DOC socket
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1. <a href>http://carment.ase.ro/rc/sem/socket/socket-doc/thunder.ocis.temple.edu/netprog5.html#sockets_concepts </a>

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2.<a href> http://beej.us/guide/bgnet/html/</a>

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3. <a href>https://www.kernel.org/doc/man-pages/ </a>
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-system calls (2); library functions (3)