malloc_hook(3) - Man pages

__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 (DEP-
       RECATED)

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), 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 defini-
       tion 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 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_ini-
       tialize_hook variable has been removed from the API, and from glibc 2.34
       onwards,  all  the  hook variables have been removed from the API.  Pro-
       grammers 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 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 6.9.1              2024-06-15                  __malloc_hook(3)

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