__malloc_hook(3) Library Functions Manual __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>
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), 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 al-
location, 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.
STANDARDS
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, and from
glibc 2.34 onwards, all the hook variables have been removed from the
API. Programmers should instead preempt calls to the relevant func-
tions by defining and exporting malloc(), free(), realloc(), and cal-
loc().
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)
Linux man-pages 6.03 2023-01-07 __malloc_hook(3)
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