PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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.1‐2008, Section 4.16, 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.1‐2008, Section 4.16, 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, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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.unix.org/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                 2013                           SEMOP(3P)