malloc_hook

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), 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.

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 variable 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)

COLOPHON
       This page is part of release 5.12 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



GNU                                2021-03-22                     MALLOC_HOOK(3)