SHMGET(2)                 NetBSD System Calls Manual                 SHMGET(2)

NAME
     shmget -- get shared memory segment

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/shm.h>

     int
     shmget(key_t key, size_t size, int shmflg);

DESCRIPTION
     shmget() returns the shared memory identifier associated with the key
     key.

     A shared memory segment is created if either key is equal to IPC_PRIVATE,
     or key does not have a shared memory segment identifier associated with
     it, and the IPC_CREAT bit is set in shmflg.  If both the IPC_CREAT bit
     and the IPC_EXCL bit are set in shmflg, and key has a shared memory seg-
     ment identifier associated with it already, the operation will fail.

     If a new shared memory segment is created, the data structure associated
     with it (the shmid_ds structure, see shmctl(2)) is initialized as fol-
     lows:

     o   shm_perm.cuid and shm_perm.uid are set to the effective uid of the
         calling process.

     o   shm_perm.gid and shm_perm.cgid are set to the effective gid of the
         calling process.

     o   shm_perm.mode is set to the lower 9 bits of shmflg.

     o   shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.

     o   shm_ctime is set to the current time.

     o   shm_segsz is set to the value of size.

RETURN VALUES
     Upon successful completion a positive shared memory segment identifier is
     returned.  Otherwise, -1 is returned and the global variable errno is set
     to indicate the error.

ERRORS
     [EACCES]           A shared memory segment is already associated with key
                        and the caller has no permission to access it.

     [EEXIST]           Both IPC_CREAT and IPC_EXCL are set in shmflg, and a
                        shared memory segment is already associated with key.

     [EINVAL]           No shared memory segment is to be created, and a
                        shared memory segment exists for key, but the size of
                        the segment associated with it is less than size,
                        which is non-zero.

                        A shared memory segment is to be created, and size is
                        less than the system imposed minimum, or greater than
                        the system imposed maximum (refer to the kern.ipc val-
                        ues in sysctl(7)).

     [ENOENT]           IPC_CREAT is not set in shmflg and no shared memory
                        segment associated with key was found.

     [ENOMEM]           There is not enough memory left to create a shared
                        memory segment of the requested size.

     [ENOSPC]           A new shared memory identifier could not be created
                        because the system limit for the number of shared mem-
                        ory identifiers has been reached.

SEE ALSO
     ipcrm(1), ipcs(1), mmap(2), shmat(2), shmctl(2), ftok(3), sysctl(7)

STANDARDS
     The shmget system call conforms to X/Open System Interfaces and Headers
     Issue 5 (``XSH5'').

HISTORY
     Shared memory segments appeared in the first release of AT&T System V
     UNIX.

NetBSD 5.0_STABLE              October 27, 2008              NetBSD 5.0_STABLE
