NAME

       mtrace, muntrace - malloc tracing

LIBRARY

       Standard C library (libc, -lc)

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 │
       └─────────────────────────────────────────────────────────────┴───────────────┴───────────┘

STANDARDS

       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 <stdio.h>
           #include <stdlib.h>

           int
           main(void)
           {
               mtrace();

               for (unsigned 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)