linux fcntl - unsetting flag

Question

How do i unset a already set flag using fcntl?

For e.g. I can set the socket to nonblocking mode using

fcntl(sockfd, F_SETFL, flags | O_NONBLOCK)

Now, i want to unset the O_NONBLOCK flag.

I tried fcntl(sockfd, F_SETFL, flags | ~O_NONBLOCK). It gave me error EINVAL

Answer 1

int oldfl;
oldfl = fcntl(sockfd, F_GETFL);
if (oldfl == -1) {
    /* handle error */
}
fcntl(sockfd, F_SETFL, oldfl & ~O_NONBLOCK);

Untested, but hope this helps. :-)

Answer 2

Tried unsetting all flags:

fcntl(sockfd, F_SETFL, 0);

Also OR-ing the flags with ~O_NONBLOCK is of no use, you need to AND it, since what you want is to unset the O_NONBLOCK bit(s).