将相同的套接字fd两次添加到fd_set，然后调用select

Question

I'm been reading the man page for select (from difference sources) and can't seem to get a straight explanation.

Lets say I have an already connected socket like this:

s1 = socket(...);
connect(s1, ...)...

Now lets say I add the socket twice (eg: mistakenly) to the same fd_set like so:

fd_set readfds;


FD_ZERO(&readfds);
FD_SET(s1, &readfds);
....
FD_SET(s1, &readfds);

Now I call select:

int rv = select(n, &readfds, NULL, NULL, &tv);

if (rv == -1) {
   perror("select"); // error occurred in select()
}
else if (rv == 0) {
   printf("Timeout occurred!  No data after 10.5 seconds.\n");
}
else {
   // one the descriptors have data
   .....
}

If data is send from the socket, will select have set both FDs as ready or only the first one that I've added?

Answer 1

Since FD_SET is a set (in the mathematical meaning of the word), any file descriptor is either in it or it is not . Adding the same descriptor to the set more than once has no effect.