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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).

     allbox;  lbw20  lb  lb  l  l l.  Interface Attribute Value T{ mtrace(),
     muntrace() T}   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)).