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

PROLOG

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

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.1-2008, Section 4.16, Semaphore).  It is unspeci-
       fied 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 associ-
       ated 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 Descrip-
       tion.

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 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

       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.1-2008, Section 4.16, 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, 2013 Edition, Standard for Information Technol-
       ogy  --	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 Stan-
       dard 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)