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12 .TH SEMAPHORE 3C "Feb 5, 2008"
13 .SH NAME
14 semaphore, sema_init, sema_destroy, sema_wait, sema_trywait, sema_post \-
15 semaphores
16 .SH SYNOPSIS
17 .LP
18 .nf
19 cc [ \fIflag\fR... ] \fIfile\fR... -lthread -lc [ \fIlibrary\fR... ]
20 #include <synch.h>
21
22 \fBint\fR \fBsema_init\fR(\fBsema_t *\fR\fIsp\fR, \fBunsigned int\fR \fIcount\fR, \fBint\fR \fItype\fR,
23 \fBvoid *\fR \fIarg\fR);
24 .fi
25
26 .LP
27 .nf
28 \fBint\fR \fBsema_destroy\fR(\fBsema_t *\fR\fIsp\fR);
29 .fi
30
31 .LP
32 .nf
33 \fBint\fR \fBsema_wait\fR(\fBsema_t *\fR\fIsp\fR);
34 .fi
35
36 .LP
37 .nf
38 \fBint\fR \fBsema_trywait\fR(\fBsema_t *\fR\fIsp\fR);
39 .fi
40
41 .LP
42 .nf
43 \fBint\fR \fBsema_post\fR(\fBsema_t *\fR\fIsp\fR);
44 .fi
45
46 .SH DESCRIPTION
47 .sp
48 .LP
49 A semaphore is a non-negative integer count and is generally used to coordinate
50 access to resources. The initial semaphore count is set to the number of free
51 resources, then threads slowly increment and decrement the count as resources
52 are added and removed. If the semaphore count drops to 0, which means no
53 available resources, threads attempting to decrement the semaphore will block
54 until the count is greater than 0.
55 .sp
56 .LP
57 Semaphores can synchronize threads in this process and other processes if they
58 are allocated in writable memory and shared among the cooperating processes
59 (see \fBmmap\fR(2)), and have been initialized for this purpose.
60 .sp
61 .LP
62 Semaphores must be initialized before use; semaphores pointed to by \fIsp\fR to
63 \fIcount\fR are initialized by \fBsema_init()\fR. The \fItype\fR argument can
64 assign several different types of behavior to a semaphore. No current type uses
65 \fIarg\fR, although it may be used in the future.
66 .sp
67 .LP
68 The \fItype\fR argument may be one of the following:
69 .sp
70 .ne 2
71 .na
72 \fB\fBUSYNC_PROCESS\fR \fR
73 .ad
74 .RS 18n
75 The semaphore can synchronize threads in this process and other processes.
76 Initializing the semaphore should be done by only one process. A semaphore
77 initialized with this type must be allocated in memory shared between
78 processes, either in Sys V shared memory (see \fBshmop\fR(2)), or in memory
79 mapped to a file (see \fBmmap\fR(2)). It is illegal to initialize the object
80 this way and not allocate it in such shared memory. \fIarg\fR is ignored.
81 .RE
82
83 .sp
84 .ne 2
85 .na
86 \fB\fBUSYNC_THREAD\fR \fR
87 .ad
88 .RS 18n
89 The semaphore can synchronize threads only in this process. The \fIarg\fR
90 argument is ignored. \fBUSYNC_THREAD\fR does not support multiple mappings to
91 the same logical synch object. If you need to \fBmmap()\fR a synch object to
92 different locations within the same address space, then the synch object should
93 be initialized as a shared object \fBUSYNC_PROCESS\fR for Solaris threads and
94 \fBPTHREAD_PROCESS_PRIVATE\fR for POSIX threads.
95 .RE
96
97 .sp
98 .LP
99 A semaphore must not be simultaneously initialized by multiple threads, nor
100 re-initialized while in use by other threads.
101 .sp
102 .LP
103 Default semaphore initialization (intra-process):
104 .sp
105 .in +2
106 .nf
107 sema_t sp;
108 int count = 1;
109 sema_init(&sp, count, NULL, NULL);
110 .fi
111 .in -2
112
113 .sp
114 .LP
115 or
116 .sp
117 .in +2
118 .nf
119 sema_init(&sp, count, USYNC_THREAD, NULL);
120 .fi
121 .in -2
122
123 .sp
124 .LP
125 Customized semaphore initialization (inter-process):
126 .sp
127 .in +2
128 .nf
129 \fBsema_t sp;
130 int count = 1;
131 sema_init(&sp, count, USYNC_PROCESS, NULL);\fR
132 .fi
133 .in -2
134
135 .sp
136 .LP
137 The \fBsema_destroy()\fR function destroys any state related to the semaphore
138 pointed to by \fIsp\fR. The semaphore storage space is not released.
139 .sp
140 .LP
141 The \fBsema_wait()\fR function blocks the calling thread until the semaphore
142 count pointed to by \fIsp\fR is greater than 0, and then it atomically
143 decrements the count.
144 .sp
145 .LP
146 The \fBsema_trywait()\fR function atomically decrements the semaphore count
147 pointed to by \fIsp\fR, if the count is greater than 0; otherwise, it returns
148 an error.
