (INTERNAL_SIZE_T)chunk2mem(mm) & MALLOC_ALIGN_MASK;
if (front_misalign > 0) {
correction = MALLOC_ALIGNMENT - front_misalign;
p = (mchunkptr)(mm + correction);
p->prev_size = correction;
set_head(p, (size - correction) |IS_MMAPPED);
else {
p = (mchunkptr)mm;
p->prev_size = 0;
set_head(p, size|IS_MMAPPED);
/* update statistics */
if (++av->n_mmaps > av->max_n_mmaps)
av->max_n_mmaps = av->n_mmaps;
sum = av->mmapped_mem += size;
if (sum > (CHUNK_SIZE_T)(av->max_mmapped_mem))
av->max_mmapped_mem = sum;
sum += av->sbrked_mem;
if (sum > (CHUNK_SIZE_T)(av->max_total_mem))
av->max_total_mem = sum;
return chunk2mem(p);
/* 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) ||
((CHUNK_SIZE_T) (old_size) >= MINSIZE &&
/* Precondition: not enough current space to satisfy nb request */
assert((CHUNK_SIZE_T)(old_size) < (CHUNK_SIZE_T)(nb + MINSIZE));
/* Precondition: all fastbins are consolidated */
/* Request enough space for nb + pad + overhead */
size = nb + av->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 = (size + pagemask) & ~pagemask;
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));
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" in
address space, so sbrk cannot extend to give contiguous space, but
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.
if (brk == (char*)(MORECORE_FAILURE)) {
/* Cannot merge with old top, so add its size back in */
if (contiguous(av))
size = (size + old_size + pagemask) & ~pagemask;
/* If we are relying on mmap as backup, then use larger units */
/* Don't try if size wraps around 0 */
if ((CHUNK_SIZE_T)(size) > (CHUNK_SIZE_T)(nb)) {
brk = (char*)(MMAP(0, size, PROT_READ|PROT_WRITE, MAP_PRIVATE));
if (brk != (char*)(MORECORE_FAILURE)) {
/* We do not need, and cannot use, another sbrk call to find end */
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 incorrectly
bridge regions.
if (brk != (char*)(MORECORE_FAILURE)) {
av->sbrked_mem += size;
If MORECORE extends previous space, we can likewise extend top size.
if (brk == old_end && snd_brk == (char*)(MORECORE_FAILURE)) {
set_head(old_top, (size + old_size) | PREV_INUSE);
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;
If MORECORE returns an address lower than we have seen before,
we know it isn't really contiguous. This and some subsequent
checks help cope with non-conforming MORECORE functions and
the presence of "foreign" calls to MORECORE from outside of
malloc or by other threads. We cannot guarantee to detect
these in all cases, but cope with the ones we do detect.
if (contiguous(av) && old_size != 0 && brk < old_end) {
/* handle contiguous cases */
if (contiguous(av)) {
We can tolerate forward non-contiguities here (usually due
to foreign calls) but treat them as part of our space for
stats reporting.
if (old_size != 0)
av->sbrked_mem += brk - old_end;
/* Guarantee alignment of first new chunk made from this space */
front_misalign = (INTERNAL_SIZE_T)chunk2mem(brk) & MALLOC_ALIGN_MASK;
if (front_misalign > 0) {
Skip over some bytes to arrive at an aligned position.
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.
correction = MALLOC_ALIGNMENT - front_misalign;
aligned_brk += correction;
If this isn't adjacent to existing space, then we will not
be able to merge with old_top space, so must add to 2nd request.
correction += old_size;
/* Extend the end address to hit a page boundary */
end_misalign = (INTERNAL_SIZE_T)(brk + size + correction);
correction += ((end_misalign + pagemask) & ~pagemask) - end_misalign;
assert(correction >= 0);
snd_brk = (char*)(MORECORE(correction));
if (snd_brk == (char*)(MORECORE_FAILURE)) {
If can't allocate correction, try to at least find out current
brk. It might be enough to proceed without failing.
correction = 0;
snd_brk = (char*)(MORECORE(0));
else if (snd_brk < brk) {
If the second call gives noncontiguous space even though
it says it won't, the only course of action is to ignore
results of second call, and conservatively estimate where
the first call left us. Also set noncontiguous, so this
won't happen again, leaving at most one hole.
Note that this check is intrinsically incomplete. Because
MORECORE is allowed to give more space than we ask for,
there is no reliable way to detect a noncontiguity
producing a forward gap for the second call.
snd_brk = brk + size;
correction = 0;
/* handle non-contiguous cases */
else {
/* MORECORE/mmap must correctly align */
/* Find out current end of memory */
if (snd_brk == (char*)(MORECORE_FAILURE)) {
snd_brk = (char*)(MORECORE(0));
av->sbrked_mem += snd_brk - brk - size;
/* Adjust top based on results of second sbrk */
if (snd_brk != (char*)(MORECORE_FAILURE)) {
av->top = (mchunkptr)aligned_brk;
set_head(av->top, (snd_brk - aligned_brk + correction) | PREV_INUSE);
av->sbrked_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 space
we don't own. These fenceposts are artificial chunks that are
marked as inuse and are in any case too small to use. We need
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 least
enough space in old_top to do this.
old_size = (old_size - 3*SIZE_SZ) & ~MALLOC_ALIGN_MASK;
set_head(old_top, old_size | PREV_INUSE);
Note that the following assignments completely overwrite
old_top when old_size was previously MINSIZE. This is
intentional. We need the fencepost, even if old_top otherwise gets
chunk_at_offset(old_top, old_size)->size =
chunk_at_offset(old_top, old_size + SIZE_SZ)->size =
If possible, release the rest, suppressing trimming.
