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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
  key	    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.

  nsems	    Specifies the number of semaphores to create in the semaphore
	    set.

  semflg    Specifies the creation flags.  Possible values are as follows:

	    IPC_CREAT 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.

	    IPC_EXCL  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 parameter.

  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 individual 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:

  [EACCES]  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.

  [EEXIST]  A semaphore ID already exists for the key parameter, but
	    IPC_CREAT and IPC_EXCL were used for the semflg parameter.

  [EINVAL]  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).

  [ENOENT]  A semaphore ID does not exist for the key parameter and IPC_CREAT
	    was not set.

  [ENOSPC]  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)

Index for Section 2

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