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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 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  informa‐
       tion 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).

       β”Œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”¬β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”
       β”‚Interface                                                                                                                           β”‚ Attribute     β”‚ Value     β”‚
       β”œβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”Όβ”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€
       β”‚mtrace(), muntrace()                                                                                                                β”‚ Thread safety β”‚ MT-Unsafe β”‚
       β””β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”΄β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”€β”˜

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

GNU                                                                            2021-03-22                                                                      MTRACE(3)