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

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       found at https://www.kernel.org/doc/man-pages/.