man3/malloc_hook.3

   1 .\" Copyright 2002 Walter Harms (walter.harms@informatik.uni-oldenburg.de)
   2 .\" Distributed under GPL
   3 .\" Heavily based on glibc documentation
   4 .\" Polished, added docs, removed glibc doc bug, 2002-07-20, aeb
   5 .TH MALLOC_HOOK 3 2002-07-20 "GNU" "Linux Programmer's Manual"
   6 .SH NAME
   7 __malloc_hook, __malloc_initialize_hook,
   8 __memalign_hook, __free_hook, __realloc_hook,
   9 __after_morecore_hook \- malloc debugging variables
  10 .SH SYNOPSIS
  11 .sp
  12 .BR "#include <malloc.h>"
  13 .sp
  14 .BI "void *(*__malloc_hook)(size_t " size ,
  15 .BI "const void *" caller );
  16 .sp
  17 .BI "void *(*__realloc_hook)(void *" ptr ,
  18 .BI "size_t " size ,
  19 .BI "const void *" caller );
  20 .sp
  21 .BI "void *(*__memalign_hook)(size_t " alignment ,
  22 .BI "size_t " size ,
  23 .BI "const void *" caller );
  24 .sp
  25 .BI "void (*__free_hook)(void *" ptr ,
  26 .BI "const void *" caller );
  27 .sp
  28 .BI "void (*__malloc_initialize_hook)(void);"
  29 .sp
  30 .BI "void (*__after_morecore_hook)(void);"
  31 .SH DESCRIPTION
  32 The GNU C library lets you modify the behavior of
  33 .IR malloc (),
  34 .IR realloc (),
  35 and
  36 .IR free ()
  37 by specifying appropriate hook functions.  You can use these hooks
  38 to help you debug programs that use dynamic memory allocation,
  39 for example.
  40 .LP
  41 The variable
  42 .B __malloc_initialize_hook
  43 points at a function that is called once when the malloc implementation
  44 is initialized.  This is a weak variable, so it can be overridden in
  45 the application with a definition like the following:
  46 .br
  47 .nf
  48     void (*__malloc_initialize_hook)(void) = my_init_hook;
  49 .fi
  50 .br
  51 Now the function
  52 .IR my_init_hook ()
  53 can do the initialization of all hooks.
  54 .LP
  55 The four functions pointed to by
  56 .BR __malloc_hook ,
  57 .BR __realloc_hook ,
  58 .BR __memalign_hook ,
  59 .BR __free_hook
  60 have a prototype like the functions
  61 .IR malloc (),
  62 .IR realloc (),
  63 .IR memalign (),
  64 .IR free (),
  65 respectively, except that they have a final argument
  66 .I caller
  67 that gives the address of the caller of
  68 .IR malloc (),
  69 etc.
  70 .LP
  71 The variable
  72 .B __after_morecore_hook
  73 points at a function that is called each time after
  74 .IR sbrk ()
  75 was asked for more core.
  76 .SH "EXAMPLE"
  77 Here a short example how to use these variables.
  78 .sp
  79 .nf
  80 #include <stdio.h>
  81 #include <malloc.h>
  82
  83 /* Prototypes for our hooks.  */
  84 static void my_init_hook(void);
  85 static void *my_malloc_hook(size_t, const void *);
  86
  87 /* Variables to save original hooks. */
  88 static void *(*old_malloc_hook)(size_t, const void *);
  89
  90 /* Override initialising hook from the C library. */
  91 void (*__malloc_initialize_hook) (void) = my_init_hook;
  92
  93 static void
  94 my_init_hook(void) {
  95     old_malloc_hook = __malloc_hook;
  96     __malloc_hook = my_malloc_hook;
  97 }
  98
  99 static void *
 100 my_malloc_hook (size_t size, const void *caller) {
 101     void *result;
 102
 103     /* Restore all old hooks */
 104     __malloc_hook = old_malloc_hook;
 105
 106     /* Call recursively */
 107     result = malloc (size);
 108
 109     /* Save underlying hooks */
 110     old_malloc_hook = __malloc_hook;
 111
 112     /* `printf' might call `malloc', so protect it too. */
 113     printf ("malloc(%u) called from %p returns %p\n",
 114         (unsigned int) size, caller, result);
 115
 116     /* Restore our own hooks */
 117     __malloc_hook = my_malloc_hook;
 118
 119     return result;
 120 }
 121 .fi
 122 .SH "SEE ALSO"
 123 .BR mallinfo (3),
 124 .BR malloc (3),
 125 .BR mcheck (3),
 126 .BR mtrace (3)