Gets a shared memory segment.

    Format

    #include <shm.h>

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

1  –  Arguments

 key
    The key for which the associated shared memory identifier is
    returned.

 size
    Shared memory segment size in bytes.

 shmflg
    Flag used to initialize the low order 9 bits of the shm_perm.mode
    member of the shmid_ds data structure associated with the new
    shared memory segment. For more information, see the Description.

2  –  Description

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

    A shared memory identifier with its associated shmid_ds data
    structure is created for a key if one of the following is TRUE:

    o  The key argument is equal to IPC_PRIVATE.
    o  The key argument does not already have a shared memory
       identifier associated with it and (shmflg and IPC_CREAT) is
       non-zero.

    If shmflg specifies both IPC_CREAT and IPC_EXCL and a shared memory
    segment already exists for a key, shmget() function fails with
    errno set to EEXIST.

    When it is created, the shmid_ds data structure associated with the
    new shared memory identifier is initialized as follows:

    o  The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and
       shm_perm.gid are set equal to the effective user ID and effective
       group ID, respectively, of the calling process.
    o  The low order 9 bits of shm_perm.mode are set equal to the low
       order 9 bits of the shmflg argument.
    o  The variable shm_segsz is set equal to the value of the size
       argument.
    o  The variables shm_lpid, shm_nattch, shm_atime, and shm_dtime are
       set equal to zero, the variable shm_cpid is set equal to the
       Process ID of the segment creator, and the variable shm_ctime is
       set equal to the current time.

3  –  Return Values

    n                 Successful completion. The function returns a
                      non-negative integer shared memory identifier.

    -1                Indicates an error. The function sets errno to one
                      of the following values:

                      o  EACCESS - A shared memory identifier exists
                         for the key, but the operation permission as
                         specified by the low order 9 bits of shmflg
                         are not granted.

                      o  EEXIST - A shared memory identifier exists for
                         a key but ((shmflg & IPC_CREAT) &&
                         (shmflg & IPC_EXCL)) is non-zero.

                      o  EINVAL - The value of size is either less than
                         SHMMIN, greater than the SHMMAX, or a
                         shared memory identifier exists for a key, but
                         the size is greater than the size of that
                         segment.
                         With current implementation SHMMIN is defined
                         to 1 byte and SHMMAX is defined to 512 MB.

                      O  ENOENT - A shared memory identifier does not
                         exist for a key and (shmflg & IPC_CREAT)
                         is equal to zero.

                      O  ENOSPC - A shared memory identifier is to be
                         created but the system-imposed limit on the
                         maximum number of allowed shared memory
                         segments system-wide is exceeded.

                      o  EVMSERR - OpenVMS specific non-translatable
                         error code.