man3/sem_wait.3

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  26 .TH SEM_WAIT 3 2017-09-15 "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 .PP
  33 .BI "int sem_wait(sem_t *" sem );
  34 .PP
  35 .BI "int sem_trywait(sem_t *" sem );
  36 .PP
  37 .BI "int sem_timedwait(sem_t *" sem ", const struct timespec *" abs_timeout );
  38 .fi
  39 .PP
  40 Link with \fI\-pthread\fP.
  41 .PP
  42 .in -4n
  43 Feature Test Macro Requirements for glibc (see
  44 .BR feature_test_macros (7)):
  45 .in
  46 .PP
  47 .BR sem_timedwait ():
  48 _POSIX_C_SOURCE\ >=\ 200112L
  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 .PP
  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 .PP
  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 .PP
  83 .in +4n
  84 .EX
  85 struct timespec {
  86     time_t tv_sec;      /* Seconds */
  87     long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */
  88 };
  89 .EE
  90 .in
  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 .PP
 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 EXAMPLE
 160 .PP
 161 The (somewhat trivial) program shown below operates on an
 162 unnamed semaphore.
 163 The program expects two command-line arguments.
 164 The first argument specifies a seconds value that is used to
 165 set an alarm timer to generate a
 166 .B SIGALRM
 167 signal.
 168 This handler performs a
 169 .BR sem_post (3)
 170 to increment the semaphore that is being waited on in
 171 .I main()
 172 using
 173 .BR sem_timedwait ().
 174 The second command-line argument specifies the length
 175 of the timeout, in seconds, for
 176 .BR sem_timedwait ().
 177 The following shows what happens on two different runs of the program:
 178 .PP
 179 .in +4n
 180 .EX
 181 .RB "$" " ./a.out 2 3"
 182 About to call sem_timedwait()
 183 sem_post() from handler
 184 sem_timedwait() succeeded
 185 .RB "$" " ./a.out 2 1"
 186 About to call sem_timedwait()
 187 sem_timedwait() timed out
 188 .EE
 189 .in
 190 .SS Program source
 191 \&
 192 .EX
 193 #include <unistd.h>
 194 #include <stdio.h>
 195 #include <stdlib.h>
 196 #include <semaphore.h>
 197 #include <time.h>
 198 #include <assert.h>
 199 #include <errno.h>
 200 #include <signal.h>
 201
 202 sem_t sem;
 203
 204 #define handle_error(msg) \\
 205     do { perror(msg); exit(EXIT_FAILURE); } while (0)
 206
 207 static void
 208 handler(int sig)
 209 {
 210     write(STDOUT_FILENO, "sem_post() from handler\\n", 24);
 211     if (sem_post(&sem) == \-1) {
 212         write(STDERR_FILENO, "sem_post() failed\\n", 18);
 213         _exit(EXIT_FAILURE);
 214     }
 215 }
 216
 217 int
 218 main(int argc, char *argv[])
 219 {
 220     struct sigaction sa;
 221     struct timespec ts;
 222     int s;
 223
 224     if (argc != 3) {
 225         fprintf(stderr, "Usage: %s <alarm\-secs> <wait\-secs>\\n",
 226                 argv[0]);
 227         exit(EXIT_FAILURE);
 228     }
 229
 230     if (sem_init(&sem, 0, 0) == \-1)
 231         handle_error("sem_init");
 232
 233     /* Establish SIGALRM handler; set alarm timer using argv[1] */
 234
 235     sa.sa_handler = handler;
 236     sigemptyset(&sa.sa_mask);
 237     sa.sa_flags = 0;
 238     if (sigaction(SIGALRM, &sa, NULL) == \-1)
 239         handle_error("sigaction");
 240
 241     alarm(atoi(argv[1]));
 242
 243     /* Calculate relative interval as current time plus
 244        number of seconds given argv[2] */
 245
 246     if (clock_gettime(CLOCK_REALTIME, &ts) == \-1)
 247         handle_error("clock_gettime");
 248
 249     ts.tv_sec += atoi(argv[2]);
 250
 251     printf("main() about to call sem_timedwait()\\n");
 252     while ((s = sem_timedwait(&sem, &ts)) == \-1 && errno == EINTR)
 253         continue;       /* Restart if interrupted by handler */
 254
 255     /* Check what happened */
 256
 257     if (s == \-1) {
 258         if (errno == ETIMEDOUT)
 259             printf("sem_timedwait() timed out\\n");
 260         else
 261             perror("sem_timedwait");
 262     } else
 263         printf("sem_timedwait() succeeded\\n");
 264
 265     exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);
 266 }
 267 .EE
 268 .SH SEE ALSO
 269 .BR clock_gettime (2),
 270 .BR sem_getvalue (3),
 271 .BR sem_post (3),
 272 .BR sem_overview (7),
 273 .BR time (7)