__malloc_hook(3)

NAME
__malloc_hook, __malloc_initialize_hook, __memalign_hook, __free_hook, __realloc_hook, __after_morecore_hook - malloc debugging variables (DEPRECATED)

LIBRARY
Standard C library (libc, -lc)

SYNOPSIS
#include <malloc.h>

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

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

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

typeof(void *(void *p, const void *caller))
    *volatile  __free_hook;

typeof(void (void))            *__malloc_initialize_hook;
typeof(void (void)) *volatile   __after_mrecore_hook;

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:

    typeof(void (void))  *__malloc_initialize_hook = 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), memalign(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.

STANDARDS
GNU.

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 variable has been removed from the API, and from glibc 2.34 onwards, all the hook variables have been removed from the API. Programmers should instead preempt calls to the relevant functions by defining and exporting malloc(), free(), realloc(), and calloc().

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 typeof(void *(size_t, const void *))  *old_malloc_hook;

    /* Override initializing hook from the C library */
    typeof(void (void))  *__malloc_initialize_hook = 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)