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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 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.
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 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).
BUGS
       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.
EXAMPLE
       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 mem-
       ory 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 mal-
       loc(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 4.15 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                               2017-09-15                         MTRACE(3)