MTRACE(3) Linux Programmer's Manual MTRACE(3)
mtrace, muntrace - malloc tracing
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 deal-
location. 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 vari-
able MALLOC_TRACE, which should contain the pathname of a file in which
the tracing information is to be recorded. If the pathname is success-
fully 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
|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 com-
piled with debugging enabled, so that line-number information is
recorded in the executable.
The tracing performed by mtrace() incurs a performance penalty (if MAL-
LOC_TRACE points to a valid, writable pathname).
The line-number information produced by mtrace(1) is not always pre-
cise: 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
main(int argc, char *argv)
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 mtrace() diagnosed mem-
ory leaks at two different locations in the program:
$ cc -g t_mtrace.c -o t_mtrace
$ export MALLOC_TRACE=/tmp/t
$ 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 mal-
loc(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)
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