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 .BR malloc (3),
  34 .BR realloc (3),
  35 and
  36 .BR free (3)
  37 by specifying appropriate hook functions.
  38 You can use these hooks
  39 to help you debug programs that use dynamic memory allocation,
  40 for example.
  41 .LP
  42 The variable
  43 .B __malloc_initialize_hook
  44 points at a function that is called once when the malloc implementation
  45 is initialized.
  46 This is a weak variable, so it can be overridden in
  47 the application with a definition like the following:
  48 .br
  49 .nf
  50     void (*__malloc_initialize_hook)(void) = my_init_hook;
  51 .fi
  52 .br
  53 Now the function
  54 .IR my_init_hook ()
  55 can do the initialization of all hooks.
  56 .LP
  57 The four functions pointed to by
  58 .BR __malloc_hook ,
  59 .BR __realloc_hook ,
  60 .BR __memalign_hook ,
  61 .BR __free_hook
  62 have a prototype like the functions
  63 .BR malloc (3),
  64 .BR realloc (3),
  65 .BR memalign (3),
  66 .BR free (3),
  67 respectively, except that they have a final argument
  68 .I caller
  69 that gives the address of the caller of
  70 .BR malloc (3),
  71 etc.
  72 .LP
  73 The variable
  74 .B __after_morecore_hook
  75 points at a function that is called each time after
  76 .BR sbrk (2)
  77 was asked for more memory.
  78 .SH "CONFORMING TO"
  79 These functions are GNU extensions.
  80 .SH "EXAMPLE"
  81 Here is a short example of how to use these variables.
  82 .sp
  83 .nf
  84 #include <stdio.h>
  85 #include <malloc.h>
  86
  87 /* Prototypes for our hooks.  */
  88 static void my_init_hook(void);
  89 static void *my_malloc_hook(size_t, const void *);
  90
  91 /* Variables to save original hooks. */
  92 static void *(*old_malloc_hook)(size_t, const void *);
  93
  94 /* Override initializing hook from the C library. */
  95 void (*__malloc_initialize_hook) (void) = my_init_hook;
  96
  97 static void
  98 my_init_hook(void)
  99 {
 100     old_malloc_hook = __malloc_hook;
 101     __malloc_hook = my_malloc_hook;
 102 }
 103
 104 static void *
 105 my_malloc_hook(size_t size, const void *caller)
 106 {
 107     void *result;
 108
 109     /* Restore all old hooks */
 110     __malloc_hook = old_malloc_hook;
 111
 112     /* Call recursively */
 113     result = malloc(size);
 114
 115     /* Save underlying hooks */
 116     old_malloc_hook = __malloc_hook;
 117
 118     /* `printf' might call `malloc', so protect it too. */
 119     printf("malloc(%u) called from %p returns %p\\n",
 120             (unsigned int) size, caller, result);
 121
 122     /* Restore our own hooks */
 123     __malloc_hook = my_malloc_hook;
 124
 125     return result;
 126 }
 127 .fi
 128 .SH "SEE ALSO"
 129 .BR mallinfo (3),
 130 .BR malloc (3),
 131 .BR mcheck (3),
 132 .BR mtrace (3)