Scroll to navigation

SEMGET(2) System Calls Manual SEMGET(2)


semgetobtain a semaphore id


Standard C Library (libc, -lc)


#include <sys/sem.h>

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


Based on the values of key and flag, () 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:

  • IPC_PRIVATE may be specified, in which case a new IPC object will be created.
  • 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.
  • 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 ORing these constants into the flag argument:

Read access for user.
Alter access for user.
Read access for group.
Alter access for group.
Read access for other.
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.


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.


The semget() system call will fail if:

Access permission failure.
IPC_CREAT and IPC_EXCL were specified, and a semaphore set corresponding to key already exists.
The number of semaphores requested exceeds the system imposed maximum per set.
A semaphore set corresponding to key already exists and contains fewer semaphores than nsems.
A semaphore set corresponding to key does not exist and nsems is 0 or negative.
Insufficiently many semaphores are available.
The kernel could not allocate a struct semid_ds.
No semaphore set was found corresponding to key, and IPC_CREAT was not specified.


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

March 4, 2018 Debian