semget(2) [netbsd man page]

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

semget(2)							System Calls Manual							 semget(2)

NAME

       semget - Returns (and possibly creates) a semaphore ID

SYNOPSIS

       #include <sys/sem.h>

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

       Application developers may want to specify #include statements for <sys/types.h> and <sys/ipc.h> before the one for <sys/sem.h> if programs
       are being developed for multiple platforms. The additional #include statements are not required on Tru64 UNIX systems or by ISO	or  X/Open
       standards, but may be required on other vendors' systems that conform to these standards.

STANDARDS

       Interfaces documented on this reference page conform to industry standards as follows:

       semget():  XPG4, XPG4-UNIX

       Refer to the standards(5) reference page for more information about industry standards and associated tags.

PARAMETERS

       Specifies  the  key  that identifies the semaphore set.	The value for the key parameter can be IPC_PRIVATE or a random number other than 0
       (zero).	To ensure that you receive a new, unused entry in the semaphore table, specify IPC_PRIVATE as the value  of  key.   Specifies  the
       number  of  semaphores to create in the semaphore set.  Specifies the creation flags.  Possible values are as follows: If the key specified
       does not exist, the semget function creates a semaphore ID using the specified key.

	      If the specified key exists, and IPC_EXCL is not set, the semaphore id for the specified key is returned.

	      If the specified key exists and IPC_EXCL is set, the semget function fails and returns an error.	Specifies that you want  exclusive
	      access to the semaphore set.

	      If the specified key already exists, the semget function fails and returns an error notification.

DESCRIPTION

       The  system defines sets of semaphores in a system-wide table, with each set being an entry in the table.  The semget() function returns an
       ID that identifies the semaphore set's entry in the table.  You determine which semaphore set's ID is returned by specifying the key param-
       eter.

       The semget() function creates a semaphore set containing nsems semaphores and returns its ID in the following situations: The key parameter
       is IPC_PRIVATE.	The key parameter does not already exist as an entry in the semaphore table and the IPC_CREAT flag is set.

       To create a semaphore set, the semget() function creates and initializes a structure of type semid_ds, which is defined as follows:

       struct semid_ds {
	       struct ipc_perm	   sem_perm;
	       struct sem	   *sem_base;
	       ushort_t 	   sem_nsems;
	       time_t		   sem_otime;
	       time_t		   sem_ctime; };

       For a complete description of this  structure,  see  semid_ds(4).   The	semget()  function  initializes  the  structure  as  follows:  The
       sem_perm.cuid  and  sem_perm.uid  fields are set equal to the effective user ID of the calling process.	The sem_perm.cgid and sem_perm.gid
       fields are set equal to the effective group ID of the calling process.  The low-order nine bits of sem_perm.mode are set equal to the  low-
       order  nine  bits of semflg.  The sem_nsems field is set equal to the value of nsems.  The sem_otime field is set equal to zero (0) and the
       sem_ctime field is set equal to the current time.

       The individual semaphores within a set are implemented using the sem structure.	(For  more  information  about	this  structure,  see  the
       <sys/sem.h>  header file.)  The semget function does not initialize the sem structure associated with each semaphore in the set.  The indi-
       vidual semaphores are initialized when you call the semctl function with the SETVAL or SETALL command.

RETURN VALUES

       Upon successful completion, the semget() function returns a semaphore identifier.  If the semget function fails, it returns a value  of	-1
       and sets errno to indicate the error.

ERRORS

       The  semget  function sets errno to the specified values for the following conditions: A semaphore ID already exists for the key parameter,
       but operation permission as specified by the low-order nine bits of the semflg parameter was not granted.  A semaphore  ID  already  exists
       for  the  key  parameter,  but IPC_CREAT and IPC_EXCL were used for the semflg parameter.  The value of the nsems parameter is less than or
       equal to 0 (zero) or greater than the system-defined limit.  Or, a semaphore ID already exists for the key parameter,  but  the	number	of
       semaphores  in  the  set is less than the nsems parameter, and the nsems parameter is not equal to 0 (zero).  A semaphore ID does not exist
       for the key parameter and IPC_CREAT was not set.  An attempt to create a new semaphore ID exceeded the system-wide limit on the size of the
       semaphore table.

RELATED INFORMATION

       Functions: semctl(2), semop(2), table(2), ftok(3)

       Files: semid_ds(4), sysconfigtab(4)

       Standards: standards(5) delim off

																	 semget(2)