Manpage of MALLOC_HOOK

MALLOC_HOOK

Section: Linux Programmer's Manual (3)
Updated: 2010-10-13
Index
 

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), 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_hookpoints 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_hookhave a prototype like the functions malloc(3), realloc(3), memalign(3), free(3), respectively, except that they have a final argument callerthat gives the address of the caller of malloc(3), etc.

The variable __after_morecore_hookpoints 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. Programmers 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_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(%u) called from %p returns %p\n",
            (unsigned int) size, caller, result);

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

    return result;
}
 

SEE ALSO

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


 

Index

NAME
SYNOPSIS
DESCRIPTION
CONFORMING TO
NOTES
EXAMPLE
SEE ALSO

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Time: 22:27:57 GMT, June 20, 2016