semop(2)                                                           semop(2)




 NAME
      semop - semaphore operations

 SYNOPSIS
      #include <sys/sem.h>

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

 DESCRIPTION
      semop() is used to atomically perform an array of semaphore operations
      on the set of semaphores associated with the semaphore identifier
      specified by semid.  sops is a pointer to the array of semaphore-
      operation structures.  nsops is the number of such structures in the
      array.  The contents of each structure includes the following members:

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

      Each semaphore operation specified by sem_op is performed on the
      corresponding semaphore specified by semid and sem_num.  Semaphore
      array operations are atomic in that none of the semaphore operations
      are performed until blocking conditions on all of the semaphores in
      the array have been removed.

      sem_op specifies one of three semaphore operations as follows:

           If sem_op is a negative integer, one of the following occurs:

                If semval (see semaphore identifier in glossary(9)) is
                greater than or equal to the absolute value of sem_op, the
                absolute value of sem_op is subtracted from semval.  Also,
                if (sem_flg & SEM_UNDO) is ``true'', the absolute value of
                sem_op is added to the calling process's semadj value (see
                glossary(9) and exit(2)) for the specified semaphore.

                If semval is less than the absolute value of sem_op and
                (sem_flg & IPC_NOWAIT) is ``true'', semop() returns
                immediately.

                If semval is less than the absolute value of sem_op and
                (sem_flg & IPC_NOWAIT) is ``false'', semop() increments the
                semncnt associated with the specified semaphore and suspend
                execution of the calling process until one of the following
                conditions occur:





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 semop(2)                                                           semop(2)




                     semval becomes greater than or equal to the absolute
                     value of sem_op.  When this occurs, the value of
                     semncnt associated with the specified semaphore is
                     decremented, the absolute value of sem_op is subtracted
                     from semval and, if (sem_flg & SEM_UNDO) is ``true'',
                     the absolute value of sem_op is added to the calling
                     process's semadj value for the specified semaphore.

                     The semid for which the calling process is awaiting
                     action is removed from the system (see semctl(2)).
                     When this occurs, errno is set equal to EIDRM, and a
                     value of -1 is returned.

                     The calling process receives a signal that is to be
                     caught.  When this occurs, the value of semncnt
                     associated with the specified semaphore is decremented,
                     and the calling process resumes execution in the manner
                     prescribed in signal(5).

           If sem_op is a positive integer, the value of sem_op is added to
           semval and, if (sem_flg & SEM_UNDO) is ``true'', the value of
           sem_op is subtracted from the calling process's semadj value for
           the specified semaphore.

           If sem_op is zero, one of the following occurs:

                If semval is zero, semop() proceeds to the next semaphore
                operation specified by sops, or returns immediately if this
                is the last operation.

                If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is
                ``true'', semop() returns immediately.

                If semval is not equal to zero and (sem_flg & IPC_NOWAIT) is
                ``false'', semop() increments the semzcnt associated with
                the specified semaphore and suspends execution of the
                calling process until one of the following occurs:

                     semval becomes zero, at which time the value of semzcnt
                     associated with the specified semaphore is decremented.

                     The semid for which the calling process is awaiting
                     action is removed from the system.  When this occurs,
                     errno is set equal to EIDRM, and a value of -1 is
                     returned.

                     The calling process receives a signal that is to be
                     caught.  When this occurs, the value of semzcnt
                     associated with the specified semaphore is decremented,
                     and the calling process resumes execution in the manner
                     prescribed in signal(5).



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 semop(2)                                                           semop(2)




 EXAMPLES
      The following call to semop() atomically performs a "P" or "get"
      operation on the second semaphore in the semaphore set and a "V" or
      "release" operation on the third semaphore in the set.  This example
      assumes the process has a valid semid which represents a set of 4
      semaphores as shown on the semget(2) manual page.  It also assumes
      that the semvals of the semaphores in the set have been initialized as
      shown in the semctl(2) manual entry.

           struct sembuf sops[4];

           sops[0].sem_num = 1;
           sops[0].sem_op  = -1;      /* P (get) */
           sops[0].sem_flg = 0;
           sops[1].sem_num = 2;
           sops[1].sem_op  =  1;      /* V (release) */
           sops[1].sem_flg = 0;

           semop (mysemid, sops, 2);

 RETURN VALUE
      If semop() returns due to the receipt of a signal, a value of -1 is
      returned to the calling process and errno is set to EINTR.  If it
      returns due to the removal of a semid from the system, a value of -1
      is returned and errno is set to EIDRM.

      Upon successful completion, a non-negative value is returned.
      Otherwise, a value of -1 is returned and errno is set to indicate the
      error.

 ERRORS
      semop() fails if one or more of the following is true for any of the
      semaphore operations specified by sops:

           [EINVAL]       semid is not a valid semaphore identifier.

           [EFBIG]        sem_num is less than zero or greater than or equal
                          to the number of semaphores in the set associated
                          with semid.

           [E2BIG]        nsops is greater than the system-imposed maximum.

           [EACCES]       Operation permission is denied to the calling
                          process (see glossary(9)).

           [EAGAIN]       The operation would result in suspension of the
                          calling process but (sem_flg & IPC_NOWAIT) is
                          ``true''.

           [ENOSPC]       The limit on the number of individual  processes
                          requesting an SEM_UNDO would be exceeded.



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 semop(2)                                                           semop(2)




           [EINVAL]       The number of individual semaphores for which the
                          calling process requests a SEM_UNDO would exceed
                          the limit.

           [ERANGE]       An operation would cause a semval to overflow the
                          system-imposed limit.

           [ERANGE]       An operation would cause a semadj value to
                          overflow the system-imposed limit.

           [EFAULT]       sops points to an illegal address.  The reliable
                          detection of this error will be implementation
                          dependent.

           Upon successful completion, the value of sempid for each
           semaphore specified in the array pointed to by sops is set equal
           to the process ID of the calling process.  The value of sem_otime
           in the data structure associated with the semaphore identifier
           will be set to the current time.

 WARNINGS
      Check all references to signal(5) for appropriateness on systems that
      support sigvector(2).  sigvector(2) can affect the behavior described
      on this page.

 SEE ALSO
      ipcs(1), exec(2), exit(2), fork(2), semctl(2), semget(2), stdipc(3C),
      signal(5).

 STANDARDS CONFORMANCE
      semop(): SVID2, SVID3, XPG2, XPG3, XPG4























 Hewlett-Packard Company            - 4 -    HP-UX Release 10.20:  July 1996