how to set file descriptor non-blocking?

Question

I have two ways to set file descriptor non-blocking.

fcntl(conn_fd, F_SETFD, (fcntl(conn_fd, F_GETFD, 0)|O_NONBLOCK));

or

 fcntl(conn_fd, F_SETFD, (fcntl(conn_fd, F_GETFD)|O_NONBLOCK));

and

void setnonblocking(int sock) {
    int opt;

    opt = fcntl(sock, F_GETFL);
    if (opt < 0) {
        printf("fcntl(F_GETFL) fail.");
    }
    opt |= O_NONBLOCK;
    if (fcntl(sock, F_SETFL, opt) < 0) {
        printf("fcntl(F_SETFL) fail.");
    }
}

why the function setnonblocking can set file descriptor non-blocking .but the other can't.I use this when epoll get a new connection.

Answer 1

O_NONBLOCK is a file status flag, not a file descriptor flag.

Maybe, in your fcntl(conn_fd, F_SETFD, (fcntl(conn_fd, F_GETFD)|O_NONBLOCK)); , you need to change F_GETFD to F_GETFL and F_SETFD to F_SETFL , as you need to modify the file status flags, not the file descriptor flags.