if (old_size >= MINSIZE) {
INTERNAL_SIZE_T tt = av->trim_threshold;
av->trim_threshold = (INTERNAL_SIZE_T)(-1);
av->trim_threshold = tt;
/* Update statistics */
sum = av->sbrked_mem;
if (sum > (CHUNK_SIZE_T)(av->max_sbrked_mem))
av->max_sbrked_mem = sum;
sum += av->mmapped_mem;
if (sum > (CHUNK_SIZE_T)(av->max_total_mem))
av->max_total_mem = sum;
/* finally, do the allocation */
p = av->top;
size = chunksize(p);
/* check that one of the above allocation paths succeeded */
if ((CHUNK_SIZE_T)(size) >= (CHUNK_SIZE_T)(nb + MINSIZE)) {
remainder_size = size - nb;
remainder = chunk_at_offset(p, nb);
av->top = remainder;
set_head(p, nb | PREV_INUSE);
set_head(remainder, remainder_size | PREV_INUSE);
check_malloced_chunk(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.
#if __STD_C
static int sYSTRIm(size_t pad, mstate av)
static int sYSTRIm(pad, av) 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 pagesz;
pagesz = av->pagesize;
top_size = chunksize(av->top);
/* Release in pagesize units, keeping at least one page */
extra = ((top_size - pad - MINSIZE + (pagesz-1)) / pagesz - 1) * pagesz;
if (extra > 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.)
new_brk = (char*)(MORECORE(0));
if (new_brk != (char*)MORECORE_FAILURE) {
released = (long)(current_brk - new_brk);
if (released != 0) {
/* Success. Adjust top. */
av->sbrked_mem -= released;
set_head(av->top, (top_size - released) | PREV_INUSE);
return 1;
return 0;
------------------------------ malloc ------------------------------
#if __STD_C
Void_t* mALLOc(size_t bytes)
Void_t* mALLOc(bytes) size_t bytes;
mstate av = get_malloc_state();
INTERNAL_SIZE_T nb; /* normalized request size */
unsigned int idx; /* associated bin index */
mbinptr bin; /* associated bin */
mfastbinptr* fb; /* associated fastbin */
mchunkptr victim; /* inspected/selected chunk */
INTERNAL_SIZE_T size; /* its size */
int victim_index; /* its bin index */
mchunkptr remainder; /* remainder from a split */
CHUNK_SIZE_T 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 */
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);
Bypass search if no frees yet
if (!have_anychunks(av)) {
if (av->max_fast == 0) /* initialization check */
goto use_top;
If the size qualifies as a fastbin, first check corresponding bin.
if ((CHUNK_SIZE_T)(nb) <= (CHUNK_SIZE_T)(av->max_fast)) {
fb = &(av->fastbins[(fastbin_index(nb))]);
if ((victim = *fb) != 0) {
*fb = victim->fd;
check_remalloced_chunk(victim, nb);
return chunk2mem(victim);
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) {
bck = victim->bk;
set_inuse_bit_at_offset(victim, nb);
bin->bk = bck;
bck->fd = bin;
check_malloced_chunk(victim, nb);
return chunk2mem(victim);
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))
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.
while ((victim = unsorted_chunks(av)->bk) != unsorted_chunks(av)) {
bck = victim->bk;
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 &&
(CHUNK_SIZE_T)(size) > (CHUNK_SIZE_T)(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);
set_head(victim, nb | PREV_INUSE);
set_head(remainder, remainder_size | PREV_INUSE);
set_foot(remainder, remainder_size);
check_malloced_chunk(victim, nb);
return chunk2mem(victim);
/* 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);
check_malloced_chunk(victim, nb);
return chunk2mem(victim);
/* 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;
if (fwd != bck) {
/* if smaller than smallest, place first */
if ((CHUNK_SIZE_T)(size) < (CHUNK_SIZE_T)(bck->bk->size)) {
fwd = bck;
bck = bck->bk;
else if ((CHUNK_SIZE_T)(size) >=
/* maintain large bins in sorted order */
size |= PREV_INUSE; /* Or with inuse bit to speed comparisons */
while ((CHUNK_SIZE_T)(size) < (CHUNK_SIZE_T)(fwd->size))
fwd = fwd->fd;
bck = fwd->bk;
mark_bin(av, victim_index);
victim->bk = bck;
victim->fd = fwd;
fwd->bk = victim;
bck->fd = victim;
If a large request, scan through the chunks of current bin to
find one that fits. (This will be the smallest that fits unless
FIRST_SORTED_BIN_SIZE has been changed from default.) This is
the only step where an unbounded number of chunks might be
scanned without doing anything useful with them. However the
lists tend to be short.
if (!in_smallbin_range(nb)) {
bin = bin_at(av, idx);
for (victim = last(bin); victim != bin; victim = victim->bk) {
size = chunksize(victim);
if ((CHUNK_SIZE_T)(size) >= (CHUNK_SIZE_T)(nb)) {
remainder_size = size - nb;
unlink(victim, bck, fwd);
/* Exhaust */
if (remainder_size < MINSIZE) {
set_inuse_bit_at_offset(victim, size);
check_malloced_chunk(victim, nb);
return chunk2mem(victim);
/* Split */
else {
remainder = chunk_at_offset(victim, nb);
unsorted_chunks(av)->bk = unsorted_chunks(av)->fd = remainder;
remainder->bk = remainder->fd = unsorted_chunks(av);
set_head(victim, nb | PREV_INUSE);
set_head(remainder, remainder_size | PREV_INUSE);
set_foot(remainder, remainder_size);
check_malloced_chunk(victim, nb);
return chunk2mem(victim);
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.
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((CHUNK_SIZE_T)(size) >= (CHUNK_SIZE_T)(nb));
remainder_size = size - nb;