* socket 함수 (linux - ubuntu 18.04)
#include <sys/types/h>
#include <sys/socket.h>
int socket(int domain, int type, int protocol)
* int domain
- specifies a communication domain
- selects the protocol family which will be used for communication
- these failies are defined in <sys/socket.h>
* int type
- specifies the communication semantics