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SEMCTL(2)			     BSD System Calls Manual				SEMCTL(2)

NAME
     semctl -- semaphore control operations

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/sem.h>

     int
     semctl(int semid, int semnum, int cmd, ...);

DESCRIPTION
     The semctl() system call provides a number of control operations on the semaphore specified
     by semnum and semid.  The operation to be performed is specified in cmd (see below).  The
     fourth argument is optional and depends upon the operation requested.  If required, it is a
     union of the following fields:

	 int	 val;		 /* value for SETVAL */
	 struct  semid_ds *buf;  /* buffer for IPC_{STAT,SET} */
	 u_short *array;	 /* array for GETALL & SETALL */

     The semid_ds structure used in the IPC_SET and IPC_STAT commands is defined in <sys/sem.h>
     and contains the following members:

	 struct ipc_perm sem_perm; /* operation permissions */
	 unsigned short sem_nsems; /* number of sems in set */
	 time_t sem_otime;	   /* last operation time */
	 time_t sem_ctime;	   /* last change time */

     The ipc_perm structure used inside the semid_ds structure is defined in <sys/ipc.h> and con-
     tains the following members:

	 uid_t cuid;  /* creator user id */
	 gid_t cgid;  /* creator group id */
	 uid_t uid;   /* user id */
	 gid_t gid;   /* group id */
	 mode_t mode; /* permission (lower 9 bits) */

     semctl() provides the following operations:

     GETVAL	Return the value of the semaphore.

     SETVAL	Set the value of the semaphore to arg.val, where arg is the fourth argument to
		semctl().

     GETPID	Return the pid of the last process that did an operation on this semaphore.

     GETNCNT	Return the number of processes waiting to acquire the semaphore.

     GETZCNT	Return the number of processes waiting for the value of the semaphore to reach 0.

     GETALL	Return the values of all the semaphores associated with semid.

     SETALL	Set the values of all the semaphores that are associated with the semaphore iden-
		tifier semid to the corresponding values in arg.array, where arg is the fourth
		argument to semctl().

     IPC_STAT	Gather information about a semaphore and place the information in the structure
		pointed to by arg.buf, where arg is the fourth argument to semctl().

     IPC_SET	Set the value of the sem_perm.uid, sem_perm.gid and sem_perm.mode fields in the
		structure associated with the semaphore.  The values are taken from the corre-
		sponding fields in the structure pointed to by arg.buf, there arg is the fourth
		argument to semctl().  This operation can only be executed by the super-user, or
		a process that has an effective user id equal to either sem_perm.cuid or
		sem_perm.uid in the data structure associated with the semaphore.

     IPC_RMID	Remove the semaphores associated with semid from the system and destroy the data
		structures associated with it.	Only the super-user or a process with an effec-
		tive uid equal to the sem_perm.cuid or sem_perm.uid values in the data structure
		associated with the semaphore can do this.

     The permission to read or change a semaphore (see semop(2)) is determined by the
     sem_perm.mode field in the same way as is done with files (see chmod(2)), but the effective
     uid can match either the sem_perm.cuid field or the sem_perm.uid field, and the effective
     gid can match either sem_perm.cgid or sem_perm.gid.

RETURN VALUES
     For the GETVAL, GETPID, GETNCNT, and GETZCNT operations, semctl() returns one of the values
     described above if successful.  All other operations will make semctl() return 0 if no
     errors occur.  Otherwise -1 is returned and errno set to reflect the error.

ERRORS
     semctl() will fail if:

     [EACCES]		The caller has no operation permission for this semaphore.

     [EFAULT]		arg.buf or arg.array specifies an invalid address.

     [EINVAL]		semid is not a valid message semaphore identifier.

			cmd is not a valid command.

     [EPERM]		cmd is equal to IPC_SET or IPC_RMID and the caller is not the super-user,
			nor does the effective uid match either the sem_perm.uid or sem_perm.cuid
			fields of the data structure associated with the semaphore.

     [ERANGE]		cmd is equal to SETVAL or SETALL and the value to be set is greater than
			the system semaphore maximum value.

SEE ALSO
     semget(2), semop(2)

STANDARDS
     The semctl system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'').

HISTORY
     Semaphores appeared in the first release of AT&T System V UNIX.

BSD					 August 25, 1999				      BSD
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