Manual browser: semget(2)

SEMGET(2) System Calls Manual SEMGET(2)


semgetget set of semaphores


Standard C Library (libc, -lc)


#include <sys/sem.h>

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


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 follows:

  • sem_perm.cuid and sem_perm.uid are set to the effective uid of the calling process.
  • sem_perm.gid and sem_perm.cgid are set to the effective gid of the calling process.
  • sem_perm.mode is set to the lower 9 bits of semflg.
  • sem_nsems is set to the value of nsems.
  • sem_ctime is set to the current time.
  • sem_otime is set to 0.


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


The caller has no permission to access a semaphore set already associated with key.
Both IPC_CREAT and IPC_EXCL are set in semflg, and a semaphore set is already associated with key.
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.

A new set of semaphores could not be created because the system limit for the number of semaphores or the number of semaphore sets has been reached.
IPC_CREAT is not set in semflg and no semaphore set associated with key was found.


The semget system call conforms to X/Open System Interfaces and Headers Issue 5 (“XSH5”).


Semaphores appeared in the first release of AT&T System V UNIX.
May 13, 2004 NetBSD 7.0