sem_wait(3)

1SEM_WAIT(3)                Linux Programmer's Manual               SEM_WAIT(3)
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NAME

6       sem_wait - lock a semaphore
7

SYNOPSIS

9       #include <semaphore.h>
10
11       int sem_wait(sem_t *sem);
12       int sem_trywait(sem_t *sem);
13
14       #define _XOPEN_SOURCE 600
15       #include <semaphore.h>
16
17       int sem_timedwait(sem_t *sem, const struct timespec *abs_timeout);
18

DESCRIPTION

20       sem_wait()  decrements (locks) the semaphore pointed to by sem.  If the
21       semaphore's value is greater than zero, then  the  decrement  proceeds,
22       and  the function returns, immediately.  If the semaphore currently has
23       the value zero, then the call blocks until either it  becomes  possible
24       to  perform the decrement (i.e., the semaphore value rises above zero),
25       or a signal handler interrupts the call.
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27       sem_trywait() is the same as sem_wait(), except that if  the  decrement
28       cannot  be immediately performed, then call returns an error (errno set
29       to EAGAIN) instead of blocking.
30
31       sem_timedwait() is the same  as  sem_wait(),  except  that  abs_timeout
32       specifies  a  limit on the amount of time that the call should block if
33       the decrement cannot be immediately performed.  The  abs_timeout  argu‐
34       ment  points  to a structure that specifies an absolute timeout in sec‐
35       onds and nanoseconds since the Epoch (00:00:00, 1 January 1970).   This
36       structure is defined as follows:
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38         struct timespec {
39             time_t tv_sec;      /* Seconds */
40             long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */
41         };
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43
44       If  the  timeout  has  already expired by the time of the call, and the
45       semaphore could not be locked immediately, then  sem_timedwait()  fails
46       with a timeout error (errno set to ETIMEDOUT).
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48       If  the  operation  can  be performed immediately, then sem_timedwait()
49       never fails with a timeout error, regardless of the value of  abs_time‐
50       out.   Furthermore,  the validity of abs_timeout is not checked in this
51       case.
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RETURN VALUE

54       All of these functions return 0 on success; on error, the value of  the
55       semaphore  is left unchanged, -1 is returned, and errno is set to indi‐
56       cate the error.
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ERRORS

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

77       A signal handler always interrupts a blocked call to one of these func‐
78       tions, regardless of the use of the sigaction(2) SA_RESTART flag.
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CONFORMING TO

81       POSIX.1-2001.
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EXAMPLE

84       The (somewhat trivial) program shown below operates on an unnamed sema‐
85       phore.  The program expects  two  command-line  arguments.   The  first
86       argument  specifies  a seconds value that is used to set an alarm timer
87       to generate a SIGALRM signal.  This handler performs  a  sem_post()  to
88       increment  the  semaphore  that  is  being  waited  on  in main() using
89       sem_timedwait().  The second command-line argument specifies the length
90       of  the  timeout, in seconds, for sem_timedwait().  The following shows
91       what happens on two different runs of the program:
92
93            $ ./a.out 2 3
94            About to call sem_timedwait()
95            sem_post() from handler
96            sem_getvalue() from handler; value = 1
97            sem_timedwait() succeeded
98            $ ./a.out 2 1
99            About to call sem_timedwait()
100            sem_timedwait() timed out
101
102       The source code of the program is as follows:
103
104       #include <unistd.h>
105       #include <stdio.h>
106       #include <stdlib.h>
107       #include <semaphore.h>
108       #include <time.h>
109       #include <assert.h>
110       #include <errno.h>
111       #include <signal.h>
112
113       #define die(msg) { perror(msg); exit(EXIT_FAILURE); }
114
115       sem_t sem;
116
117       static void
118       handler(int sig)
119       {
120           int sval;
121
122           printf("sem_post() from handler\n");
123           if (sem_post(&sem) == -1) die("sem_post");
124
125           if (sem_getvalue(&sem, &sval) == -1) die("sem_getvalue");
126           printf("sem_getvalue() from handler; value = %d\n", sval);
127       } /* handler */
128
129       int
130       main(int argc, char *argv[])
131       {
132           struct sigaction sa;
133           struct timespec ts;
134           int s;
135
136           assert(argc == 3);  /* Usage: ./a.out alarm-secs wait-secs */
137
138           if (sem_init(&sem, 0, 0) == -1) die("sem_init");
139
140           /* Establish SIGALRM handler; set alarm timer using argv[1] */
141
142           sa.sa_handler = handler;
143           sigemptyset(&sa.sa_mask);
144           sa.sa_flags = 0;
145           if (sigaction(SIGALRM, &sa, NULL) == -1) die("sigaction");
146
147           alarm(atoi(argv[1]));
148
149           /* Calculate relative interval as current time plus
150              number of seconds given argv[2] */
151
152           if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
153               die("clock_gettime");
154           ts.tv_sec += atoi(argv[2]);
155
156           printf("main() about to call sem_timedwait()\n");
157           while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR)
158               continue;       /* Restart when interrupted by handler */
159
160           /* Check what happened */
161
162           if (s == -1) {
163               if (errno == ETIMEDOUT)
164                   printf("sem_timedwait() timed out\n");
165               else
166                   die("sem_timedwait");
167           } else
168               printf("sem_timedwait() succeeded\n");
169
170           exit(EXIT_SUCCESS);
171       }
172