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(libc.info)Local Namespace Concepts

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Local Namespace Concepts
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   In the local namespace socket addresses are file names.  You can
specify any file name you want as the address of the socket, but you
must have write permission on the directory containing it.  It's common
to put these files in the `/tmp' directory.

   One peculiarity of the local namespace is that the name is only used
when opening the connection; once open the address is not meaningful and
may not exist.

   Another peculiarity is that you cannot connect to such a socket from
another machine-not even if the other machine shares the file system
which contains the name of the socket.  You can see the socket in a
directory listing, but connecting to it never succeeds.  Some programs
take advantage of this, such as by asking the client to send its own
process ID, and using the process IDs to distinguish between clients.
However, we recommend you not use this method in protocols you design,
as we might someday permit connections from other machines that mount
the same file systems.  Instead, send each new client an identifying
number if you want it to have one.

   After you close a socket in the local namespace, you should delete
the file name from the file system.  Use `unlink' or `remove' to do
this; see Note: Deleting Files.

   The local namespace supports just one protocol for any communication
style; it is protocol number `0'.