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 IEEE Std 1003.1-2001, Section 4.15, Semaphore).  It	is
       unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Realtime .

       The  semop()  function  shall  perform atomically a user-defined array of semaphore operations 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
						     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 calling process'  semadj  value
	      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 associ-
		 ated  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 calling process' semadj value for the  speci-
		 fied semaphore.

	       * The  semid for which the calling thread is awaiting action is removed from the system. When this occurs, errno shall be set equal
		 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 calling process' semadj value for the specified sema-
	   phore.

	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  equal
		 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  equal  to  the
       process ID of the calling process.

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 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 less than 0 or 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;

	      /* 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 a call to 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.

       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 <sys/types.h>
	      #include <stdio.h>
	      #include <sys/ipc.h>
	      #include <sys/sem.h>
	      #include <sys/stat.h>
	      #include <errno.h>
	      #include <unistd.h>
	      #include <stdlib.h>
	      #include <pwd.h>
	      #include <fcntl.h>
	      #include <limits.h>
	      ...
	      key_t semkey;
	      int semid, pfd, fv;
	      struct sembuf sbuf;
	      char *lgn;
	      char filename[PATH_MAX+1];
	      struct stat outstat;
	      struct passwd *pw;
	      ...
	      /* 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. */
		      sbuf.sem_num = 0;
		      sbuf.sem_op = 2;	/* This is the number of runs without queuing. */
		      sbuf.sem_flg = 0;
		      if (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);
		      }
		  }
		  else {
		      perror("IPC error 2: semget"); 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 XSI Interprocess Communication can be easily  modified
       to use the alternative interfaces.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       XSI  Interprocess  Communication,  Realtime,  exec()  ,	exit(),  fork(),  semctl(),  semget(), sem_close(), sem_destroy(), sem_getvalue(),
       sem_init(),  sem_open(),  sem_post(),  sem_unlink(),  sem_wait(),  the  Base  Definitions  volume  of  IEEE Std 1003.1-2001,   <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, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .

IEEE
/The Open Group						       2003								 SEMOP(3P)