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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | ATTRIBUTES | STANDARDS | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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mtrace(3) Library Functions Manual mtrace(3)
mtrace, muntrace - malloc tracing
Standard C library (libc, -lc)
#include <mcheck.h>
void mtrace(void);
void muntrace(void);
The mtrace() function installs hook functions for the memory-
allocation functions (malloc(3), realloc(3) memalign(3), 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.
The muntrace() function disables the hook functions installed by
mtrace(), so that tracing information is no longer recorded for
the memory-allocation functions. If no hook functions were
successfully installed by mtrace(), muntrace() does nothing.
When mtrace() is called, it checks the value of the environment
variable MALLOC_TRACE, 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.
If MALLOC_TRACE is not set, or the pathname it specifies is
invalid or not writable, then no hook functions are installed, and
mtrace() has no effect. In set-user-ID and set-group-ID programs,
MALLOC_TRACE is ignored, and mtrace() has no effect.
For an explanation of the terms used in this section, see
attributes(7).
┌────────────────────────────────────┬───────────────┬───────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────┼───────────────┼───────────┤
│ mtrace(), muntrace() │ Thread safety │ MT-Unsafe │
└────────────────────────────────────┴───────────────┴───────────┘
GNU.
In normal usage, mtrace() is called once at the start of execution
of a program, and muntrace() is never called.
The tracing output produced after a call to mtrace() is textual,
but not designed to be human readable. The GNU C library provides
a Perl script, 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 mtrace() incurs a performance penalty (if
MALLOC_TRACE points to a valid, writable pathname).
The line-number information produced by mtrace(1) is not always
precise: the line number references may refer to the previous or
following (nonblank) line of the source code.
The shell session below demonstrates the use of the mtrace()
function and the mtrace(1) command in a program that has memory
leaks at two different locations. The demonstration uses the
following program:
$ cat t_mtrace.c
#include <mcheck.h>
#include <stdio.h>
#include <stdlib.h>
int
main(void)
{
mtrace();
for (unsigned int j = 0; j < 2; j++)
malloc(100); /* Never freed--a memory leak */
calloc(16, 16); /* Never freed--a memory leak */
exit(EXIT_SUCCESS);
}
When we run the program as follows, we see that mtrace() diagnosed
memory leaks at two different locations in the program:
$ cc -g t_mtrace.c -o t_mtrace
$ export MALLOC_TRACE=/tmp/t
$ ./t_mtrace
$ 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
malloc(3) calls inside the for loop. The final message
corresponds to the call to calloc(3) (which in turn calls
malloc(3)).
mtrace(1), malloc(3), malloc_hook(3), mcheck(3)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 mtrace(3)
Pages that refer to this page: mtrace(1), malloc(3), malloc_hook(3), mallopt(3), mcheck(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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