man3/sem_wait.3

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  26 .TH SEM_WAIT 3 2015-08-08 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 sem_wait, sem_timedwait, sem_trywait \- lock a semaphore
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <semaphore.h>
  32 .sp
  33 .BI "int sem_wait(sem_t *" sem );
  34 .sp
  35 .BI "int sem_trywait(sem_t *" sem );
  36 .sp
  37 .BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
  38 .fi
  39 .sp
  40 Link with \fI\-pthread\fP.
  41 .sp
  42 .in -4n
  43 Feature Test Macro Requirements for glibc (see
  44 .BR feature_test_macros (7)):
  45 .in
  46 .sp
  47 .BR sem_timedwait ():
  48 _POSIX_C_SOURCE\ >=\ 200112L || _XOPEN_SOURCE\ >=\ 600
  49 .SH DESCRIPTION
  50 .BR sem_wait ()
  51 decrements (locks) the semaphore pointed to by
  52 .IR sem .
  53 If the semaphore's value is greater than zero,
  54 then the decrement proceeds, and the function returns, immediately.
  55 If the semaphore currently has the value zero,
  56 then the call blocks until either it becomes possible to perform
  57 the decrement (i.e., the semaphore value rises above zero),
  58 or a signal handler interrupts the call.
  59
  60 .BR sem_trywait ()
  61 is the same as
  62 .BR sem_wait (),
  63 except that if the decrement cannot be immediately performed,
  64 then call returns an error
  65 .RI ( errno
  66 set to
  67 .BR EAGAIN )
  68 instead of blocking.
  69
  70 .BR sem_timedwait ()
  71 is the same as
  72 .BR sem_wait (),
  73 except that
  74 .I abs_timeout
  75 specifies a limit on the amount of time that the call
  76 should block if the decrement cannot be immediately performed.
  77 The
  78 .I abs_timeout
  79 argument points to a structure that specifies an absolute timeout
  80 in seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC).
  81 This structure is defined as follows:
  82
  83 .nf
  84 .in +4n
  85 struct timespec {
  86     time_t tv_sec;      /* Seconds */
  87     long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */
  88 };
  89 .in
  90 .fi
  91 .PP
  92 If the timeout has already expired by the time of the call,
  93 and the semaphore could not be locked immediately,
  94 then
  95 .BR sem_timedwait ()
  96 fails with a timeout error
  97 .RI ( errno
  98 set to
  99 .BR ETIMEDOUT ).
 100
 101 If the operation can be performed immediately, then
 102 .BR sem_timedwait ()
 103 never fails with a timeout error, regardless of the value of
 104 .IR abs_timeout .
 105 Furthermore, the validity of
 106 .I abs_timeout
 107 is not checked in this case.
 108 .SH RETURN VALUE
 109 All of these functions return 0 on success;
 110 on error, the value of the semaphore is left unchanged,
 111 \-1 is returned, and
 112 .I errno
 113 is set to indicate the error.
 114 .SH ERRORS
 115 .TP
 116 .B EINTR
 117 The call was interrupted by a signal handler; see
 118 .BR signal (7).
 119 .TP
 120 .B EINVAL
 121 .I sem
 122 is not a valid semaphore.
 123 .PP
 124 The following additional error can occur for
 125 .BR sem_trywait ():
 126 .TP
 127 .B EAGAIN
 128 The operation could not be performed without blocking (i.e., the
 129 semaphore currently has the value zero).
 130 .PP
 131 The following additional errors can occur for
 132 .BR sem_timedwait ():
 133 .TP
 134 .B EINVAL
 135 The value of
 136 .I abs_timeout.tv_nsecs
 137 is less than 0, or greater than or equal to 1000 million.
 138 .TP
 139 .B ETIMEDOUT
 140 The call timed out before the semaphore could be locked.
 141 .\" POSIX.1-2001 also allows EDEADLK -- "A deadlock condition
 142 .\" was detected", but this does not occur on Linux(?).
 143 .SH ATTRIBUTES
 144 For an explanation of the terms used in this section, see
 145 .BR attributes (7).
 146 .TS
 147 allbox;
 148 lbw26 lb lb
 149 l l l.
