NAME | SYNOPSIS | DESCRIPTION | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

MALLOC_HOOK(3)            Linux Programmer's Manual           MALLOC_HOOK(3)

NAME         top

       __malloc_hook,       __malloc_initialize_hook,       __memalign_hook,
       __free_hook, __realloc_hook, __after_morecore_hook - malloc debugging
       variables

SYNOPSIS         top

       #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         top

       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         top

       These functions are GNU extensions.

NOTES         top

       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".

EXAMPLE         top

       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         top

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

COLOPHON         top

       This page is part of release 4.07 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                              2016-07-17                   MALLOC_HOOK(3)