Semaphores with key of 0

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semget

semget(2)							System Calls Manual							 semget(2)

Name
       semget - get set of semaphores

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

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

Description
       The  system  call  returns the semaphore identifier associated with key.  A semaphore identifier and associated data structure and set con-
       taining nsems semaphores are created for key, if one of the following is true:

       o   The key is equal to IPC_PRIVATE

       o   The key does not already have a semaphore identifier associated with it, and (semflg & IPC_CREAT ) is true.

       For further information, see

       Upon creation, the data structure associated with the new semaphore identifier is initialized as follows:

       o   The sem_perm.cuid, sem_perm.uid, sem_perm.cgid and sem_perm.gid are set equal to the effective user ID and effective group ID,  respec-
	   tively, of the calling process.

       o   The low-order nine bits of sem_perm.mode are set equal to the low-order nine bits of semflg.

       o   The sem_nsems is set equal to the value of nsems.

       o   The sem_otime is set equal to zero (0) and sem_ctime is set equal to the current time.

Return Values
       Upon  successful  completion,  a nonnegative integer, namely a semaphore identifier, is returned.  Otherwise, a value of -1 is returned and
       errno is set to indicate the error.

Diagnostics
       The semget system call fails if any of the following is true:

       [EINVAL]       The nsems is either less than or equal to zero or greater than the system-imposed limit

       [EACCES]       A semaphore identifier exists for key, but operation permission, as specified by the low-order nine bits of semflg would not
		      be granted.  For further information, see

       [EINVAL]       A  semaphore  identifier	exists	for key, but the number of semaphores in the set associated with it is less than nsems and
		      nsems is not equal to zero.

       [ENOENT]       A semaphore identifier does not exist for key and (semflg & IPC_CREAT ) is false.

       [ENOSPC]       A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphore  identifiers
		      in the system would be exceeded.

       [EEXIST]       A semaphore identifier exists for key but ((semflg & IPC_CREAT ) & (semflg & IPC_EXCL )) is true.

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

																	 semget(2)

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