mirror of
https://sourceware.org/git/glibc.git
synced 2024-12-26 20:51:11 +00:00
96 lines
3.8 KiB
C
96 lines
3.8 KiB
C
/* Allocate and initialize an object once, in a thread-safe fashion.
|
|
Copyright (C) 2018 Free Software Foundation, Inc.
|
|
This file is part of the GNU C Library.
|
|
|
|
The GNU C Library is free software; you can redistribute it and/or
|
|
modify it under the terms of the GNU Lesser General Public
|
|
License as published by the Free Software Foundation; either
|
|
version 2.1 of the License, or (at your option) any later version.
|
|
|
|
The GNU C Library is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
|
|
Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public
|
|
License along with the GNU C Library; if not, see
|
|
<http://www.gnu.org/licenses/>. */
|
|
|
|
#ifndef _ALLOCATE_ONCE_H
|
|
#define _ALLOCATE_ONCE_H
|
|
|
|
#include <atomic.h>
|
|
|
|
/* Slow path for allocate_once; see below. */
|
|
void *__libc_allocate_once_slow (void **__place,
|
|
void *(*__allocate) (void *__closure),
|
|
void (*__deallocate) (void *__closure,
|
|
void *__ptr),
|
|
void *__closure);
|
|
|
|
/* Return an a pointer to an allocated and initialized data structure.
|
|
If this function returns a non-NULL value, the caller can assume
|
|
that pointed-to data has been initialized according to the ALLOCATE
|
|
function.
|
|
|
|
It is expected that callers define an inline helper function which
|
|
adds type safety, like this.
|
|
|
|
struct foo { ... };
|
|
struct foo *global_foo;
|
|
static void *allocate_foo (void *closure);
|
|
static void *deallocate_foo (void *closure, void *ptr);
|
|
|
|
static inline struct foo *
|
|
get_foo (void)
|
|
{
|
|
return allocate_once (&global_foo, allocate_foo, free_foo, NULL);
|
|
}
|
|
|
|
(Note that the global_foo variable is initialized to zero.)
|
|
Usage of this helper function looks like this:
|
|
|
|
struct foo *local_foo = get_foo ();
|
|
if (local_foo == NULL)
|
|
report_allocation_failure ();
|
|
|
|
allocate_once first performs an acquire MO load on *PLACE. If the
|
|
result is not null, it is returned. Otherwise, ALLOCATE (CLOSURE)
|
|
is called, yielding a value RESULT. If RESULT equals NULL,
|
|
allocate_once returns NULL, and does not modify *PLACE (but another
|
|
thread may concurrently perform an allocation which succeeds,
|
|
updating *PLACE). If RESULT does not equal NULL, the function uses
|
|
a CAS with acquire-release MO to update the NULL value in *PLACE
|
|
with the RESULT value. If it turns out that *PLACE was updated
|
|
concurrently, allocate_once calls DEALLOCATE (CLOSURE, RESULT) to
|
|
undo the effect of ALLOCATE, and returns the new value of *PLACE
|
|
(after an acquire MO load). If DEALLOCATE is NULL, free (RESULT)
|
|
is called instead.
|
|
|
|
Compared to __libc_once, allocate_once has the advantage that it
|
|
does not need separate space for a control variable, and that it is
|
|
safe with regards to cancellation and other forms of exception
|
|
handling if the supplied callback functions are safe in that
|
|
regard. allocate_once passes a closure parameter to the allocation
|
|
function, too. */
|
|
static inline void *
|
|
allocate_once (void **__place, void *(*__allocate) (void *__closure),
|
|
void (*__deallocate) (void *__closure, void *__ptr),
|
|
void *__closure)
|
|
{
|
|
/* Synchronizes with the release MO CAS in
|
|
__allocate_once_slow. */
|
|
void *__result = atomic_load_acquire (__place);
|
|
if (__result != NULL)
|
|
return __result;
|
|
else
|
|
return __libc_allocate_once_slow (__place, __allocate, __deallocate,
|
|
__closure);
|
|
}
|
|
|
|
#ifndef _ISOMAC
|
|
libc_hidden_proto (__libc_allocate_once_slow)
|
|
#endif
|
|
|
|
#endif /* _ALLOCATE_ONCE_H */
|