semop(2)							System Calls Manual							  semop(2)

Name
       semop - semaphore operations

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

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

Description
       The system call is used to atomically perform an array of semaphore operations on the set of semaphores associated with the semaphore iden-
       tifier specified by The sops is a pointer to the array of semaphore-operation structures.  The nsops is the number of  such  structures	in
       the array.  The contents of each structure includes 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 and sem_num.

       The sem_op specifies one of three semaphore operations as follows:

       1.  If sem_op is a negative integer, one of the following occurs:

       o   If  semval  is greater than or equal to the absolute value of sem_op, the absolute value of sem_op is subtracted from semval.  For fur-
	   ther information, see Also, 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.  For further information, see

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

       o   If  semval  is  less than the absolute value of sem_op and (sem_flg & IPC_NOWAIT ) is false, increments the semncnt associated with the
	   specified semaphore and suspend execution of the calling process until one of the following occurs:

       o   If the 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.

	   The for which the calling process is awaiting action is removed from the system.  For further information, see When this occurs,  errno
	   is set equal to EIDRM, and a value of -1 is returned.

       The calling process receives a signal that is to be caught.  When this occurs, the value of semncnt associated with the specified semaphore
       is decremented, and the calling process resumes execution in the manner prescribed in

       2.  If sem_op is a positive integer, 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.  If sem_op is zero, one of the following occurs:

       o   If semval is zero, returns immediately.

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

       o   If  semval is not equal to zero and (sem_flg & IPC_NOWAIT) is false, increments the semzcnt associated with the specified semaphore and
	   suspend execution of the calling process, until one of the following occurs:

	   The semval became zero, at which time the value of semzcnt associated with the specified semaphore is decremented.

	   The for which the calling process is awaiting action is removed from the system.  When this occurs, errno is set equal to EIDRM, and  a
	   value of -1 is returned.

       The calling process receives a signal that is to be caught.  When this occurs, the value of semzcnt associated with the specified semaphore
       is decremented, and the calling process resumes execution in the manner prescribed in

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

Return Values
       Upon successful completion, a value of 0 is returned.  Otherwise, a value of -1 is returned, and errno is set to indicate the error.

Diagnostics
       The fails if any of the following is true for any of the semaphore operations specified by sops:

       [EINVAL]       The sempid is not a valid semaphore identifier.

       [EFBIG]	      The sem_num is less than zero or greater than or equal to the number of semaphores in the set associated with

       [E2BIG]	      The nsops is greater than the system-imposed maximum.

       [EACCESS]      Operation permission is denied to the calling process.  For further information, see

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

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

       [EINVAL]       The number of individual semaphores for which the calling process request a SEM_UNDO would exceed the limit.

       [ERANGE]       An operation would cause a semval to overflow the system-imposed limit.

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

       [EFAULT]       The sops points to an illegal address.

       [EINTR]	      The returns due to the receipt of a signal.

       [EIDRM]	      The has been removed from the system.

See Also
       execve(2), exit(2), fork(2), semctl(2), semget(2), signal(3)

																	  semop(2)