semget(2) [freebsd man page]

SEMGET(2)						      BSD System Calls Manual							 SEMGET(2)

NAME

     semget -- obtain a semaphore id

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

     int
     semget(key_t key, int nsems, int flag);

DESCRIPTION

     Based on the values of key and flag, semget() returns the identifier of a newly created or previously existing set of semaphores.	The key is
     analogous to a filename: it provides a handle that names an IPC object.  There are three ways to specify a key:

     o	 IPC_PRIVATE may be specified, in which case a new IPC object will be created.

     o	 An integer constant may be specified.	If no IPC object corresponding to key is specified and the IPC_CREAT bit is set in flag, a new one
	 will be created.

     o	 The ftok(3) function may be used to generate a key from a pathname.

     The mode of a newly created IPC object is determined by OR'ing the following constants into the flag argument:

     SEM_R	   Read access for user.

     SEM_A	   Alter access for user.

     (SEM_R>>3)    Read access for group.

     (SEM_A>>3)    Alter access for group.

     (SEM_R>>6)    Read access for other.

     (SEM_A>>6)    Alter access for other.

     If a new set of semaphores is being created, nsems is used to indicate the number of semaphores the set should contain.  Otherwise, nsems may
     be specified as 0.

RETURN VALUES

     The semget() system call returns the id of a semaphore set if successful; otherwise, -1 is returned and errno is set to indicate the error.

ERRORS

     The semget() system call will fail if:

     [EACCES]		Access permission failure.

     [EEXIST]		IPC_CREAT and IPC_EXCL were specified, and a semaphore set corresponding to key already exists.

     [EINVAL]		The number of semaphores requested exceeds the system imposed maximum per set.

     [ENOSPC]		Insufficiently many semaphores are available.

     [ENOSPC]		The kernel could not allocate a struct semid_ds.

     [ENOENT]		No semaphore set was found corresponding to key, and IPC_CREAT was not specified.

SEE ALSO

     semctl(2), semop(2), ftok(3)

BSD
								September 12, 1995							       BSD

SEMGET(2)						      BSD System Calls Manual							 SEMGET(2)

NAME

     semget -- get set of semaphores

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/sem.h>

     int
     semget(key_t key, int nsems, int semflg);

DESCRIPTION

     The semget() system call returns the semaphore identifier associated with key.

     A new set containing nsems semaphores is created if either key is equal to IPC_PRIVATE, or key does not have a semaphore set associated with
     it and the IPC_CREAT bit is set in semflg.  If both the IPC_CREAT bit and the IPC_EXCL bit are set in semflg, and key has a semaphore set
     associated with it already, the operation will fail.

     If a new set of semaphores is created, the data structure associated with it (the semid_ds structure, see semctl(2)) is initialized as fol-
     lows:

     o	 sem_perm.cuid and sem_perm.uid are set to the effective uid of the calling process.

     o	 sem_perm.gid and sem_perm.cgid are set to the effective gid of the calling process.

     o	 sem_perm.mode is set to the lower 9 bits of semflg.

     o	 sem_nsems is set to the value of nsems.

     o	 sem_ctime is set to the current time.

     o	 sem_otime is set to 0.

RETURN VALUES

     semget() returns a non-negative semaphore identifier if successful.  Otherwise, -1 is returned and errno is set to reflect the error.

ERRORS

     [EACCES]		The caller has no permission to access a semaphore set already associated with key.

     [EEXIST]		Both IPC_CREAT and IPC_EXCL are set in semflg, and a semaphore set is already associated with key.

     [EINVAL]		nsems is less than 0 or greater than the system limit for the number in a semaphore set.

			A semaphore set associated with key exists, but has fewer semaphores than the number specified in nsems.

     [ENOSPC]		A new set of semaphores could not be created because the system limit for the number of semaphores or the number of sema-
			phore sets has been reached.

     [ENOENT]		IPC_CREAT is not set in semflg and no semaphore set associated with key was found.

SEE ALSO

     ipcs(1), semctl(2), semop(2), ftok(3)

STANDARDS

     The semget 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
								   May 13, 2004 							       BSD