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SEMOP(2)                   Linux Programmer's Manual                  SEMOP(2)



NAME
       semop, semtimedop - semaphore operations

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 elements  of
       this structure are of type struct sembuf, containing the following 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 operation specifies SEM_UNDO, it will be auto-
       matically 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 complete 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 semaphore 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.

       If sem_op is zero, the process must have read permission on the semaphore set.  This is a "wait-for-zero" oper-
       ation: if semval is zero, the operation can immediately proceed.  Otherwise,  if  IPC_NOWAIT  is  specified  in
       sem_flg,  semop()  fails with errno set to EAGAIN (and none of the operations in sops is performed).  Otherwise
       semzcnt (the count of processes waiting until this semaphore's value becomes zero) is incremented  by  one  and
       the process sleeps until one of the following occurs:

       ?  semval becomes 0, at which time the value of semzcnt is decremented.

       ?  The semaphore set is removed: semop() fails, with errno set to EIDRM.

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

       ?  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 specified for this operation, the system updates the pro-
       cess 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:

       ?  semval  becomes greater than or equal to the absolute value of sem_op, at which time the value of semncnt is
          decremented, the absolute 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.

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

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

       ?  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 structure 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 timeout 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 operations allowed per system call.

       EACCES The calling process does not have the permissions required to perform  the  specified  semaphore  opera-
              tions, and does not have the CAP_IPC_OWNER capability.

       EAGAIN An  operation  could  not proceed immediately and either IPC_NOWAIT was specified in sem_flg or the time
              limit specified in timeout expired.

       EFAULT An address specified in either the sops or the timeout argument isn't accessible.

       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 non-positive 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 backported 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 produced 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).

       The implementation has no intrinsic limits for the adjust on exit maximum value (SEMAEM), the system wide maxi-
       mum number of undo structures (SEMMNU) and the per-process maximum number of undo entries system parameters.

BUGS
       When  a  process  terminates,  its set of associated semadj structures is used to undo the effect of all of the
       semaphore operations it performed with the SEM_UNDO flag.  This raises a difficulty: if one (or more) of  these
       semaphore  adjustments  would  result  in an attempt to decrease a semaphore's value below zero, what should an
       implementation do?  One possible approach would be to block until all the semaphore adjustments could  be  per-
       formed.   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 altogether  (somewhat  analo-
       gously  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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-10-04                          SEMOP(2)