96 lines
3.8 KiB
C
96 lines
3.8 KiB
C
/* Allocate and initialize an object once, in a thread-safe fashion.
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Copyright (C) 2018-2019 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#ifndef _ALLOCATE_ONCE_H
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#define _ALLOCATE_ONCE_H
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#include <atomic.h>
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/* Slow path for allocate_once; see below. */
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void *__libc_allocate_once_slow (void **__place,
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void *(*__allocate) (void *__closure),
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void (*__deallocate) (void *__closure,
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void *__ptr),
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void *__closure);
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/* Return an a pointer to an allocated and initialized data structure.
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If this function returns a non-NULL value, the caller can assume
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that pointed-to data has been initialized according to the ALLOCATE
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function.
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It is expected that callers define an inline helper function which
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adds type safety, like this.
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struct foo { ... };
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struct foo *global_foo;
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static void *allocate_foo (void *closure);
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static void *deallocate_foo (void *closure, void *ptr);
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static inline struct foo *
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get_foo (void)
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{
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return allocate_once (&global_foo, allocate_foo, free_foo, NULL);
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}
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(Note that the global_foo variable is initialized to zero.)
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Usage of this helper function looks like this:
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struct foo *local_foo = get_foo ();
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if (local_foo == NULL)
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report_allocation_failure ();
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allocate_once first performs an acquire MO load on *PLACE. If the
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result is not null, it is returned. Otherwise, ALLOCATE (CLOSURE)
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is called, yielding a value RESULT. If RESULT equals NULL,
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allocate_once returns NULL, and does not modify *PLACE (but another
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thread may concurrently perform an allocation which succeeds,
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updating *PLACE). If RESULT does not equal NULL, the function uses
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a CAS with acquire-release MO to update the NULL value in *PLACE
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with the RESULT value. If it turns out that *PLACE was updated
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concurrently, allocate_once calls DEALLOCATE (CLOSURE, RESULT) to
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undo the effect of ALLOCATE, and returns the new value of *PLACE
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(after an acquire MO load). If DEALLOCATE is NULL, free (RESULT)
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is called instead.
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Compared to __libc_once, allocate_once has the advantage that it
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does not need separate space for a control variable, and that it is
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safe with regards to cancellation and other forms of exception
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handling if the supplied callback functions are safe in that
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regard. allocate_once passes a closure parameter to the allocation
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function, too. */
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static inline void *
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allocate_once (void **__place, void *(*__allocate) (void *__closure),
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void (*__deallocate) (void *__closure, void *__ptr),
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void *__closure)
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{
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/* Synchronizes with the release MO CAS in
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__allocate_once_slow. */
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void *__result = atomic_load_ptr_acquire (__place);
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if (__result != NULL)
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return __result;
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else
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return __libc_allocate_once_slow (__place, __allocate, __deallocate,
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__closure);
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}
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#ifndef _ISOMAC
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libc_hidden_proto (__libc_allocate_once_slow)
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#endif
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#endif /* _ALLOCATE_ONCE_H */
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