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

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

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

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

       void (*__malloc_initialize_hook)(void);

       void (*volatile __after_morecore_hook)(void);

DESCRIPTION
       The  GNU  C library lets you modify the behavior of malloc(3), realloc(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 definition 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, __memalign_hook, __free_hook have a prototype like the functions malloc(3), realloc(3), mema‐
       lign(3), free(3), respectively, except that they have a final argument caller that gives the address of the caller of malloc(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 __malloc_initialize_hook  vari‐
       able  has been removed from the API.  Programmers should instead preempt calls to the relevant functions by defining and exporting functions such as "malloc" and
       "free".

EXAMPLES
       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_malloc_hook)(size_t, const void *);

       /* Override initializing hook from the C library */
       void (*__malloc_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(%zu) called from %p returns %p\n",
                   size, caller, result);

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

           return result;
       }

SEE ALSO
       mallinfo(3), malloc(3), mcheck(3), mtrace(3)

GNU                                                                            2021-03-22                                                                 MALLOC_HOOK(3)