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



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

NAME
       semop -- XSI semaphore operations

SYNOPSIS
       #include <sys/sem.h>

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

DESCRIPTION
       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().

        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().

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

ERRORS
       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
   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 aa 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
       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
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       exec, exit(), fork(), semctl(), semget(), sem__close(), sem__destroy(),
       sem__getvalue(), sem__init(), sem__open(), sem__post(), sem__trywait(),
       sem__unlink()

       The Base Definitions volume of POSIX.1-2017, Section 4.17, Semaphore,
       <sys_ipc.h>, <sys_sem.h>, <sys_types.h>

COPYRIGHT
       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(3)
