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SHMGET(2)                                  BSD System Calls Manual                                 SHMGET(2)

NAME
     shmget -- get shared memory area identifier

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

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

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

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

     •   shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0

     •   shm_ctime is set to the current time.

     •   shm_segsz is set to the value of size.

     •   The ftok(3) function may be used to generate a key from a pathname.

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
     The shmget() system call will fail if:

     [EACCES]           A shared memory segment is already associated with key and the caller has no permis-sion permission
                        sion 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. nonzero.
                        zero.

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

     [ENOMEM]           There is not enough memory left to created 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 memory identifiers has been reached.

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

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

     All of these include files are necessary.  The type of size has changed.

SEE ALSO
     ftok(3), shmat(2), shmctl(2), shmdt(2), compat(5)

BSD                                            August 17, 1995                                           BSD