semop(3p) — Linux manual page

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SEMOP(3P)               POSIX Programmer's Manual              SEMOP(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       semop — XSI semaphore operations

SYNOPSIS         top

       #include <sys/sem.h>

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

DESCRIPTION         top

       The semop() function operates on XSI semaphores (see the Base
       Definitions volume of POSIX.1‐2017, Section 4.17, Semaphore).  It
       is unspecified whether this function interoperates with the
       realtime interprocess communication facilities defined in Section
       2.8, Realtime.

       The semop() function shall perform atomically a user-defined
       array of semaphore operations in array order on the set of
       semaphores associated with the semaphore identifier specified by
       the argument semid.

       The argument sops is a pointer to a user-defined array of
       semaphore operation structures. The implementation shall not
       modify elements of this array unless the application uses
       implementation-defined extensions.

       The argument nsops is the number of such structures in the array.

       Each structure, sembuf, includes the following members:

          ┌───────────────┬───────────────┬──────────────────────────┐
          │ Member Type   Member Name   Description        │
          ├───────────────┼───────────────┼──────────────────────────┤
          │unsigned short 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.

       The variable sem_op specifies one of three semaphore operations:

        1. If sem_op is a negative integer and the calling process has
           alter permission, one of the following shall occur:

            *  If semval(see <sys/sem.h>) 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 non-zero, the absolute value of sem_op
               shall be added to the semadj value of the calling process
               for the specified semaphore.

            *  If semval is less than the absolute value of sem_op and
               (sem_flg &IPC_NOWAIT) is non-zero, semop() shall return
               immediately.

            *  If semval is less than the absolute value of sem_op and
               (sem_flg &IPC_NOWAIT) is 0, semop() shall increment the
               semncnt associated with the specified semaphore and
               suspend execution of the calling thread until one of the
               following conditions occurs:

               --  The value of 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 shall be decremented, the absolute value of
                   sem_op shall be subtracted from semval and, if
                   (sem_flg &SEM_UNDO) is non-zero, the absolute value
                   of sem_op shall be added to the semadj value of the
                   calling process for the specified semaphore.

               --  The semid for which the calling thread is awaiting
                   action is removed from the system. When this occurs,
                   errno shall be set to [EIDRM] and -1 shall be
                   returned.

               --  The calling thread receives a signal that is to be
                   caught. When this occurs, the value of semncnt
                   associated with the specified semaphore shall be
                   decremented, and the calling thread shall resume
                   execution in the manner prescribed in sigaction(3p).

        2. If sem_op is a positive integer and the calling process has
           alter permission, the value of sem_op shall be added to
           semval and, if (sem_flg &SEM_UNDO) is non-zero, the value of
           sem_op shall be subtracted from the semadj value of the
           calling process for the specified semaphore.

        3. If sem_op is 0 and the calling process has read permission,
           one of the following shall occur:

            *  If semval is 0, semop() shall return immediately.

            *  If semval is non-zero and (sem_flg &IPC_NOWAIT) is non-
               zero, semop() shall return immediately.

            *  If semval is non-zero and (sem_flg &IPC_NOWAIT) is 0,
               semop() shall increment the semzcnt associated with the
               specified semaphore and suspend execution of the calling
               thread until one of the following occurs:

               --  The value of semval becomes 0, at which time the
                   value of semzcnt associated with the specified
                   semaphore shall be decremented.

               --  The semid for which the calling thread is awaiting
                   action is removed from the system. When this occurs,
                   errno shall be set to [EIDRM] and -1 shall be
                   returned.

               --  The calling thread receives a signal that is to be
                   caught. When this occurs, the value of semzcnt
                   associated with the specified semaphore shall be
                   decremented, and the calling thread shall resume
                   execution in the manner prescribed in sigaction(3p).

       Upon successful completion, the value of sempid for each
       semaphore specified in the array pointed to by sops shall be set
       to the process ID of the calling process. Also, the sem_otime
       timestamp shall be set to the current time, as described in
       Section 2.7.1, IPC General Description.

RETURN VALUE         top

       Upon successful completion, semop() shall return 0; otherwise, it
       shall return -1 and set errno to indicate the error.

ERRORS         top

       The semop() function shall fail if:

       E2BIG  The value of nsops is greater than the system-imposed
              maximum.

       EACCES Operation permission is denied to the calling process; see
              Section 2.7, XSI Interprocess Communication.

       EAGAIN The operation would result in suspension of the calling
              process but (sem_flg &IPC_NOWAIT) is non-zero.

       EFBIG  The value of sem_num is greater than or equal to the
              number of semaphores in the set associated with semid.

       EIDRM  The semaphore identifier semid is removed from the system.

       EINTR  The semop() function was interrupted by a signal.

       EINVAL The value of semid is not a valid semaphore identifier, or
              the number of individual semaphores for which the calling
              process requests a SEM_UNDO would exceed the system-
              imposed limit.

       ENOSPC The limit on the number of individual processes requesting
              a SEM_UNDO would be exceeded.

       ERANGE An operation would cause a semval to overflow the system-
              imposed limit, or an operation would cause a semadj value
              to overflow the system-imposed limit.

