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.\" Copyright (c) 2012 by Michael Kerrisk <>
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date. The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein. The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.TH MTRACE 3 2012-03-22 "GNU" "Linux Programmer's Manual"
mtrace, muntrace \- malloc tracing
.B "#include <mcheck.h>"
.B "void mtrace(void);"
.B "void muntrace(void);"
.BR mtrace ()
function installs hook functions for the memory-allocation functions
.RB ( malloc (3),
.BR realloc (3)
.BR memalign (3),
.BR free (3)).
These hook functions record tracing information about memory allocation
and deallocation.
The tracing information can be used to discover memory leaks and
attempts to free nonallocated memory in a program.
.BR muntrace ()
function disables the hook functions installed by
.BR mtrace (),
so that tracing information is no longer recorded
for the memory-allocation functions.
If no hook functions were successfully installed by
.BR mtrace (),
.BR muntrace ()
does nothing.
.BR mtrace (3)
is called, it checks the value of the environment variable
which should contain the pathname of a file in which
the tracing information is to be recorded.
If the pathname is successfully opened, it is truncated to zero length.
is not set,
or the pathname it specifies is invalid or not writable,
then no hook functions are installed, and
.BR mtrace ()
has no effect.
In set-user-ID and set-group-ID programs,
is ignored, and
.BR mtrace ()
has no effect.
These functions are GNU extensions.
In normal usage,
.BR mtrace ()
is called once at the start of execution of a program, and
.BR muntrace ()
is never called.
The tracing output produced after a call to
.BR mtrace ()
is textual, but not designed to be human readable.
The GNU C library provides a Perl script,
.BR mtrace (1),
that interprets the trace log and produces human-readable output.
For best results,
the traced program should be compiled with debugging enabled,
so that line-number information is recorded in the executable.
The tracing performed by
.BR mtrace ()
incurs a performance penalty (if
points to a valid, writable pathname).
The line-number information produced by
.BR mtrace (1)
is not always precise:
the line number references may refer to the previous or following (non-blank)
line of the source code.
The shell session below demonstrates the use of the
.BR mtrace ()
function and the
.BR mtrace (1)
command in a program that has memory leaks at two different locations.
The demonstration uses the following program:
.in +4
.RB "$ " "cat t_mtrace.c"
#include <mcheck.h>
#include <stdlib.h>
#include <stdio.h>
main(int argc, char *argv[])
int j;
for (j = 0; j < 2; j++)
malloc(100); /* Never freed\-\-a memory leak */
calloc(16, 16); /* Never freed\-\-a memory leak */
When we run the program as follows, we see that
.BR mtrace ()
diagnosed memory leaks at two different locations in the program:
.in +4n
.RB "$ " "cc \-g t_mtrace.c \-o t_mtrace"
.RB "$ " "export MALLOC_TRACE=/tmp/t"
.RB "$ " "./t_mtrace"
.RB "$ " "mtrace ./t_mtrace $MALLOC_TRACE"
Memory not freed:
Address Size Caller
0x084c9378 0x64 at /home/cecilia/t_mtrace.c:12
0x084c93e0 0x64 at /home/cecilia/t_mtrace.c:12
0x084c9448 0x100 at /home/cecilia/t_mtrace.c:16
The first two messages about unfreed memory correspond to the two
.BR malloc (3)
calls inside the
.I for
The final message corresponds to the call to
.BR calloc (3)
(which in turn calls
.BR malloc (3)).
.BR mtrace (1),
.BR malloc (3),
.BR malloc_hook (3)
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