semop(2)							   System Calls 							  semop(2)

NAME

       semop, semtimedop - semaphore operations

SYNOPSIS

       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>

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

       int semtimedop(int semid, struct sembuf *sops, size_t nsops, const struct timespec *timeout);

DESCRIPTION

       The  semop() function is used to perform atomically an array of semaphore operations on the set of semaphores associated with the semaphore
       identifier specified by semid. The sops argument is a pointer to the array of semaphore-operation structures. The  nsops  argument  is  the
       number of such structures in the array.

       Each sembuf structure contains the following members:

       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 permission
       required for a semaphore operation is given as {token}, where token is the type of permission needed.  The types of permission  are  inter-
       preted as follows:

       00400	READ by user
       00200	ALTER by user
       00040	READ by group
       00020	ALTER by group
       00004	READ by others
       00002	ALTER by others

       See the Semaphore Operation Permissions section of intro(2) for more information.

       A  process  maintains  a value, semadj, for each semaphore it modifies. This value contains the cumulative effect of operations the process
       has performed on an individual semaphore with the SEM_UNDO flag set (so that they can be undone if the  process	terminates  unexpectedly).
       The  value of semadj can affect the behavior of calls to semop(), semtimedop(), exit(), and _exit() (the latter two functions documented on
       exit(2)), but is otherwise unobservable.  See below for details.

       The sem_op member specifies one of three semaphore operations:

       1.  The sem_op member is a negative integer; {ALTER}

	     o	If semval (see intro(2)) 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  true, the absolute value of sem_op is added to the calling process's semadj value (see
		exit(2)) for the specified semaphore.

	     o	If semval is less than the absolute value of sem_op and (sem_flg&IPC_NOWAIT) is true, semop() returns immediately.

	     o	If semval is less than the absolute value of sem_op and (sem_flg&IPC_NOWAIT) is false, semop() increments the  semncnt	associated
		with the specified semaphore and suspends execution of the calling thread until one of the following conditions occur:

		  o  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 is decremented, the absolute value of  sem_op  is	subtracted  from  semval  and,	if
		     (sem_flg&SEM_UNDO)  is  true,  the  absolute value of sem_op is added to the calling process's semadj value for the specified
		     semaphore.

		  o
		     The semid for which the calling thread is awaiting action is removed from the system (see semctl(2)). When this occurs, errno
		     is set to EIDRM and -1 is returned.

		  o  The  calling thread receives a signal that is to be caught. When this occurs, the value of semncnt associated with the speci-
		     fied semaphore is decremented, and the calling thread resumes execution in the manner prescribed in sigaction(2).

       2.  The sem_op member is a positive integer; {ALTER}

	   The value of sem_op is added to semval and, if (sem_flg&SEM_UNDO) is true, the value of sem_op is subtracted from the calling process's
	   semadj value for the specified semaphore.

       3.  The sem_op member is 0; {READ}

	     o	If semval is 0, semop() returns immediately.

	     o	If semval is not equal to 0 and (sem_flg&IPC_NOWAIT) is true, semop() returns immediately.

	     o	If  semval is not equal to 0 and (sem_flg&IPC_NOWAIT) is false, semop() increments the semzcnt associated with the specified sema-
		phore and suspends execution of the calling thread until one of the following occurs:

		  o  The value of semval becomes 0, at which time the value of semzcnt associated with the specified semaphore is set to 0 and all
		     processes waiting on semval to become 0 are awakened.

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

		  o  The calling thread receives a signal that is to be caught. When this occurs, the value of semzcnt associated with the  speci-
		     fied semaphore is decremented, and the calling thread resumes execution in the manner prescribed in sigaction(2).

       Upon  successful  completion,  the value of sempid for each semaphore specified in the array pointed to by sops is set to the process ID of
       the calling process.

       The semtimedop() function behaves as semop() except when it must suspend execution of the calling process to complete  its  operation.	If
       semtimedop()  must  suspend  the  calling process after the time interval specified in timeout expires, or if the timeout expires while the
       process is suspended, semtimedop() returns with an error.  If the timespec structure pointed to by timeout is zero-valued and  semtimedop()
       needs  to suspend the calling process to complete the requested operation(s), it returns immediately with an error.  If timeout is the NULL
       pointer, the behavior of semtimedop() is identical to that of semop().

RETURN VALUES

       Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

       The semop() and semtimedop() functions will fail if:

       E2BIG	       The nsops argument is greater than the system-imposed maximum. See NOTES.

       EACCES	       Operation permission is denied to the calling process (see intro(2)).

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

       EFAULT	       The sops argument points to an illegal address.

       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	       A semid was removed from the system.

       EINTR	       A signal was received.

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

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

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

       The semtimedop() function will fail if:

       EAGAIN	       The timeout expired before the requested operation could be completed.

       The semtimedop() function will fail if one of the following is detected:

       EFAULT	       The timeout argument points to an illegal address.

       EINVAL	       The timeout argument specified a tv_sec or tv_nsec value less than 0, or a tv_nsec value greater than or equal to 1000 mil-
		       lion.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |semop() is Standard.	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       ipcs(1), rctladm(1M), intro(2), exec(2), exit(2), fork(2), semctl(2), semget(2), setrctl(2), sigaction(2), attributes(5), standards(5)

NOTES

       The system-imposed maximum on nsops for a semaphore identifier is the minimum enforced value of the process.max-sem-ops resource control of
       the creating process at the time semget(2) was used to allocate the identifier.

	See rctladm(1M) and setrctl(2) for information about using resource controls.

SunOS 5.10							    13 May 2003 							  semop(2)