1 /* $NetBSD: pthread_specific.c,v 1.4 2003/01/21 23:29:22 nathanw Exp $ */ 2 3 /*- 4 * Copyright (c) 2001 The NetBSD Foundation, Inc. 5 * All rights reserved. 6 * 7 * This code is derived from software contributed to The NetBSD Foundation 8 * by Nathan J. Williams. 9 * 10 * Redistribution and use in source and binary forms, with or without 11 * modification, are permitted provided that the following conditions 12 * are met: 13 * 1. Redistributions of source code must retain the above copyright 14 * notice, this list of conditions and the following disclaimer. 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 3. All advertising materials mentioning features or use of this software 19 * must display the following acknowledgement: 20 * This product includes software developed by the NetBSD 21 * Foundation, Inc. and its contributors. 22 * 4. Neither the name of The NetBSD Foundation nor the names of its 23 * contributors may be used to endorse or promote products derived 24 * from this software without specific prior written permission. 25 * 26 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 27 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 28 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 29 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 30 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 31 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 32 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 33 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 34 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 35 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 36 * POSSIBILITY OF SUCH DAMAGE. 37 */ 38 39 /* Functions and structures dealing with thread-specific data */ 40 #include <assert.h> 41 #include <errno.h> 42 #include <sys/cdefs.h> 43 44 #include "pthread.h" 45 #include "pthread_int.h" 46 47 static pthread_mutex_t tsd_mutex = PTHREAD_MUTEX_INITIALIZER; 48 static int nextkey; 49 int pthread__tsd_alloc[PTHREAD_KEYS_MAX]; 50 void (*pthread__tsd_destructors[PTHREAD_KEYS_MAX])(void *); 51 52 __strong_alias(__libc_thr_keycreate,pthread_key_create) 53 __strong_alias(__libc_thr_setspecific,pthread_setspecific) 54 __strong_alias(__libc_thr_getspecific,pthread_getspecific) 55 __strong_alias(__libc_thr_keydelete,pthread_key_delete) 56 57 int 58 pthread_key_create(pthread_key_t *key, void (*destructor)(void *)) 59 { 60 int i; 61 62 /* Get a lock on the allocation list */ 63 pthread_mutex_lock(&tsd_mutex); 64 65 /* Find an avaliable slot */ 66 /* 1. Search from "nextkey" to the end of the list. */ 67 for (i = nextkey; i < PTHREAD_KEYS_MAX; i++) 68 if (pthread__tsd_alloc[i] == 0) 69 break; 70 71 if (i == PTHREAD_KEYS_MAX) { 72 /* 2. If that didn't work, search from the start 73 * of the list back to "nextkey". 74 */ 75 for (i = 0; i < nextkey; i++) 76 if (pthread__tsd_alloc[i] == 0) 77 break; 78 79 if (i == nextkey) { 80 /* If we didn't find one here, there isn't one 81 * to be found. 82 */ 83 pthread_mutex_unlock(&tsd_mutex); 84 return EAGAIN; 85 } 86 } 87 88 /* Got one. */ 89 pthread__tsd_alloc[i] = 1; 90 nextkey = (i + 1) % PTHREAD_KEYS_MAX; 91 pthread__tsd_destructors[i] = destructor; 92 pthread_mutex_unlock(&tsd_mutex); 93 *key = i; 94 95 return 0; 96 } 97 98 int 99 pthread_key_delete(pthread_key_t key) 100 { 101 102 /* 103 * This is tricky. The standard says of pthread_key_create() 104 * that new keys have the value NULL associated with them in 105 * all threads. According to people who were present at the 106 * standardization meeting, that requirement was written 107 * before pthread_key_delete() was introduced, and not 108 * reconsidered when it was. 109 * 110 * See David Butenhof's article in comp.programming.threads: 111 * Subject: Re: TSD key reusing issue 112 * Message-ID: <u97d8.29$fL6.200@news.cpqcorp.net> 113 * Date: Thu, 21 Feb 2002 09:06:17 -0500 114 * http://groups.google.com/groups?hl=en&selm=u97d8.29%24fL6.200%40news.cpqcorp.net 115 * 116 * Given: 117 * 118 * 1: Applications are not required to clear keys in all 119 * threads before calling pthread_key_delete(). 120 * 2: Clearing pointers without running destructors is a 121 * memory leak. 122 * 3: The pthread_key_delete() function is expressly forbidden 123 * to run any destructors. 124 * 125 * Option 1: Make this function effectively a no-op and 126 * prohibit key reuse. This is a possible resource-exhaustion 127 * problem given that we have a static storage area for keys, 128 * but having a non-static storage area would make 129 * pthread_setspecific() expensive (might need to realloc the 130 * TSD array). 