mtrace

MTRACE(3)                   Linux Programmer's Manual                  MTRACE(3)



NAME
       mtrace, muntrace - malloc tracing

SYNOPSIS
       #include <mcheck.h>

       void mtrace(void);

       void muntrace(void);

DESCRIPTION
       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.

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌─────────────────────┬───────────────┬───────────┐
       │Interface            Attribute     Value     │
       ├─────────────────────┼───────────────┼───────────┤
       │mtrace(), muntrace() │ Thread safety │ MT-Unsafe │
       └─────────────────────┴───────────────┴───────────┘
CONFORMING TO
       These functions are GNU extensions.

NOTES
       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).

BUGS
       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.

EXAMPLES
       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[])
           {
               mtrace();

               for (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)).

SEE ALSO
       mtrace(1), malloc(3), malloc_hook(3), mcheck(3)

COLOPHON
       This page is part of release 5.09 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                                2020-11-01                          MTRACE(3)