semop(2) - NetBSD Manual Pages

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SEMOP(2)                  NetBSD System Calls Manual                  SEMOP(2)


NAME

     semop, semtimedop -- semaphore operations


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <sys/sem.h>

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

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


DESCRIPTION

     semop() provides a number of atomic operations on a set of semaphores.
     The semaphore set is specified by semid, sops is an array of semaphore
     operations, and nsops is the number of operations in this array.  The
     sembuf structures in the array contain the following members:

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

     Each operation (specified in sem_op) is applied to semaphore number
     sem_num in the set of semaphores specified by semid.  The value of sem_op
     determines the action taken in the following way:

     ·   sem_op is less than 0.  The current process is blocked until the
         value of the semaphore is greater than or equal to the absolute value
         of sem_op.  The absolute value of sem_op is then subtracted from the
         value of the semaphore, and the calling process continues.  Negative
         values of sem_op are thus used to enter critical regions.

     ·   sem_op is greater than 0.  Its value is added to the value of the
         specified semaphore.  This is used to leave critical regions.

     ·   sem_op is equal to 0.  The calling process is blocked until the value
         of the specified semaphore reaches 0.

     The behaviour of each operation is influenced by the flags set in sem_flg
     in the following way:

     IPC_NOWAIT   In the case where the calling process would normally block,
                  waiting for a semaphore to reach a certain value, IPC_NOWAIT
                  makes the call return immediately, returning a value of -1
                  and setting errno to EAGAIN.

     SEM_UNDO     Keep track of the changes that this call makes to the value
                  of a semaphore, so that they can be undone when the calling
                  process terminates.  This is useful to prevent other pro-
                  cesses waiting on a semaphore to block forever, should the
                  process that has the semaphore locked terminate in a criti-
                  cal section.

     semtimedop() is similar to semop(), but it also allows specifying a time-
     out.  When the semaphore is not available, the thread typically sleeps
     until the semaphore is available.  semtimedop() allows specifying a maxi-
     mum amount of time in timeout argument that a thread should sleep while
     waiting for the semaphore to be available.  If the specified time limit
     has been reached, semtimedop() fails with EAGAIN (and none of the opera-
     tions in sops are performed).  If timeout is NULL, semtimedop() behaves
     exactly like semop().


RETURN VALUES

     Upon successful completion both semop() and semtimedop() return a value
     of 0. Otherwise, -1 is returned and the global variable errno is set to
     indicate the error.


ERRORS

     semop() will fail if:

     [EINVAL]           There is no semaphore associated with semid.

     [EIDRM]            The semaphore set was removed while the process was
                        waiting for one of its semaphores to reach a certain
                        value.

     [EACCES]           The calling process has no permission to access the
                        specified semaphore set.

     [E2BIG]            The value of nsops is too big.  The maximum is defined
                        as MAX_SOPS in <sys/sem.h>.

     [EFBIG]            sem_num in one of the sem_buf structures is less than
                        0, or greater than the actual number of semaphores in
                        the set specified by semid.

     [ENOSPC]           SEM_UNDO was requested, and there is not enough space
                        left in the kernel to store the undo information.

     [EAGAIN]           The requested operation can not immediately be per-
                        formed, and IPC_NOWAIT was set in sem_flg.

     [EFAULT]           sops points to an illegal address.

     [EINTR]            While blocked in this system call, the thread caught a
                        signal.

     In addition, semtimedop() will fail if:

     [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 the timeout argument isn't
                        accessible.


EXAMPLES

     The following example shows how to perform a semaphore operation with a
     timeout:


           /* Performs a semaphore operation with a 5 sec timeout*/

           struct sembuf sops[1];        /* Semaphore operation structure */
           struct timespec timeout;      /* Timeout structure */

           /* Create semaphore set with 1 semaphore */
           int semid = semget(key, 1, 0666 | IPC_CREAT);

           /* Initialize semaphore to 0 */
           if (semctl(semid, 0, SETVAL, 0) == -1) {
               warn("semctl SETVAL");
               exit(EXIT_FAILURE);
           }

           sops[0].sem_num = 0;          /* Operation on semaphore 0 */
           sops[0].sem_op = -1;          /* Decrement semaphore by 1 */
           sops[0].sem_flg = 0;          /* No flags */

           timeout.tv_sec = 5;           /* 5 seconds */
           timeout.tv_nsec = 0;          /* 0 nanoseconds */

           if (semtimedop(semid, sops, 1, &timeout) == -1) {
               warn("semtimedop");       /* Print error message */
           }


SEE ALSO

     semctl(2), semget(2)


STANDARDS

     The semop, system call conforms to X/Open System Interfaces and Headers
     Issue 5 (``XSH5'').


HISTORY

     Semaphores appeared in the first release of AT&T System V UNIX.

NetBSD 10.99                    October 3, 2024                   NetBSD 10.99