131 * 132 * Option 2: Ignore the specified behavior of 133 * pthread_key_create() and leave the old values. If an 134 * application deletes a key that still has non-NULL values in 135 * some threads... it's probably a memory leak and hence 136 * incorrect anyway, and we're within our rights to let the 137 * application lose. However, it's possible (if unlikely) that 138 * the application is storing pointers to non-heap data, or 139 * non-pointers that have been wedged into a void pointer, so 140 * we can't entirely write off such applications as incorrect. 141 * This could also lead to running (new) destructors on old 142 * data that was never supposed to be associated with that 143 * destructor. 144 * 145 * Option 3: Follow the specified behavior of 146 * pthread_key_create(). Either pthread_key_create() or 147 * pthread_key_delete() would then have to clear the values in 148 * every thread's slot for that key. In order to guarantee the 149 * visibility of the NULL value in other threads, there would 150 * have to be synchronization operations in both the clearer 151 * and pthread_getspecific(). Putting synchronization in 152 * pthread_getspecific() is a big performance lose. But in 153 * reality, only (buggy) reuse of an old key would require 154 * this synchronization; for a new key, there has to be a 155 * memory-visibility propagating event between the call to 156 * pthread_key_create() and pthread_getspecific() with that 157 * key, so setting the entries to NULL without synchronization 158 * will work, subject to problem (2) above. However, it's kind 159 * of slow. 160 * 161 * Note that the argument in option 3 only applies because we 162 * keep TSD in ordinary memory which follows the pthreads 163 * visibility rules. The visibility rules are not required by 164 * the standard to apply to TSD, so this arguemnt doesn't 165 * apply in general, just to this implementation. 166 */ 167 168 /* For the momemt, we're going with option 1. */ 169 pthread_mutex_lock(&tsd_mutex); 170 pthread__tsd_destructors[key] = NULL; 171 pthread_mutex_unlock(&tsd_mutex); 172 173 return 0; 174 } 175 176 int 177 pthread_setspecific(pthread_key_t key, const void *value) 178 { 179 pthread_t self; 180 181 if (pthread__tsd_alloc[key] == 0) 182 return EINVAL; 183 184 self = pthread__self(); 185 /* 186 * We can't win here on constness. Having been given a 187 * "const void *", we can only assign it to other const void *, 188 * and return it from functions that are const void *, without 189 * generating a warning. 190 */ 191 /*LINTED const cast*/ 192 self->pt_specific[key] = (void *) value; 193 194 return 0; 195 } 196 197 void* 198 pthread_getspecific(pthread_key_t key) 199 { 200 pthread_t self; 201 202 if (pthread__tsd_alloc[key] == 0) 203 return NULL; 204 205 self = pthread__self(); 206 return (self->pt_specific[key]); 207 } 208 209 /* Perform thread-exit-time destruction of thread-specific data. */ 210 void 211 pthread__destroy_tsd(pthread_t self) 212 { 213 int i, done, iterations; 214 void *val; 215 void (*destructor)(void *); 216 217 /* Butenhof, section 5.4.2 (page 167): 218 * 219 * ``Also, Pthreads sets the thread-specific data value for a 220 * key to NULL before calling that key's destructor (passing 221 * the previous value of the key) when a thread terminates [*]. 222 * ... 223 * [*] That is, unfortunately, not what the standard 224 * says. This is one of the problems with formal standards - 225 * they say what they say, not what they were intended to 226 * say. Somehow, an error crept in, and the sentence 227 * specifying that "the implementation clears the 228 * thread-specific data value before calling the destructor" 229 * was deleted. Nobody noticed, and the standard was approved 230 * with the error. So the standard says (by omission) that if 231 * you want to write a portable application using 232 * thread-specific data, that will not hang on thread 233 * termination, you must call pthread_setspecific within your 234 * destructor function to change the value to NULL. This would 235 * be silly, and any serious implementation of Pthreads will 236 * violate the standard in this respect. Of course, the 237 * standard will be fixed, probably by the 1003.1n amendment 238 * (assorted corrections to 1003.1c-1995), but that will take 239 * a while.'' 240 */ 241 242 iterations = PTHREAD_DESTRUCTOR_ITERATIONS; 243 do { 244 done = 1; 245 for (i = 0; i < PTHREAD_KEYS_MAX; i++) { 246 if (self->pt_specific[i] != NULL) { 247 pthread_mutex_lock(&tsd_mutex); 248 destructor = pthread__tsd_destructors[i]; 249 pthread_mutex_unlock(&tsd_mutex); 250 if (destructor != NULL) { 251 done = 0; 252 val = self->pt_specific[i]; 253 self->pt_specific[i] = NULL; /* see above */ 254 (*destructor)(val); 255 } 256 } 257 } 258 } while (!done && iterations--); 259 } 260