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(ipc.info)perms

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Permissions
===========

   Each resource has an associated `ipc_perm' struct which defines the
creator, owner and access perms for the resource.

     struct ipc_perm
             key_t key;    /* set by creator */
             ushort uid;   /* owner euid and egid */
             ushort gid;
             ushort cuid;  /* creator euid and egid */
             ushort cgid;
             ushort mode;  /* access modes in lower 9 bits */
             ushort seq;   /* sequence number */

   The creating process is the default owner. The owner can be
reassigned by the creator and has creator perms. Only the owner,
creator or super-user can delete the resource.

   The lowest nine bits of the flags parameter supplied by the user to
the system call are compared with the values stored in `ipc_perms.mode'
to determine if the requested access is allowed. In the case that the
system call creates the resource, these bits are initialized from the
user supplied value.

   As for files, access permissions are specified as read, write and
exec for user, group or other (though the exec perms are unused). For
example 0624 grants read-write to owner, write-only to group and
read-only access to others.

   For shared memory, note that read-write access for segments is
determined by a separate flag which is not stored in the `mode' field.
Shared memory segments attached with write access can be read.

   The `cuid', `cgid', `key' and `seq' fields cannot be changed by the
user.