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 mem-
       ory-allocation functions.  If no hook functions were successfully installed by mtrace(), muntrace() does nothing.

       When mtrace(3) 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 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 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 3.44 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/.

GNU
								    2012-04-18								 MTRACE(3)