mcheck(3)

NAME

   mcheck,  mcheck_check_all,  mcheck_pedantic,  mprobe - heap consistency
   checking

SYNOPSIS

   #include <mcheck.h>

   int mcheck(void (*abortfunc)(enum mcheck_status mstatus));

   int mcheck_pedantic(void (*abortfunc)(enum mcheck_status mstatus));

   void mcheck_check_all(void);

   enum mcheck_status mprobe(void *ptr);

DESCRIPTION

   The mcheck() function  installs  a  set  of  debugging  hooks  for  the
   malloc(3)  family  of  memory-allocation  functions.  These hooks cause
   certain consistency checks to be performed on the state  of  the  heap.
   The  checks  can  detect  application errors such as freeing a block of
   memory more than once or corrupting  the  bookkeeping  data  structures
   that immediately precede a block of allocated memory.

   To  be effective, the mcheck() function must be called before the first
   call to malloc(3) or a  related  function.   In  cases  where  this  is
   difficult  to  ensure,  linking  the  program  with -lmcheck inserts an
   implicit call to mcheck() (with a NULL argument) before the first  call
   to a memory-allocation function.

   The  mcheck_pedantic()  function  is  similar to mcheck(), but performs
   checks on all allocated blocks whenever one  of  the  memory-allocation
   functions is called.  This can be very slow!

   The  mcheck_check_all()  function  causes  an  immediate  check  on all
   allocated blocks.  This call is effective only if  mcheck()  is  called
   beforehand.

   If the system detects an inconsistency in the heap, the caller-supplied
   function pointed to by abortfunc is invoked  with  a  single  argument,
   mstatus,  that  indicates  what type of inconsistency was detected.  If
   abortfunc is NULL, a default function prints an error message on stderr
   and calls abort(3).

   The  mprobe()  function  performs  a  consistency check on the block of
   allocated memory pointed to by ptr.  The mcheck()  function  should  be
   called beforehand (otherwise mprobe() returns MCHECK_DISABLED).

   The  following list describes the values returned by mprobe() or passed
   as the mstatus argument when abortfunc is invoked:

   MCHECK_DISABLED (mprobe() only)
          mcheck() was not  called  before  the  first  memory  allocation
          function was called.  Consistency checking is not possible.

   MCHECK_OK (mprobe() only)
          No inconsistency detected.

   MCHECK_HEAD
          Memory preceding an allocated block was clobbered.

   MCHECK_TAIL
          Memory following an allocated block was clobbered.

   MCHECK_FREE
          A block of memory was freed twice.

RETURN VALUE

   mcheck() and mcheck_pedantic() return 0 on success, or -1 on error.

VERSIONS

   The  mcheck_pedantic()  and  mcheck_check_all() functions are available
   since glibc 2.2.  The mcheck() and mprobe() functions are present since
   at least glibc 2.0

ATTRIBUTES

   For   an   explanation   of   the  terms  used  in  this  section,  see
   attributes(7).

   
   Interface                     Attribute      Value                 
   
   mcheck(), mcheck_pedantic(),  Thread safety  MT-Unsafe race:mcheck 
   mcheck_check_all(), mprobe()                 const:malloc_hooks    
   

CONFORMING TO

   These functions are GNU extensions.

NOTES

   Linking a program with -lmcheck and using the MALLOC_CHECK_ environment
   variable (described in mallopt(3)) cause the same kinds of errors to be
   detected.  But, using MALLOC_CHECK_ does not require the application to
   be relinked.

EXAMPLE

   The  program  below  calls mcheck() with a NULL argument and then frees
   the  same  block  of  memory  twice.   The  following   shell   session
   demonstrates what happens when running the program:

       $ ./a.out
       About to free

       About to free a second time
       block freed twice
       Aborted (core dumped)

   Program source

   #include <stdlib.h>
   #include <stdio.h>
   #include <mcheck.h>

   int
   main(int argc, char *argv[])
   {
       char *p;

       if (mcheck(NULL) != 0) {
           fprintf(stderr, "mcheck() failed\n");

           exit(EXIT_FAILURE);
       }

       p = malloc(1000);

       fprintf(stderr, "About to free\n");
       free(p);
       fprintf(stderr, "\nAbout to free a second time\n");
       free(p);

       exit(EXIT_SUCCESS);
   }

SEE ALSO

   malloc(3), mallopt(3), mtrace(3)

COLOPHON

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   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

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