SHMGET(2)                            SysV                            SHMGET(2)



NAME
     shmget - get shared memory segment identifier

SYNOPSIS
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>

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

DESCRIPTION
     shmget returns the shared memory identifier associated with key.

     A shared memory identifier and associated data structure and shared
     memory segment of at least size bytes (see intro(2)) are created for key
     if one of the following are true:

     ⊕  key is equal to IPC_PRIVATE.

     ⊕  key does not already have a shared memory identifier associated with
        it, and (shmflg & IPC_CREAT) is "true".

     Upon creation, the data structure associated with the new shared memory
     identifier is initialized as follows:

     1.   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.

     2.   The low-order nine bits of shm_perm.mode are set equal to the low-
          order nine bits of shmflg.  shm_segsz is set equal to the value of
          size.

     3.   shm_lpid, shm_nattch, shm_atime, and shm_dtime are set equal to 0.

     4.   ahm_ctime is set equal to the current time.

ERRORS
     shmget will fail if one or more of the following are true:

     [EINVAL]   size is less than the system-imposed minimum or greater than
                the system-imposed maximum.

     [EACCES]   A shared memory identifier exists for key but operation
                permission (see intro(2)) as specified by the low-order nine
                bits of shmflg would not be granted.

     [EINVAL]   A shared memory identifier exists for key but the size of the
                segment associated with it is less than size and size is not
                equal to 0.

     [ENOENT]   A shared memory identifier does not exist for key and (shmflg
                & IPC_CREAT) is "false".

     [ENOSPC]   A shared memory identifier is to be created but the system-
                imposed limit on the maximum number of allowed shared memory
                identifiers system wide would be exceeded.

     [ENOMEM]   A shared memory identifier and associated shared memory

                segment are to be created but the amount of available memory
                is not sufficient to fill the request.

     [EEXIST]   A shared memory identifier exists for key but ((shmflg &
                IPC_CREAT) and (shmflg& IPC_EXCL)) is "true".

SEE ALSO
     intro(2), shmctl(2), shmop(2).

DIAGNOSTICS
     Upon successful completion, a non-negative integer, namely a shared
     memory identifier, is returned.  Otherwise, a value of -1 is returned and
     errno is set to as indicated under "Errors."

NOTES
     The user must explicitly remove shared memory segments after the last
     reference to them has been removed.