mirror of https://gitee.com/Nocallback/glibc.git
Florian Weimer
8 years ago
6 changed files with 363 additions and 4 deletions
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/* Allocate and initialize an object once, in a thread-safe fashion.
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Copyright (C) 2018 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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|
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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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|
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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_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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@ -0,0 +1,59 @@ |
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/* Concurrent allocation and initialization of a pointer.
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Copyright (C) 2018 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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|
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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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|
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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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#include <allocate_once.h> |
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#include <stdlib.h> |
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#include <stdbool.h> |
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void * |
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__libc_allocate_once_slow (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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void *result = allocate (closure); |
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if (result == NULL) |
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return NULL; |
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/* This loop implements a strong CAS on *place, with acquire-release
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MO semantics, from a weak CAS with relaxed-release MO. */ |
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while (true) |
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{ |
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/* Synchronizes with the acquire MO load in allocate_once. */ |
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void *expected = NULL; |
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if (atomic_compare_exchange_weak_release (place, &expected, result)) |
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return result; |
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/* The failed CAS has relaxed MO semantics, so perform another
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acquire MO load. */ |
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void *other_result = atomic_load_acquire (place); |
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if (other_result == NULL) |
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/* Spurious failure. Try again. */ |
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continue; |
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/* We lost the race. Free what we allocated and return the
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other result. */ |
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if (deallocate == NULL) |
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free (result); |
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else |
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deallocate (closure, result); |
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return other_result; |
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} |
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return result; |
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} |
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libc_hidden_def (__libc_allocate_once_slow) |
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@ -0,0 +1,181 @@ |
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/* Test the allocate_once function.
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Copyright (C) 2018 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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|
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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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#include <allocate_once.h> |
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#include <mcheck.h> |
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#include <string.h> |
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#include <support/check.h> |
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#include <support/support.h> |
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/* Allocate a new string. */ |
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static void * |
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allocate_string (void *closure) |
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{ |
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return xstrdup (closure); |
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} |
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/* Allocation and deallocation functions which are not expected to be
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called. */ |
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static void * |
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allocate_not_called (void *closure) |
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{ |
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FAIL_EXIT1 ("allocation function called unexpectedly (%p)", closure); |
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} |
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static void |
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deallocate_not_called (void *closure, void *ptr) |
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{ |
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FAIL_EXIT1 ("deallocate function called unexpectedly (%p, %p)", |
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closure, ptr); |
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} |
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/* Counter for various function calls. */ |
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static int function_called; |
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/* An allocation function which returns NULL and records that it has
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been called. */ |
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static void * |
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allocate_return_null (void *closure) |
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{ |
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/* The function should only be called once. */ |
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TEST_COMPARE (function_called, 0); |
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++function_called; |
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return NULL; |
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} |
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/* The following is used to check the retry logic, by causing a fake
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race condition. */ |
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static void *fake_race_place; |
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static char fake_race_region[3]; /* To obtain unique addresses. */ |
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static void * |
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fake_race_allocate (void *closure) |
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{ |
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TEST_VERIFY (closure == &fake_race_region[0]); |
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TEST_COMPARE (function_called, 0); |
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++function_called; |
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/* Fake allocation by another thread. */ |
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fake_race_place = &fake_race_region[1]; |
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return &fake_race_region[2]; |
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} |
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static void |
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fake_race_deallocate (void *closure, void *ptr) |
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{ |
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/* Check that the pointer returned from fake_race_allocate is
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deallocated (and not the one stored in fake_race_place). */ |
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TEST_VERIFY (ptr == &fake_race_region[2]); |
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TEST_VERIFY (fake_race_place == &fake_race_region[1]); |
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TEST_VERIFY (closure == &fake_race_region[0]); |
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TEST_COMPARE (function_called, 1); |
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++function_called; |
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} |
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/* Similar to fake_race_allocate, but expects to be paired with free
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as the deallocation function. */ |
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static void * |
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fake_race_allocate_for_free (void *closure) |
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{ |
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TEST_VERIFY (closure == &fake_race_region[0]); |
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TEST_COMPARE (function_called, 0); |
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++function_called; |
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/* Fake allocation by another thread. */ |
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fake_race_place = &fake_race_region[1]; |
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return xstrdup ("to be freed"); |
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} |
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static int |
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do_test (void) |
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{ |
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mtrace (); |
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/* Simple allocation. */ |
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void *place1 = NULL; |
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char *string1 = allocate_once (&place1, allocate_string, |
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deallocate_not_called, |
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(char *) "test string 1"); |
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TEST_VERIFY_EXIT (string1 != NULL); |
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TEST_VERIFY (strcmp ("test string 1", string1) == 0); |
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/* Second call returns the first pointer, without calling any
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callbacks. */ |
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TEST_VERIFY (string1 |
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== allocate_once (&place1, allocate_not_called, |
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deallocate_not_called, |
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(char *) "test string 1a")); |
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/* Different place should result in another call. */ |
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void *place2 = NULL; |
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char *string2 = allocate_once (&place2, allocate_string, |
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deallocate_not_called, |
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(char *) "test string 2"); |
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TEST_VERIFY_EXIT (string2 != NULL); |
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TEST_VERIFY (strcmp ("test string 2", string2) == 0); |
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TEST_VERIFY (string1 != string2); |
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/* Check error reporting (NULL return value from the allocation
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function). */ |
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void *place3 = NULL; |
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char *string3 = allocate_once (&place3, allocate_return_null, |
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deallocate_not_called, NULL); |
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TEST_VERIFY (string3 == NULL); |
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TEST_COMPARE (function_called, 1); |
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/* Check that the deallocation function is called if the race is
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lost. */ |
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function_called = 0; |
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TEST_VERIFY (allocate_once (&fake_race_place, |
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fake_race_allocate, |
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fake_race_deallocate, |
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&fake_race_region[0]) |
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== &fake_race_region[1]); |
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TEST_COMPARE (function_called, 2); |
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function_called = 3; |
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TEST_VERIFY (allocate_once (&fake_race_place, |
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fake_race_allocate, |
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fake_race_deallocate, |
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&fake_race_region[0]) |
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== &fake_race_region[1]); |
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TEST_COMPARE (function_called, 3); |
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/* Similar, but this time rely on that free is called. */ |
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function_called = 0; |
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fake_race_place = NULL; |
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TEST_VERIFY (allocate_once (&fake_race_place, |
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fake_race_allocate_for_free, |
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NULL, |
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&fake_race_region[0]) |
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== &fake_race_region[1]); |
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TEST_COMPARE (function_called, 1); |
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function_called = 3; |
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TEST_VERIFY (allocate_once (&fake_race_place, |
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fake_race_allocate_for_free, |
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NULL, |
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&fake_race_region[0]) |
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== &fake_race_region[1]); |
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TEST_COMPARE (function_called, 3); |
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free (place2); |
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free (place1); |
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return 0; |
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} |
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#include <support/test-driver.c> |
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