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In C/C++, how would I turn a blocking socket into a non blocking socket in both WinSocks and *nix; so that select() would work correctly. You can use the pre-processor for the platform specific code.
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Change a socket to nonblocking mode using the ioctl() call that specifies command FIONBIO and a fullword (four byte) argument with a nonzero binary value. Any succeeding socket calls against the involved socket descriptor are nonblocking calls.
To mark a socket as non-blocking, we use the fcntl system call. Here's an example: int flags = guard(fcntl(socket_fd, F_GETFL), "could not get file flags"); guard(fcntl(socket_fd, F_SETFL, flags | O_NONBLOCK), "could not set file flags"); Here's a complete example.
A socket can be in "blocking mode" or "nonblocking mode." The functions of sockets in blocking (or synchronous) mode do not return until they can complete their action. This is called blocking because the socket whose function was called cannot do anything — is blocked — until the call returns.
If no pending connections are present on the queue, and the socket is not marked as nonblocking, accept() blocks the caller until a connection is present. If the socket is marked nonblocking and no pending connections are present on the queue, accept() fails with the error EAGAIN or EWOULDBLOCK.
On linux:
fcntl(fd, F_SETFL, O_NONBLOCK);
Windows:
u_long on = 1;
ioctlsocket(fd, FIONBIO, &on);
select() is supposed to work on blocking sockets. It returns when a read() would return immediately, which is always the case with non-blocking sockets.
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