 150 Interface       Attribute       Value
 151 T{
 152 .BR sem_wait (),
 153 .BR sem_trywait (),
 154 .BR sem_timedwait ()
 155 T}      Thread safety   MT-Safe
 156 .TE
 157 .SH CONFORMING TO
 158 POSIX.1-2001, POSIX.1-2008.
 159 .SH NOTES
 160 A signal handler always interrupts a blocked call to
 161 one of these functions, regardless of the use of the
 162 .BR sigaction (2)
 163 .B SA_RESTART
 164 flag.
 165 .\" sem_wait() is always interrupted on most other implementations,
 166 .\" but on FreeBSD 5.4 SA_RESTART does cause restarting.
 167 .SH EXAMPLE
 168 .PP
 169 The (somewhat trivial) program shown below operates on an
 170 unnamed semaphore.
 171 The program expects two command-line arguments.
 172 The first argument specifies a seconds value that is used to
 173 set an alarm timer to generate a
 174 .B SIGALRM
 175 signal.
 176 This handler performs a
 177 .BR sem_post (3)
 178 to increment the semaphore that is being waited on in
 179 .I main()
 180 using
 181 .BR sem_timedwait ().
 182 The second command-line argument specifies the length
 183 of the timeout, in seconds, for
 184 .BR sem_timedwait ().
 185 The following shows what happens on two different runs of the program:
 186
 187 .in +4n
 188 .nf
 189 .RB "$" " ./a.out 2 3"
 190 About to call sem_timedwait()
 191 sem_post() from handler
 192 sem_timedwait() succeeded
 193 .RB "$" " ./a.out 2 1"
 194 About to call sem_timedwait()
 195 sem_timedwait() timed out
 196 .fi
 197 .in
 198 .SS Program source
 199 \&
 200 .nf
 201 #include <unistd.h>
 202 #include <stdio.h>
 203 #include <stdlib.h>
 204 #include <semaphore.h>
 205 #include <time.h>
 206 #include <assert.h>
 207 #include <errno.h>
 208 #include <signal.h>
 209
 210 sem_t sem;
 211
 212 #define handle_error(msg) \\
 213     do { perror(msg); exit(EXIT_FAILURE); } while (0)
 214
 215 static void
 216 handler(int sig)
 217 {
 218     write(STDOUT_FILENO, "sem_post() from handler\\n", 24);
 219     if (sem_post(&sem) == \-1) {
 220         write(STDERR_FILENO, "sem_post() failed\\n", 18);
 221         _exit(EXIT_FAILURE);
 222     }
 223 }
 224
 225 int
 226 main(int argc, char *argv[])
 227 {
 228     struct sigaction sa;
 229     struct timespec ts;
 230     int s;
 231
 232     if (argc != 3) {
 233         fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\\n",
 234                 argv[0]);
 235         exit(EXIT_FAILURE);
 236     }
 237
 238     if (sem_init(&sem, 0, 0) == \-1)
 239         handle_error("sem_init");
 240
 241     /* Establish SIGALRM handler; set alarm timer using argv[1] */
 242
 243     sa.sa_handler = handler;
 244     sigemptyset(&sa.sa_mask);
 245     sa.sa_flags = 0;
 246     if (sigaction(SIGALRM, &sa, NULL) == \-1)
 247         handle_error("sigaction");
 248
 249     alarm(atoi(argv[1]));
 250
 251     /* Calculate relative interval as current time plus
 252        number of seconds given argv[2] */
 253
 254     if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
 255         handle_error("clock_gettime");
 256
 257     ts.tv_sec += atoi(argv[2]);
 258
 259     printf("main() about to call sem_timedwait()\\n");
 260     while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
 261         continue;       /* Restart if interrupted by handler */
 262
 263     /* Check what happened */
 264
 265     if (s == \-1) {
 266         if (errno == ETIMEDOUT)
 267             printf("sem_timedwait() timed out\\n");
 268         else
 269             perror("sem_timedwait");
 270     } else
 271         printf("sem_timedwait() succeeded\\n");
 272
 273     exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
 274 }
 275 .fi
 276 .SH SEE ALSO
 277 .BR clock_gettime (2),
 278 .BR sem_getvalue (3),
 279 .BR sem_post (3),
 280 .BR sem_overview (7),
 281 .BR time (7)