man3/sem_wait.3

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  23 .TH SEM_WAIT 3 2007-07-26 "Linux" "Linux Programmer's Manual"
  24 .SH NAME
  25 sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <semaphore.h>
  29 .sp
  30 .BI "int sem_wait(sem_t *" sem );
  31 .BI "int sem_trywait(sem_t *" sem );
  32 .BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
  33 .fi
  34 .sp
  35 .in -4n
  36 Feature Test Macro Requirements for glibc (see
  37 .BR feature_test_macros (7)):
  38 .in
  39 .sp
  40 .ad l
  41 .BR sem_timedwait ():
  42 _POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600
  43 .ad p
  44 .SH DESCRIPTION
  45 .BR sem_wait ()
  46 decrements (locks) the semaphore pointed to by
  47 .IR sem .
  48 If the semaphore's value is greater than zero,
  49 then the decrement proceeds, and the function returns, immediately.
  50 If the semaphore currently has the value zero,
  51 then the call blocks until either it becomes possible to perform
  52 the decrement (i.e., the semaphore value rises above zero),
  53 or a signal handler interrupts the call.
  54
  55 .BR sem_trywait ()
  56 is the same as
  57 .BR sem_wait (),
  58 except that if the decrement cannot be immediately performed,
  59 then call returns an error
  60 .RI ( errno
  61 set to
  62 .BR EAGAIN )
  63 instead of blocking.
  64
  65 .BR sem_timedwait ()
  66 is the same as
  67 .BR sem_wait (),
  68 except that
  69 .I abs_timeout
  70 specifies a limit on the amount of time that the call
  71 should block if the decrement cannot be immediately performed.
  72 The
  73 .I abs_timeout
  74 argument points to a structure that specifies an absolute timeout
  75 in seconds and nanoseconds since the Epoch (00:00:00, 1 January 1970).
  76 This structure is defined as follows:
  77
  78 .nf
  79 .in +0.5i
  80 struct timespec {
  81     time_t tv_sec;      /* Seconds */
  82     long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */
  83 };
  84 .in -0.5i
  85 .fi
  86 .PP
  87 If the timeout has already expired by the time of the call,
  88 and the semaphore could not be locked immediately,
  89 then
  90 .BR sem_timedwait ()
  91 fails with a timeout error
  92 .RI ( errno
  93 set to
  94 .BR ETIMEDOUT ).
  95
  96 If the operation can be performed immediately, then
  97 .BR sem_timedwait ()
  98 never fails with a timeout error, regardless of the value of
  99 .IR abs_timeout .
 100 Furthermore, the validity of
 101 .I abs_timeout
 102 is not checked in this case.
 103 .SH RETURN VALUE
 104 All of these functions return 0 on success;
 105 on error, the value of the semaphore is left unchanged,
 106 \-1 is returned, and
 107 .I errno
 108 is set to indicate the error.
 109 .SH ERRORS
 110 .TP
 111 .B EINTR
 112 The call was interrupted by a signal handler.
 113 .TP
 114 .B EINVAL
 115 .I sem
 116 is not a valid semaphore.
 117 .PP
 118 The following additional error can occur for
 119 .BR sem_trywait ():
 120 .TP
 121 .B EAGAIN
 122 The operation could not be performed without blocking (i.e., the
 123 semaphore currently has the value zero).
 124 .PP
 125 The following additional errors can occur for
 126 .BR sem_timedwait ():
 127 .TP
 128 .B EINVAL
 129 The value of
 130 .I abs_timeout.tv_nsecs
 131 is less than 0, or greater than or equal to 1000 million.
 132 .TP
 133 .B ETIMEDOUT
 134 The call timed out before the semaphore could be locked.
 135 .\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
 136 .\" was detected", but this does not occur on Linux(?).
 137 .SH CONFORMING TO
 138 POSIX.1-2001.
 139 .SH NOTES
 140 A signal handler always interrupts a blocked call to
 141 one of these functions, regardless of the use of the
 142 .BR sigaction (2)
 143 .B SA_RESTART
 144 flag.
