Making recvfrom() function non-blocking

Question

I am working on a UDP server/client application.

For finding out if any of the client is down, the server sends a handshake message to the client. Then, the server waits for the response of client to send some data to assure that the client is active. For this the server blocks in call to recvfrom() unless the client replies back, but if the client is down, the server blocks infinitely in call to recvfrom().

I want to implement such a functionality on my server so that it waits in the call to recvfrom() for a specific time (say 2 secs). If no data was received from the client within 2 seconds, the client is considered to be dead and recvfrom() returns.

Is there any way to do it? I searched internet but found solutions like setting MSG_DONTWAIT flag that returns immediately when no data received, but in my case, I don't want recvfrom() to return immediately but wait for data for a specific duration of time, and when no data received for that specific duration, the recvfrom() function should return.

Answer 1

The easiest way would be to use setsockopt() to set a receive time-out for the socket in question.

SO_RCVTIMEO is used for this.

If a time-out has been set for a socket passed to recvfrom(), the function returns after this time-out if no data had been received.

For instance, to set a 10 μs read time-out (add error-checking on the value returned by setsockopt() as necessary):

#include <sys/types.h> 
#include <sys/socket.h>

...

struct timeval read_timeout;
read_timeout.tv_sec = 0;
read_timeout.tv_usec = 10;
setsockopt(socketfd, SOL_SOCKET, SO_RCVTIMEO, &read_timeout, sizeof read_timeout);

For details on Windows please see here, and on Linux see here and/or here (POSIX).