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 (),
  34 .BR realloc (),
  35 and
  36 .BR free ()
  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 (),
  64 .BR realloc (),
  65 .BR memalign (),
  66 .BR free (),
  67 respectively, except that they have a final argument
  68 .I caller
  69 that gives the address of the caller of
  70 .BR malloc (),
  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 ()
  77 was asked for more memory.
  78 .SH "EXAMPLE"
  79 Here is a short example of how to use these variables.
  80 .sp
  81 .nf
  82 #include <stdio.h>
  83 #include <malloc.h>
  84
  85 /* Prototypes for our hooks.  */
  86 static void my_init_hook(void);
  87 static void *my_malloc_hook(size_t, const void *);
  88
  89 /* Variables to save original hooks. */
  90 static void *(*old_malloc_hook)(size_t, const void *);
  91
  92 /* Override initialising hook from the C library. */
  93 void (*__malloc_initialize_hook) (void) = my_init_hook;
  94
  95 static void
  96 my_init_hook(void)
  97 {
  98     old_malloc_hook = __malloc_hook;
  99     __malloc_hook = my_malloc_hook;
 100 }
 101
 102 static void *
 103 my_malloc_hook(size_t size, const void *caller)
 104 {
 105     void *result;
 106
 107     /* Restore all old hooks */
 108     __malloc_hook = old_malloc_hook;
 109
 110     /* Call recursively */
 111     result = malloc(size);
 112
 113     /* Save underlying hooks */
 114     old_malloc_hook = __malloc_hook;
 115
 116     /* `printf' might call `malloc', so protect it too. */
 117     printf("malloc(%u) called from %p returns %p\\n",
 118             (unsigned int) size, caller, result);
 119
 120     /* Restore our own hooks */
 121     __malloc_hook = my_malloc_hook;
 122
 123     return result;
 124 }
 125 .fi
 126 .SH "CONFORMING TO"
 127 These functions are GNU extensions.
 128 .SH "SEE ALSO"
 129 .BR mallinfo (3),
 130 .BR malloc (3),
 131 .BR mcheck (3),
 132 .BR mtrace (3)