mtrace(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

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

NAME         top

       mtrace, muntrace - malloc tracing

SYNOPSIS         top

       #include <mcheck.h>

       void mtrace(void);
       void muntrace(void);

DESCRIPTION         top

       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         top

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

       ┌────────────────────────────────────┬───────────────┬───────────┐
       │Interface                           Attribute     Value     │
       ├────────────────────────────────────┼───────────────┼───────────┤
       │mtrace(), muntrace()                │ Thread safety │ MT-Unsafe │
       └────────────────────────────────────┴───────────────┴───────────┘

CONFORMING TO         top

       These functions are GNU extensions.

NOTES         top

       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         top

       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         top

       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         top

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

COLOPHON         top

       This page is part of release 5.12 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                            2021-03-22                      MTRACE(3)

Pages that refer to this page: mtrace(1)malloc(3)malloc_hook(3)mallopt(3)mcheck(3)