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MALLOC_HOOK(3) Linux Programmer's Manual MALLOC_HOOK(3)

NAME

     __malloc_hook,  __malloc_initialize_hook, __memalign_hook, __free_hook,
     __realloc_hook, __after_morecore_hook - malloc debugging variables

SYNOPSIS

     #include <malloc.h>

     void *(*__malloc_hook)(size_t size, const void *caller);

     void *(*__realloc_hook)(void *ptr, size_t size, const void *caller);

     void *(*__memalign_hook)(size_t alignment, size_t size,
                              const void *caller);

     void (*__free_hook)(void *ptr, const void *caller);

     void (*__malloc_initialize_hook)(void);

     void (*__after_morecore_hook)(void);

DESCRIPTION

     The GNU C library lets you modify  the  behavior  of  malloc(3),  real-
     loc(3),  and free(3) by specifying appropriate hook functions.  You can
     use these hooks to help you debug  programs  that  use  dynamic  memory
     allocation, for example.

     The  variable  __malloc_initialize_hook  points  at  a function that is
     called once when the malloc implementation is initialized.  This  is  a
     weak  variable, so it can be overridden in the application with a defi-
     nition like the following:

         void (*__malloc_initialize_hook)(void) = my_init_hook;

     Now the function my_init_hook() can do the initialization of all hooks.

     The four functions pointed to by __malloc_hook, __realloc_hook, __mema-
     lign_hook, __free_hook have a prototype like the  functions  malloc(3),
     realloc(3), memalign(3), free(3), respectively, except that they have a
     final argument caller that gives the address  of  the  caller  of  mal-
     loc(3), etc.

     The  variable __after_morecore_hook points at a function that is called
     each time after sbrk(2) was asked for more memory.

CONFORMING TO

     These functions are GNU extensions.

NOTES

     The use of these hook functions is not safe in multithreaded  programs,
     and  they  are  now  deprecated.   From  glibc 2.24 onwards, the __mal-
     loc_initialize_hook variable has been removed from the  API.   Program-
     mers should instead preempt calls to the relevant functions by defining
     and exporting functions such as "malloc" and "free".

EXAMPLE

     Here is a short example of how to use these variables.

     #include <stdio.h> #include <malloc.h>

     /* Prototypes for our hooks.  */ static void my_init_hook(void); static
     void *my_malloc_hook(size_t, const void *);

     /*  Variables  to  save  original  hooks.  */  static  void *(*old_mal-
     loc_hook)(size_t, const void *);

     /* Override initializing hook from the  C  library.  */  void  (*__mal-
     loc_initialize_hook) (void) = my_init_hook;

     static void my_init_hook(void) {
         old_malloc_hook = __malloc_hook;
         __malloc_hook = my_malloc_hook; }

     static void * my_malloc_hook(size_t size, const void *caller) {
         void *result;

         /* Restore all old hooks */
         __malloc_hook = old_malloc_hook;

         /* Call recursively */
         result = malloc(size);

         /* Save underlying hooks */
         old_malloc_hook = __malloc_hook;

         /* printf() might call malloc(), so protect it too. */
         printf("malloc(%u) called from %p returns %p\n",
                 (unsigned int) size, caller, result);

         /* Restore our own hooks */
         __malloc_hook = my_malloc_hook;

         return result; }