 145 .\" sem_wait() is always interrupted on most other implementations,
 146 .\" but on FreeBSD 5.4 SA_RESTART does cause restarting.
 147 .SH EXAMPLE
 148 .PP
 149 The (somewhat trivial) program shown below operates on an
 150 unnamed semaphore.
 151 The program expects two command-line arguments.
 152 The first argument specifies a seconds value that is used to
 153 set an alarm timer to generate a
 154 .B SIGALRM
 155 signal.
 156 This handler performs a
 157 .BR sem_post (3)
 158 to increment the semaphore that is being waited on in
 159 .I main()
 160 using
 161 .BR sem_timedwait ().
 162 The second command-line argument specifies the length
 163 of the timeout, in seconds, for
 164 .BR sem_timedwait ().
 165 The following shows what happens on two different runs of the program:
 166
 167 .in +0.5i
 168 .nf
 169 $ ./a.out 2 3
 170 About to call sem_timedwait()
 171 sem_post() from handler
 172 sem_getvalue() from handler; value = 1
 173 sem_timedwait() succeeded
 174 $ ./a.out 2 1
 175 About to call sem_timedwait()
 176 sem_timedwait() timed out
 177 .fi
 178 .in -0.5i
 179 .PP
 180 The source code of the program is as follows:
 181 .nf
 182
 183 #include <unistd.h>
 184 #include <stdio.h>
 185 #include <stdlib.h>
 186 #include <semaphore.h>
 187 #include <time.h>
 188 #include <assert.h>
 189 #include <errno.h>
 190 #include <signal.h>
 191
 192 sem_t sem;
 193
 194 static void
 195 handler(int sig)
 196 {
 197     write(STDOUT_FILENO, "sem_post() from handler\\n", 24);
 198     if (sem_post(&sem) == \-1) {
 199         write(STDERR_FILENO, "sem_post() failed\\n", 18);
 200         _exit(EXIT_FAILURE);
 201     }
 202 }
 203
 204 int
 205 main(int argc, char *argv[])
 206 {
 207     struct sigaction sa;
 208     struct timespec ts;
 209     int s;
 210
 211     if (argc != 3) {
 212         fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\\n",
 213                 argv[0]);
 214         exit(EXIT_FAILURE);
 215     }
 216
 217     if (sem_init(&sem, 0, 0) == \-1) {
 218         perror("sem_init");
 219         exit(EXIT_FAILURE);
 220     }
 221
 222     /* Establish SIGALRM handler; set alarm timer using argv[1] */
 223
 224     sa.sa_handler = handler;
 225     sigemptyset(&sa.sa_mask);
 226     sa.sa_flags = 0;
 227     if (sigaction(SIGALRM, &sa, NULL) == \-1) {
 228         perror("sigaction");
 229         exit(EXIT_FAILURE);
 230     }
 231
 232     alarm(atoi(argv[1]));
 233
 234     /* Calculate relative interval as current time plus
 235        number of seconds given argv[2] */
 236
 237     if (clock_gettime(CLOCK_REALTIME, &ts) == \-1) {
 238         perror("clock_gettime");
 239         exit(EXIT_FAILURE);
 240     }
 241     ts.tv_sec += atoi(argv[2]);
 242
 243     printf("main() about to call sem_timedwait()\\n");
 244     while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
 245         continue;       /* Restart if interrupted by handler */
 246
 247     /* Check what happened */
 248
 249     if (s == \-1) {
 250         if (errno == ETIMEDOUT)
 251             printf("sem_timedwait() timed out\\n");
 252         else
 253             perror("sem_timedwait");
 254     } else
 255         printf("sem_timedwait() succeeded\\n");
 256
 257     exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
 258 }
 259 .fi
 260 .SH "SEE ALSO"
 261 .\" FIXME . when this page has been added to the man-pages set:
 262 .\" .BR clock_gettime (3),
 263 .BR sem_getvalue (3),
 264 .BR sem_post (3),
 265 .BR sem_overview (7)