[Socket] How to use he function fcntl()

When server is running to accept() and no client connection request comes, what it happens? When the server stops waiting for the arrival of the connection request on accept(). Similarly, when the program is running to receive(), ifno data can be read, the program will also stop receiving statements. This situation is called blocking. 
If you want to only pay attention to check whether there are customers waiting for a connection server to accept connection, otherwise the program continues to do other things, through the Socket set for non-blocking way: non-blocking socket when no customers waiting for accept calls return immediately.

# include <unistd.h>
# include <fcntl.h>
sockfd = socket (AF_INET, SOCK_STREAM, 0);
fcntl (sockfd, F_SETFL, O_NONBLOCK);

Set socket to non-blocking mode, "polling" the Socket. When no data is waiting to be processed from a non-blocking Socket ( for read data ), the function will return immediately, and the return value is set to -1 and errno set of EWOULDBLOCK
But this "polling" causes the CPU is busy waiting mode, thereby reducing performance.

There is another way to deal with non-blocking socket.
ioctl(sockfd, FIONBIO, 0); // 0 is about non-blocking mode


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