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.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: GPL-1.0-or-later
.\"
.TH __malloc_hook 3 2025-05-17 "Linux man-pages (unreleased)"
.SH NAME
__malloc_hook, __malloc_initialize_hook,
__memalign_hook, __free_hook, __realloc_hook,
__after_morecore_hook \- malloc debugging variables (DEPRECATED)
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B "#include <malloc.h>"
.P
.BI "typeof(void *(size_t " size ", const void *" caller ))
.B " *volatile __malloc_hook;"
.P
.BI "typeof(void *(void *" p ", size_t " size ", const void *" caller ))
.B " *volatile __realloc_hook;"
.P
.BI "typeof(void *(size_t " align ", size_t " size ", const void *" caller ))
.B " *volatile __memalign_hook;"
.P
.BI "typeof(void *(void *" p ", const void *" caller ))
.B " *volatile __free_hook;"
.P
.B "typeof(void (void)) *__malloc_initialize_hook;"
.B "typeof(void (void)) *volatile __after_mrecore_hook;"
.fi
.SH DESCRIPTION
The GNU C library lets you modify the behavior of
.BR malloc (3),
.BR realloc (3),
and
.BR free (3)
by specifying appropriate hook functions.
You can use these hooks
to help you debug programs that use dynamic memory allocation,
for example.
.P
The variable
.B __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:
.P
.in +4n
.EX
typeof(void (void)) *__malloc_initialize_hook = my_init_hook;
.EE
.in
.P
Now the function
.IR my_init_hook ()
can do the initialization of all hooks.
.P
The four functions pointed to by
.BR __malloc_hook ,
.BR __realloc_hook ,
.BR __memalign_hook ,
.B __free_hook
have a prototype like the functions
.BR malloc (3),
.BR realloc (3),
.BR memalign (3),
.BR free (3),
respectively, except that they have a final argument
.I caller
that gives the address of the caller of
.BR malloc (3),
etc.
.P
The variable
.B __after_morecore_hook
points at a function that is called each time after
.BR sbrk (2)
was asked for more memory.
.SH STANDARDS
GNU.
.SH NOTES
The use of these hook functions is not safe in multithreaded programs,
and they are now deprecated.
From glibc 2.24 onwards, the
.B __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.
.\" https://bugzilla.redhat.com/show_bug.cgi?id=450187
.\" http://sourceware.org/bugzilla/show_bug.cgi?id=9957
Programmers should instead preempt calls to the relevant functions
by defining and exporting
.BR malloc (),
.BR free (),
.BR realloc (),
and
.BR calloc ().
.SH EXAMPLES
Here is a short example of how to use these variables.
.P
.EX
#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 typeof(void *(size_t, const void *)) *old_malloc_hook;
\&
/* Override initializing hook from the C library */
typeof(void (void)) *__malloc_initialize_hook = 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\[rs]n",
size, caller, result);
\&
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;
\&
return result;
}
.EE
.SH SEE ALSO
.BR mallinfo (3),
.BR malloc (3),
.BR mcheck (3),
.BR mtrace (3)
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