NAME

       semget — obtain a semaphore id

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #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 ORing these constants into the flag argument:

       0400
         Read access for user.

       0200
         Alter access for user.

       0040
         Read access for group.

       0020
         Alter access for group.

       0004
         Read access for other.

       0002
         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)

Debian                                            March 4, 2018                                        SEMGET(2)