Nonblocking Sockets

Some applications require sockets that do not block. For example, requests that cannot complete immediately and would cause the process to be suspended (awaiting completion) are not executed. An error code would be returned. After a socket is created and any connection to another socket is made, it can be made nonblocking by issuing a fcntl() call, as shown in Example 2-14.

Example 2-14 Set Nonblocking Socket

#include <fcntl.h>
#include <sys/file.h>
...
int fileflags;
int s;
...
s = socket(AF_INET, SOCK_STREAM, 0);
...
if (fileflags = fcntl(s, F_GETFL, 0) == -1)
		perror("fcntl F_GETFL");
		exit(1);
}
if (fcntl(s, F_SETFL, fileflags | FNDELAY) == -1)
		perror("fcntl F_SETFL, FNDELAY");
		exit(1);
}
...

When doing I/O on a nonblocking socket, check for the error EWOULDBLOCK (in errno.h), which occurs when an operation would normally block. accept(), connect(), send(), recv(), read(), and write() can all return EWOULDBLOCK. If an operation such as a send() cannot be done in its entirety, but partial writes work (such as when using a stream socket), the data that can be sent immediately are processed, and the return value is the amount actually sent.