semtimedop


SYNOPSIS
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>

       int semop(int semid, struct sembuf *sops, unsigned nsops);

       int semtimedop(int semid, struct sembuf *sops, unsigned nsops,
                      struct timespec *timeout);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       semtimedop(): _GNU_SOURCE

DESCRIPTION
       Each semaphore in a semaphore set has the following associated values:

           unsigned short  semval;   /* semaphore value */
           unsigned short  semzcnt;  /* # waiting for zero */
           unsigned short  semncnt;  /* # waiting for increase */
           pid_t           sempid;   /* process that did last op */

       semop() performs operations on selected semaphores in the set indicated
       by semid.  Each of the nsops elements in the array pointed to  by  sops
       specifies an operation to be performed on a single semaphore.  The ele-
       ments of this structure are of type struct sembuf, containing the  fol-
       lowing members:

           unsigned short sem_num;  /* semaphore number */
           short          sem_op;   /* semaphore operation */
           short          sem_flg;  /* operation flags */

       Flags  recognized in sem_flg are IPC_NOWAIT and SEM_UNDO.  If an opera-
       tion specifies SEM_UNDO, it  will  be  automatically  undone  when  the
       process terminates.

       The  set  of  operations contained in sops is performed in array order,
       and atomically, that is, the operations are performed either as a  com-
       plete  unit, or not at all.  The behavior of the system call if not all
       operations can be performed immediately depends on the presence of  the
       IPC_NOWAIT flag in the individual sem_flg fields, as noted below.

       Each  operation  is  performed on the sem_num-th semaphore of the sema-
       phore set, where the first semaphore of the set is numbered  0.   There
       are three types of operation, distinguished by the value of sem_op.

       If  sem_op  is a positive integer, the operation adds this value to the
       semaphore value (semval).  Furthermore, if SEM_UNDO  is  specified  for
       this  operation, the system updates the process undo count (semadj) for
       this semaphore.  This operation can always proceed--it never  forces  a
       process to wait.  The calling process must have alter permission on the
       semaphore set.


       o  The calling process catches a signal: the value of semzcnt is decre-
          mented and semop() fails, with errno set to EINTR.

       o  The time limit specified by timeout in a semtimedop() call  expires:
          semop() fails, with errno set to EAGAIN.

       If  sem_op is less than zero, the process must have alter permission on
       the semaphore set.  If semval is greater than or equal to the  absolute
       value  of  sem_op,  the operation can proceed immediately: the absolute
       value of sem_op is subtracted from semval, and, if SEM_UNDO  is  speci-
       fied  for  this  operation,  the  system updates the process undo count
       (semadj) for this semaphore.   If  the  absolute  value  of  sem_op  is
       greater  than  semval,  and IPC_NOWAIT is specified in sem_flg, semop()
       fails, with errno set to EAGAIN (and none of the operations in sops  is
       performed).   Otherwise  semncnt  (the counter of processes waiting for
       this semaphore's value to increase)  is  incremented  by  one  and  the
       process sleeps until one of the following occurs:

       o  semval  becomes  greater  than  or  equal  to  the absolute value of
          sem_op, at which time the value of semncnt is decremented, the abso-
          lute  value  of sem_op is subtracted from semval and, if SEM_UNDO is
          specified for this operation, the system updates  the  process  undo
          count (semadj) for this semaphore.

       o  The  semaphore  set  is removed from the system: semop() fails, with
          errno set to EIDRM.

       o  The calling process catches a signal: the value of semncnt is decre-
          mented and semop() fails, with errno set to EINTR.

       o  The  time limit specified by timeout in a semtimedop() call expires:
          the system call fails, with errno set to EAGAIN.

       On successful completion, the sempid value for each semaphore specified
       in the array pointed to by sops is set to the process ID of the calling
       process.  In addition, the sem_otime is set to the current time.

       semtimedop() behaves identically to semop() except that in those  cases
       were  the  calling  process  would sleep, the duration of that sleep is
       limited by the amount of elapsed time specified by the timespec  struc-
       ture whose address is passed in the timeout argument.  If the specified
       time limit has been reached,  semtimedop()  fails  with  errno  set  to
       EAGAIN (and none of the operations in sops is performed).  If the time-
       out argument is NULL, then semtimedop() behaves exactly like semop().

