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NAME | SYNOPSIS | DESCRIPTION | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON |
MTRACE(3) Linux Programmer's Manual MTRACE(3)
mtrace, muntrace - malloc tracing
#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 │
└─────────────────────┴───────────────┴───────────┘
These functions are GNU extensions.
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 <stdlib.h>
#include <stdio.h>
int
main(int argc, char *argv[])
{
int j;
mtrace();
for (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 release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2015-03-02 MTRACE(3)
Pages that refer to this page: mtrace(1), malloc(3), malloc_hook(3), mallopt(3), mcheck(3)