1SEM_WAIT(3)                Linux Programmer's Manual               SEM_WAIT(3)
2
3
4

NAME

6       sem_wait, sem_timedwait, sem_trywait - lock a semaphore
7

SYNOPSIS

9       #include <semaphore.h>
10
11       int sem_wait(sem_t *sem);
12
13       int sem_trywait(sem_t *sem);
14
15       int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
16
17       Link with -pthread.
18
19   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
20
21       sem_timedwait(): _POSIX_C_SOURCE >= 200112L
22

DESCRIPTION

24       sem_wait()  decrements (locks) the semaphore pointed to by sem.  If the
25       semaphore's value is greater than zero, then  the  decrement  proceeds,
26       and  the function returns, immediately.  If the semaphore currently has
27       the value zero, then the call blocks until either it  becomes  possible
28       to  perform the decrement (i.e., the semaphore value rises above zero),
29       or a signal handler interrupts the call.
30
31       sem_trywait() is the same as sem_wait(), except that if  the  decrement
32       cannot  be immediately performed, then call returns an error (errno set
33       to EAGAIN) instead of blocking.
34
35       sem_timedwait() is the same  as  sem_wait(),  except  that  abs_timeout
36       specifies  a  limit on the amount of time that the call should block if
37       the decrement cannot be immediately performed.  The  abs_timeout  argu‐
38       ment  points  to a structure that specifies an absolute timeout in sec‐
39       onds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000  (UTC).
40       This structure is defined as follows:
41
42           struct timespec {
43               time_t tv_sec;      /* Seconds */
44               long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */
45           };
46
47       If  the  timeout  has  already expired by the time of the call, and the
48       semaphore could not be locked immediately, then  sem_timedwait()  fails
49       with a timeout error (errno set to ETIMEDOUT).
50
51       If  the  operation  can  be performed immediately, then sem_timedwait()
52       never fails with a timeout error, regardless of the value of  abs_time‐
53       out.   Furthermore,  the validity of abs_timeout is not checked in this
54       case.
55

RETURN VALUE

57       All of these functions return 0 on success; on error, the value of  the
58       semaphore  is left unchanged, -1 is returned, and errno is set to indi‐
59       cate the error.
60

ERRORS

62       EINTR  The call was interrupted by a signal handler; see signal(7).
63
64       EINVAL sem is not a valid semaphore.
65
66       The following additional error can occur for sem_trywait():
67
68       EAGAIN The operation could not be performed without blocking (i.e., the
69              semaphore currently has the value zero).
70
71       The following additional errors can occur for sem_timedwait():
72
73       EINVAL The  value  of  abs_timeout.tv_nsecs  is less than 0, or greater
74              than or equal to 1000 million.
75
76       ETIMEDOUT
77              The call timed out before the semaphore could be locked.
78

ATTRIBUTES

80       For  an  explanation  of  the  terms  used   in   this   section,   see
81       attributes(7).
82
83       ┌───────────────────────────┬───────────────┬─────────┐
84Interface                  Attribute     Value   
85       ├───────────────────────────┼───────────────┼─────────┤
86sem_wait(), sem_trywait(), │ Thread safety │ MT-Safe │
87sem_timedwait()            │               │         │
88       └───────────────────────────┴───────────────┴─────────┘

CONFORMING TO

90       POSIX.1-2001, POSIX.1-2008.
91

EXAMPLE

93       The (somewhat trivial) program shown below operates on an unnamed sema‐
94       phore.   The  program  expects  two  command-line arguments.  The first
95       argument specifies a seconds value that is used to set an  alarm  timer
96       to  generate  a SIGALRM signal.  This handler performs a sem_post(3) to
97       increment the semaphore  that  is  being  waited  on  in  main()  using
98       sem_timedwait().  The second command-line argument specifies the length
99       of the timeout, in seconds, for sem_timedwait().  The  following  shows
100       what happens on two different runs of the program:
101
102           $ ./a.out 2 3
103           About to call sem_timedwait()
104           sem_post() from handler
105           sem_timedwait() succeeded
106           $ ./a.out 2 1
107           About to call sem_timedwait()
108           sem_timedwait() timed out
109
110   Program source
111
112       #include <unistd.h>
113       #include <stdio.h>
114       #include <stdlib.h>
115       #include <semaphore.h>
116       #include <time.h>
117       #include <assert.h>
118       #include <errno.h>
119       #include <signal.h>
120
121       sem_t sem;
122
123       #define handle_error(msg) \
124           do { perror(msg); exit(EXIT_FAILURE); } while (0)
125
126       static void
127       handler(int sig)
128       {
129           write(STDOUT_FILENO, "sem_post() from handler\n", 24);
130           if (sem_post(&sem) == -1) {
131               write(STDERR_FILENO, "sem_post() failed\n", 18);
132               _exit(EXIT_FAILURE);
133           }
134       }
135
136       int
137       main(int argc, char *argv[])
138       {
139           struct sigaction sa;
140           struct timespec ts;
141           int s;
142
143           if (argc != 3) {
144               fprintf(stderr, "Usage: %s <alarm-secs> <wait-secs>\n",
145                       argv[0]);
146               exit(EXIT_FAILURE);
147           }
148
149           if (sem_init(&sem, 0, 0) == -1)
150               handle_error("sem_init");
151
152           /* Establish SIGALRM handler; set alarm timer using argv[1] */
153
154           sa.sa_handler = handler;
155           sigemptyset(&sa.sa_mask);
156           sa.sa_flags = 0;
157           if (sigaction(SIGALRM, &sa, NULL) == -1)
158               handle_error("sigaction");
159
160           alarm(atoi(argv[1]));
161
162           /* Calculate relative interval as current time plus
163              number of seconds given argv[2] */
164
165           if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
166               handle_error("clock_gettime");
167
168           ts.tv_sec += atoi(argv[2]);
169
170           printf("main() about to call sem_timedwait()\n");
171           while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR)
172               continue;       /* Restart if interrupted by handler */
173
174           /* Check what happened */
175
176           if (s == -1) {
177               if (errno == ETIMEDOUT)
178                   printf("sem_timedwait() timed out\n");
179               else
180                   perror("sem_timedwait");
181           } else
182               printf("sem_timedwait() succeeded\n");
183
184           exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
185       }
186

SEE ALSO

188       clock_gettime(2),    sem_getvalue(3),   sem_post(3),   sem_overview(7),
189       time(7)
190

COLOPHON

192       This page is part of release 4.15 of the Linux  man-pages  project.   A
193       description  of  the project, information about reporting bugs, and the
194       latest    version    of    this    page,    can     be     found     at
195       https://www.kernel.org/doc/man-pages/.
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199Linux                             2017-09-15                       SEM_WAIT(3)
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