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| .TH SEM_WAIT 3 2016-03-15 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| sem_wait, sem_timedwait, sem_trywait \- lock a semaphore |
| .SH SYNOPSIS |
| .nf |
| .B #include <semaphore.h> |
| .sp |
| .BI "int sem_wait(sem_t *" sem ); |
| .sp |
| .BI "int sem_trywait(sem_t *" sem ); |
| .sp |
| .BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout ); |
| .fi |
| .sp |
| Link with \fI\-pthread\fP. |
| .sp |
| .in -4n |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .in |
| .sp |
| .BR sem_timedwait (): |
| _POSIX_C_SOURCE\ >=\ 200112L |
| .SH DESCRIPTION |
| .BR sem_wait () |
| decrements (locks) the semaphore pointed to by |
| .IR sem . |
| If the semaphore's value is greater than zero, |
| then the decrement proceeds, and the function returns, immediately. |
| If the semaphore currently has the value zero, |
| then the call blocks until either it becomes possible to perform |
| the decrement (i.e., the semaphore value rises above zero), |
| or a signal handler interrupts the call. |
| |
| .BR sem_trywait () |
| is the same as |
| .BR sem_wait (), |
| except that if the decrement cannot be immediately performed, |
| then call returns an error |
| .RI ( errno |
| set to |
| .BR EAGAIN ) |
| instead of blocking. |
| |
| .BR sem_timedwait () |
| is the same as |
| .BR sem_wait (), |
| except that |
| .I abs_timeout |
| specifies a limit on the amount of time that the call |
| should block if the decrement cannot be immediately performed. |
| The |
| .I abs_timeout |
| argument points to a structure that specifies an absolute timeout |
| in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC). |
| This structure is defined as follows: |
| |
| .nf |
| .in +4n |
| struct timespec { |
| time_t tv_sec; /* Seconds */ |
| long tv_nsec; /* Nanoseconds [0 .. 999999999] */ |
| }; |
| .in |
| .fi |
| .PP |
| If the timeout has already expired by the time of the call, |
| and the semaphore could not be locked immediately, |
| then |
| .BR sem_timedwait () |
| fails with a timeout error |
| .RI ( errno |
| set to |
| .BR ETIMEDOUT ). |
| |
| If the operation can be performed immediately, then |
| .BR sem_timedwait () |
| never fails with a timeout error, regardless of the value of |
| .IR abs_timeout . |
| Furthermore, the validity of |
| .I abs_timeout |
| is not checked in this case. |
| .SH RETURN VALUE |
| All of these functions return 0 on success; |
| on error, the value of the semaphore is left unchanged, |
| \-1 is returned, and |
| .I errno |
| is set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EINTR |
| The call was interrupted by a signal handler; see |
| .BR signal (7). |
| .TP |
| .B EINVAL |
| .I sem |
| is not a valid semaphore. |
| .PP |
| The following additional error can occur for |
| .BR sem_trywait (): |
| .TP |
| .B EAGAIN |
| The operation could not be performed without blocking (i.e., the |
| semaphore currently has the value zero). |
| .PP |
| The following additional errors can occur for |
| .BR sem_timedwait (): |
| .TP |
| .B EINVAL |
| The value of |
| .I abs_timeout.tv_nsecs |
| is less than 0, or greater than or equal to 1000 million. |
| .TP |
| .B ETIMEDOUT |
| The call timed out before the semaphore could be locked. |
| .\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition |
| .\" was detected", but this does not occur on Linux(?). |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .TS |
| allbox; |
| lbw26 lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR sem_wait (), |
| .BR sem_trywait (), |
| .BR sem_timedwait () |
| T} Thread safety MT-Safe |
| .TE |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| A signal handler always interrupts a blocked call to |
| one of these functions, regardless of the use of the |
| .BR sigaction (2) |
| .B SA_RESTART |
| flag. |
| .\" sem_wait() is always interrupted on most other implementations, |
| .\" but on FreeBSD 5.4 SA_RESTART does cause restarting. |
| .SH EXAMPLE |
| .PP |
| The (somewhat trivial) program shown below operates on an |
| unnamed semaphore. |
| The program expects two command-line arguments. |
| The first argument specifies a seconds value that is used to |
| set an alarm timer to generate a |
| .B SIGALRM |
| signal. |
| This handler performs a |
| .BR sem_post (3) |
| to increment the semaphore that is being waited on in |
| .I main() |
| using |
| .BR sem_timedwait (). |
| The second command-line argument specifies the length |
| of the timeout, in seconds, for |
| .BR sem_timedwait (). |
| The following shows what happens on two different runs of the program: |
| |
| .in +4n |
| .nf |
| .RB "$" " ./a.out 2 3" |
| About to call sem_timedwait() |
| sem_post() from handler |
| sem_timedwait() succeeded |
| .RB "$" " ./a.out 2 1" |
| About to call sem_timedwait() |
| sem_timedwait() timed out |
| .fi |
| .in |
| .SS Program source |
| \& |
| .nf |
| #include <unistd.h> |
| #include <stdio.h> |
| #include <stdlib.h> |
| #include <semaphore.h> |
| #include <time.h> |
| #include <assert.h> |
| #include <errno.h> |
| #include <signal.h> |
| |
| sem_t sem; |
| |
| #define handle_error(msg) \\ |
| do { perror(msg); exit(EXIT_FAILURE); } while (0) |
| |
| static void |
| handler(int sig) |
| { |
| write(STDOUT_FILENO, "sem_post() from handler\\n", 24); |
| if (sem_post(&sem) == \-1) { |
| write(STDERR_FILENO, "sem_post() failed\\n", 18); |
| _exit(EXIT_FAILURE); |
| } |
| } |
| |
| int |
| main(int argc, char *argv[]) |
| { |
| struct sigaction sa; |
| struct timespec ts; |
| int s; |
| |
| if (argc != 3) { |
| fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\\n", |
| argv[0]); |
| exit(EXIT_FAILURE); |
| } |
| |
| if (sem_init(&sem, 0, 0) == \-1) |
| handle_error("sem_init"); |
| |
| /* Establish SIGALRM handler; set alarm timer using argv[1] */ |
| |
| sa.sa_handler = handler; |
| sigemptyset(&sa.sa_mask); |
| sa.sa_flags = 0; |
| if (sigaction(SIGALRM, &sa, NULL) == \-1) |
| handle_error("sigaction"); |
| |
| alarm(atoi(argv[1])); |
| |
| /* Calculate relative interval as current time plus |
| number of seconds given argv[2] */ |
| |
| if (clock_gettime(CLOCK_REALTIME, &ts) == \-1) |
| handle_error("clock_gettime"); |
| |
| ts.tv_sec += atoi(argv[2]); |
| |
| printf("main() about to call sem_timedwait()\\n"); |
| while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR) |
| continue; /* Restart if interrupted by handler */ |
| |
| /* Check what happened */ |
| |
| if (s == \-1) { |
| if (errno == ETIMEDOUT) |
| printf("sem_timedwait() timed out\\n"); |
| else |
| perror("sem_timedwait"); |
| } else |
| printf("sem_timedwait() succeeded\\n"); |
| |
| exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE); |
| } |
| .fi |
| .SH SEE ALSO |
| .BR clock_gettime (2), |
| .BR sem_getvalue (3), |
| .BR sem_post (3), |
| .BR sem_overview (7), |
| .BR time (7) |