NAME

     semget — obtain a semaphore id

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

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

DESCRIPTION

     Based on the values of key and flag, semget() 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 OR'ing the following constants into
     the flag argument:

     SEM_R         Read access for user.

     SEM_A         Alter access for user.

     (SEM_R>>3)    Read access for group.

     (SEM_A>>3)    Alter access for group.

     (SEM_R>>6)    Read access for other.

     (SEM_A>>6)    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.

RETURN VALUES

     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.

ERRORS

     The semget() system call will fail if:

     [EACCES]           Access permission failure.

     [EEXIST]           IPC_CREAT and IPC_EXCL were specified, and a semaphore set corresponding
                        to key already exists.

     [EINVAL]           The number of semaphores requested exceeds the system imposed maximum per
                        set.

     [EINVAL]           A semaphore set corresponding to key already exists and contains fewer
                        semaphores than nsems.

     [EINVAL]           A semaphore set corresponding to key does not exist and nsems is 0 or
                        negative.

     [ENOSPC]           Insufficiently many semaphores are available.

     [ENOSPC]           The kernel could not allocate a struct semid_ds.

     [ENOENT]           No semaphore set was found corresponding to key, and IPC_CREAT was not
                        specified.

SEE ALSO

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