mtrace(3) - Linux manual page (original) (raw)

mtrace(3) Library Functions Manual mtrace(3)

NAME top

   mtrace, muntrace - malloc tracing

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <mcheck.h>**

   **void mtrace(void);**
   **void muntrace(void);**

DESCRIPTION top

   The **mtrace**() function installs hook functions for the memory-
   allocation functions ([malloc(3)](../man3/malloc.3.html), [realloc(3)](../man3/realloc.3.html) [memalign(3)](../man3/memalign.3.html), [free(3)](../man3/free.3.html)).
   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 top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌────────────────────────────────────┬───────────────┬───────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├────────────────────────────────────┼───────────────┼───────────┤
   │ **mtrace**(), **muntrace**()               │ Thread safety │ MT-Unsafe │
   └────────────────────────────────────┴───────────────┴───────────┘

STANDARDS top

   GNU.

NOTES top

   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)](../man1/mtrace.1.html), 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 top

   The line-number information produced by [mtrace(1)](../man1/mtrace.1.html) is not always
   precise: the line number references may refer to the previous or
   following (nonblank) line of the source code.

EXAMPLES top

   The shell session below demonstrates the use of the **mtrace**()
   function and the [mtrace(1)](../man1/mtrace.1.html) 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)](../man3/malloc.3.html) calls inside the _for_ loop.  The final message
   corresponds to the call to [calloc(3)](../man3/calloc.3.html) (which in turn calls
   [malloc(3)](../man3/malloc.3.html)).

SEE ALSO top

   [mtrace(1)](../man1/mtrace.1.html), [malloc(3)](../man3/malloc.3.html), [malloc_hook(3)](../man3/malloc%5Fhook.3.html), [mcheck(3)](../man3/mcheck.3.html)

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Linux man-pages 6.10 2024-07-23 mtrace(3)

Pages that refer to this page:mtrace(1), malloc(3), malloc_hook(3), mallopt(3), mcheck(3)