what is the difference in using O_NONBLOCK in fcntl and SOCK_NONBLOCK in socket

[jayasekar.k]

Hi friends, i would like to know the difference in using O_NONBLOCK / SOCK_NONBLOCK option in two different system calls,

1. In socket system call
socket(AF_INET,SOCK_STREAM|SOCK_NONBLOCK,0);

2. In fcntl system call

if ((flags = fcntl(sock_descriptor, F_GETFL, 0)) < 0)
{
/* Handle error */
}


if (fcntl(socket_descriptor, F_SETFL, flags | O_NONBLOCK) < 0)
{
/* Handle error */
}

Is the above given two examples serves for same purpose ??
Can anyone explain it.

[Rubberman]

You should be able to change the mode of the socket after it is opened with fcntl(). If you create the socket with SOCK_NONBLOCK, what does the flags returned by fcntl(sock, F_GETFL, 0) return? Is the O_NOBLOCK bit set?

[jayasekar.k]

Thanks for you reply Rubberman,
if i use SOCK_NONBLOCK in socket(), and fcntl(sock, F_GETFL, 0), the O_NOBLOCK flag is set.
man socket, you will see the below comments,
SOCK_NONBLOCK Set the O_NONBLOCK file status flag on the new open file description. Using this flag saves extra calls to fcntl(2) to achieve the same result

[Rubberman]

So, it appears that option #1 is the most efficient. Of course, fcntl() will work with any file descriptor, and not just with sockets, but then you can use the O_NOBLOCK flag with open() calls as well...