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     shmget — obtain a shared memory identifier


     Standard C Library (libc, -lc)


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

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


     Based on the values of key and flag, shmget() returns the identifier of a newly created or
     previously existing shared memory segment.  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) may be used to generate a key from a pathname.

     The mode of a newly created IPC object is determined by OR'ing the following constants into
     the flag argument:

     S_IRUSR       Read access for owner.

     S_IWUSR       Write access for owner.

     S_IRGRP       Read access for group.

     S_IWGRP       Write access for group.

     S_IROTH       Read access for other.

     S_IWOTH       Write access for other.

     When creating a new shared memory segment, size indicates the desired size of the new
     segment in bytes.  The size of the segment may be rounded up to a multiple convenient to the
     kernel (i.e., the page size).


     Upon successful completion, shmget() returns the positive integer identifier of a shared
     memory segment.  Otherwise, -1 is returned and errno set to indicate the error.


     The shmget() system call will fail if:

     [EINVAL]           Size specified is greater than the size of the previously existing
                        segment.  Size specified is less than the system imposed minimum, or
                        greater than the system imposed maximum.

     [ENOENT]           No shared memory segment was found matching key, and IPC_CREAT was not

     [ENOSPC]           The kernel was unable to allocate enough memory to satisfy the request.

     [EEXIST]           IPC_CREAT and IPC_EXCL were specified, and a shared memory segment
                        corresponding to key already exists.


     shmat(2), shmctl(2), shmdt(2), stat(2), ftok(3)