149 .sp
150 .LP
151 The \fBsema_post()\fR function atomically increments the semaphore count
152 pointed to by \fIsp\fR. If there are any threads blocked on the semaphore, one
153 will be unblocked.
154 .sp
155 .LP
156 The semaphore functionality described on this man page is for the Solaris
157 threads implementation. For the POSIX-conforming semaphore interface
158 documentation, see \fBsem_close\fR(3C), \fBsem_destroy\fR(3C),
159 \fBsem_getvalue\fR(3C), \fBsem_init\fR(3C), \fBsem_open\fR(3C),
160 \fBsem_post\fR(3C), \fBsem_unlink\fR(3C), and \fBsem_wait\fR(3C).
161 .SH RETURN VALUES
162 .sp
163 .LP
164 Upon successful completion, \fB0\fR is returned; otherwise, a non-zero value
165 indicates an error.
166 .SH ERRORS
167 .sp
168 .LP
169 These functions will fail if:
170 .sp
171 .ne 2
172 .na
173 \fB\fBEINVAL\fR \fR
174 .ad
175 .RS 11n
176 The \fIsp\fR argument does not refer to a valid semaphore.
177 .RE
178
179 .sp
180 .ne 2
181 .na
182 \fB\fBEFAULT\fR \fR
183 .ad
184 .RS 11n
185 Either the \fIsp\fR or \fIarg\fR argument points to an illegal address.
186 .RE
187
188 .sp
189 .LP
190 The \fBsema_wait()\fR function will fail if:
191 .sp
192 .ne 2
193 .na
194 \fB\fBEINTR\fR \fR
195 .ad
196 .RS 10n
197 The wait was interrupted by a signal or \fBfork()\fR.
198 .RE
199
200 .sp
201 .LP
202 The \fBsema_trywait()\fR function will fail if:
203 .sp
204 .ne 2
205 .na
206 \fB\fBEBUSY\fR \fR
207 .ad
208 .RS 10n
209 The semaphore pointed to by \fIsp\fR has a 0 count.
210 .RE
211
212 .sp
213 .LP
214 The \fBsema_post()\fR function will fail if:
215 .sp
216 .ne 2
217 .na
218 \fB\fBEOVERFLOW\fR \fR
219 .ad
220 .RS 14n
221 The semaphore value pointed to by \fIsp\fR exceeds \fBSEM_VALUE_MAX\fR.
222 .RE
223
224 .SH EXAMPLES
225 .LP
226 \fBExample 1 \fRThe customer waiting-line in a bank is analogous to the
227 synchronization scheme of a semaphore using \fBsema_wait()\fR and
228 \fBsema_trywait()\fR:
229 .sp
230 .in +2
231 .nf
232 /* cc [ flag \|.\|.\|. ] file \|.\|.\|. -lthread [ library \|.\|.\|. ] */
233 #include <errno.h>
234 #define TELLERS 10
235 sema_t tellers; /* semaphore */
236 int banking_hours(), deposit_withdrawal;
237 void*customer(), do_business(), skip_banking_today();
238 \&.\|.\|.
239
240 sema_init(&tellers, TELLERS, USYNC_THREAD, NULL);
241 /* 10 tellers available */
242 while(banking_hours())
243 pthread_create(NULL, NULL, customer, deposit_withdrawal);
244 \&.\|.\|.
245
246 void *
247 customer(int deposit_withdrawal)
248 {
249 int this_customer, in_a_hurry = 50;
250 this_customer = rand() % 100;
251
252 if (this_customer == in_a_hurry) {
253 if (sema_trywait(&tellers) != 0)
254 if (errno == EBUSY){ /* no teller available */
255 skip_banking_today(this_customer);
256 return;
257 } /* else go immediately to available teller and
258 decrement tellers */
259 }
260 else
261 sema_wait(&tellers); /* wait for next teller, then
262 proceed, and decrement tellers */
263
264 do_business(deposit_withdrawal);
265 sema_post(&tellers); /* increment tellers; this_customer's
266 teller is now available */
267 }
268 .fi
269 .in -2
270
271 .SH ATTRIBUTES
272 .sp
273 .LP
274 See \fBattributes\fR(5) for descriptions of the following attributes:
275 .sp
276
277 .sp
278 .TS
279 box;
280 c | c
281 l | l .
282 ATTRIBUTE TYPE ATTRIBUTE VALUE
283 _
284 MT-Level Async-Signal-Safe
285 .TE
286
287 .SH SEE ALSO
288 .sp
289 .LP
290 \fBmmap\fR(2), \fBshmop\fR(2), \fBsem_close\fR(3C), \fBsem_destroy\fR(3C),
291 \fBsem_getvalue\fR(3C), \fBsem_init\fR(3C), \fBsem_open\fR(3C),
292 \fBsem_post\fR(3C), \fBsem_unlink\fR(3C), \fBsem_wait\fR(3C),
293 \fBattributes\fR(5), \fBstandards\fR(5)
294 .SH NOTES
295 .sp
296 .LP
297 These functions are also available by way of:
298 .sp
299 .in +2
300 .nf
301 #include <thread.h>
302 .fi
303 .in -2
304
305 .sp
306 .LP
307 By default, there is no defined order of unblocking for multiple threads
308 waiting for a semaphore.