       The following sections are informative.

EXAMPLES         top

   Setting Values in Semaphores
       The following example sets the values of the two semaphores
       associated with the semid identifier to the values contained in
       the sb array.

           #include <sys/sem.h>
           ...
           int semid;
           struct sembuf sb[2];
           int nsops = 2;
           int result;

           /* Code to initialize semid. */
           ...

           /* Adjust value of semaphore in the semaphore array semid. */
           sb[0].sem_num = 0;
           sb[0].sem_op = -1;
           sb[0].sem_flg = SEM_UNDO | IPC_NOWAIT;
           sb[1].sem_num = 1;
           sb[1].sem_op = 1;
           sb[1].sem_flg = 0;

           result = semop(semid, sb, nsops);

   Creating a Semaphore Identifier
       The following example gets a unique semaphore key using the
       ftok() function, then gets a semaphore ID associated with that
       key using the semget() function (the first call also tests to
       make sure the semaphore exists).  If the semaphore does not
       exist, the program creates it, as shown by the second call to
       semget().  In creating the semaphore for the queuing process, the
       program attempts to create one semaphore with read/write
       permission for all. It also uses the IPC_EXCL flag, which forces
       semget() to fail if the semaphore already exists.

       After creating the semaphore, the program uses calls to semctl()
       and semop() to initialize it to the values in the sbuf array. The
       number of processes that can execute concurrently without queuing
       is initially set to 2. The final call to semget() creates a
       semaphore identifier that can be used later in the program.

       Processes that obtain semid without creating it check that
       sem_otime is non-zero, to ensure that the creating process has
       completed the semop() initialization.

       The final call to semop() acquires the semaphore and waits until
       it is free; the SEM_UNDO option releases the semaphore when the
       process exits, waiting until there are less than two processes
       running concurrently.

           #include <stdio.h>
           #include <sys/sem.h>
           #include <sys/stat.h>
           #include <errno.h>
           #include <stdlib.h>
           ...
           key_t semkey;
           int semid;
           struct sembuf sbuf;
           union semun {
               int val;
               struct semid_ds *buf;
               unsigned short *array;
           } arg;
           struct semid_ds ds;
           ...
           /* Get unique key for semaphore. */
           if ((semkey = ftok("/tmp", 'a')) == (key_t) -1) {
               perror("IPC error: ftok"); exit(1);
           }

           /* Get semaphore ID associated with this key. */
           if ((semid = semget(semkey, 0, 0)) == -1) {

               /* Semaphore does not exist - Create. */
               if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
                   S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
               {
                   /* Initialize the semaphore. */
                   arg.val = 0;
                   sbuf.sem_num = 0;
                   sbuf.sem_op = 2;  /* This is the number of runs without queuing. */
                   sbuf.sem_flg = 0;
                   if (semctl(semid, 0, SETVAL, arg) == -1
                       || semop(semid, &sbuf, 1) == -1) {
                       perror("IPC error: semop"); exit(1);
                   }
               }
               else if (errno == EEXIST) {
                   if ((semid = semget(semkey, 0, 0)) == -1) {
                       perror("IPC error 1: semget"); exit(1);
                   }
                   goto check_init;
               }
               else {
                   perror("IPC error 2: semget"); exit(1);
               }
           }
           else
           {
               /* Check that semid has completed initialization. */
               /* An application can use a retry loop at this point rather than
                  exiting. */
               check_init:
               arg.buf = &ds;
               if (semctl(semid, 0, IPC_STAT, arg) < 0) {
                   perror("IPC error 3: semctl"); exit(1);
               }
               if (ds.sem_otime == 0) {
                   perror("IPC error 4: semctl"); exit(1);
               }
           }
           ...
           sbuf.sem_num = 0;
           sbuf.sem_op = -1;
           sbuf.sem_flg = SEM_UNDO;
           if (semop(semid, &sbuf, 1) == -1) {
               perror("IPC Error: semop"); exit(1);
           }

APPLICATION USAGE         top

       The POSIX Realtime Extension defines alternative interfaces for
       interprocess communication. Application developers who need to
       use IPC should design their applications so that modules using
       the IPC routines described in Section 2.7, XSI Interprocess
       Communication can be easily modified to use the alternative
       interfaces.

RATIONALE         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       Section 2.7, XSI Interprocess Communication, Section 2.8,
       Realtime, exec(1p), exit(3p), fork(3p), semctl(3p), semget(3p),
       sem_close(3p), sem_destroy(3p), sem_getvalue(3p), sem_init(3p),
       sem_open(3p), sem_post(3p), sem_trywait(3p), sem_unlink(3p)

       The Base Definitions volume of POSIX.1‐2017, Section 4.17,
       Semaphore, sys_ipc.h(0p), sys_sem.h(0p), sys_types.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                         SEMOP(3P)

Pages that refer to this page: sys_sem.h(0p)ipcs(1p)exec(3p)_Exit(3p)fork(3p)sem_close(3p)semctl(3p)sem_destroy(3p)semget(3p)sem_getvalue(3p)sem_open(3p)sem_post(3p)sem_timedwait(3p)sem_trywait(3p)sem_unlink(3p)