RETURN VALUE
       If successful semop() and semtimedop() return 0; otherwise they  return
       -1 with errno indicating the error.

ERRORS
       On failure, errno is set to one of the following:

       E2BIG  The argument nsops is greater than SEMOPM, the maximum number of

       EFBIG  For  some  operation  the  value  of  sem_num  is less than 0 or
              greater than or equal to the number of semaphores in the set.

       EIDRM  The semaphore set was removed.

       EINTR  While blocked in this system call, the process caught a  signal;
              see signal(7).

       EINVAL The  semaphore set doesn't exist, or semid is less than zero, or
              nsops has a nonpositive value.

       ENOMEM The sem_flg of some operation specified SEM_UNDO and the  system
              does not have enough memory to allocate the undo structure.

       ERANGE For  some  operation  sem_op+semval  is greater than SEMVMX, the
              implementation dependent maximum value for semval.

VERSIONS
       semtimedop() first appeared in Linux 2.5.52, and was subsequently back-
       ported  into  kernel  2.4.22.   Glibc  support  for  semtimedop() first
       appeared in version 2.3.3.

CONFORMING TO
       SVr4, POSIX.1-2001.

NOTES
       The sem_undo structures of a process aren't inherited by the child pro-
       duced  by  fork(2),  but  they are inherited across an execve(2) system
       call.

       semop() is never automatically restarted after being interrupted  by  a
       signal  handler,  regardless of the setting of the SA_RESTART flag when
       establishing a signal handler.

       semadj is a per-process integer which is simply the (negative) count of
       all  semaphore operations performed specifying the SEM_UNDO flag.  When
       a semaphore's value is directly set using the SETVAL or SETALL  request
       to  semctl(2),  the  corresponding  semadj  values in all processes are
       cleared.

       The semval, sempid, semzcnt, and semnct values for a semaphore can  all
       be retrieved using appropriate semctl(2) calls.

       The  following  limits  on  semaphore  set resources affect the semop()
       call:

       SEMOPM Maximum number of operations allowed for one semop()  call  (32)
              (on  Linux,  this  limit  can be read and modified via the third
              field of /proc/sys/kernel/sem).

       SEMVMX Maximum allowable value  for  semval:  implementation  dependent
              (32767).

       could be performed.  This is however undesirable since it  could  force
       process  termination  to  block  for arbitrarily long periods.  Another
       possibility is that such semaphore adjustments could be  ignored  alto-
       gether  (somewhat  analogously  to failing when IPC_NOWAIT is specified
       for a semaphore operation).  Linux adopts a third approach:  decreasing
       the  semaphore  value  as  far as possible (i.e., to zero) and allowing
       process termination to proceed immediately.

       In kernels 2.6.x, x <= 10, there is a bug that  in  some  circumstances
       prevents a process that is waiting for a semaphore value to become zero
       from being woken up when the value does actually become zero.  This bug
       is fixed in kernel 2.6.11.

EXAMPLE
       The  following  code  segment  uses  semop() to atomically wait for the
       value of semaphore 0 to become zero, and then increment  the  semaphore
       value by one.

           struct sembuf sops[2];
           int semid;

           /* Code to set semid omitted */

           sops[0].sem_num = 0;        /* Operate on semaphore 0 */
           sops[0].sem_op = 0;         /* Wait for value to equal 0 */
           sops[0].sem_flg = 0;

           sops[1].sem_num = 0;        /* Operate on semaphore 0 */
           sops[1].sem_op = 1;         /* Increment value by one */
           sops[1].sem_flg = 0;

           if (semop(semid, sops, 2) == -1) {
               perror("semop");
               exit(EXIT_FAILURE);
           }

SEE ALSO
       semctl(2),  semget(2),  sigaction(2), capabilities(7), sem_overview(7),
       svipc(7), time(7)

COLOPHON
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



Linux                             2008-10-04                          SEMOP(2)
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