malloc_hook(3) — Linux manual page

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | STANDARDS | NOTES | EXAMPLES | SEE ALSO | COLOPHON

__malloc_hook(3)        Library Functions Manual        __malloc_hook(3)

NAME         top

       __malloc_hook, __malloc_initialize_hook, __memalign_hook,
       __free_hook, __realloc_hook, __after_morecore_hook - malloc
       debugging variables (DEPRECATED)

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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         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.

STANDARDS         top

       GNU.

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, and from glibc 2.34 onwards, all the hook variables have
       been removed from the API.  Programmers should instead preempt
       calls to the relevant functions by defining and exporting
       malloc(), free(), realloc(), and calloc().

EXAMPLES         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(%zu) called from %p returns %p\n",
                   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 the man-pages (Linux kernel and C library
       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨https://www.kernel.org/doc/man-pages/⟩.  If you have a bug report
       for this manual page, see
       ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
       fetched from
       ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
       improvements to the information in this COLOPHON (which is not
       part of the original manual page), send a mail to
       man-pages@man7.org

Linux man-pages 6.9.1          2024-06-15               __malloc_hook(3)

Pages that refer to this page: malloc_get_state(3)mallopt(3)mtrace(3)