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.

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 (non-blank) 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 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 3.54 of the Linux man-pages project.  A description of the project, and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.