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. 🙂