New to socket programming, questions regarding "select()"

Question

Currently in my degree we're starting to work with sockets.

I Have a couple of questions regarding polling for input from sockets,

using the select() function.

int select( int nfds,
    fd_set *readfds,
    fd_set *writefds,
    fd_set *exceptfds,
    const struct timespec *timeout);

1. We give select "nfds" param, which would normally would be the maximum sockets number we would like to monitor. How can i watch only one specific socket instead of the range of 0 to nfds_val sockets ?

2. What are the file descriptors objects that we use? what is their purpose, and why can't we just point "select" to the relevant socket structure?

3. I've read over the forum regarding Blocking and Non-Blocking mode of select, but couldn't understand the meaning or uses of each, nor how to implement such, would be glad if someone could explain.

4. Last but not least (only for the time being :D ) - When binding a socketaddr_in to socket number, why does one needs to cast to socketaddr * and not leave it as sockaddr_in * ? I mean except for the fact that bind method expects this kind of pointer type ;)

Would appreciate some of the experts answers here :)

Thank you guys and have a great week!

Answer 1

We give select "nfds" param, which would normally would be the maximum sockets number we would like to monitor. How can i watch only one specific socket instead of the range of 0 to nfds_val sockets ?

Edit. (sorry, the previous text here was wrong) Just provide your socket descriptor + 1. I'm pretty sure it doesn't mean OS will check all the descriptors in [0, 1... descriptor] range.

What are the file descriptors objects that we use? what is their purpose, and why can't we just point "select" to the relevant socket structure?

File descriptors are usually integer values given to the user by OS. OS uses descriptors to control physical and logical resources - one file descriptor means OS has given you something file-like to control. Since Berkeley Sockets have read and write operations defined, they are file-like and socket objects essentially are plain file descriptors.

Answering why can't we just point "select" to the relevant socket structure? - we actually can. What exactly to pass to select depends on OS and language. In C you place your socket descriptor (plain int value most probably) into a fd_set. fd_set is then passed to select.

Edit. An tiny example for Linux:

fd_set set;
FD_ZERO(&set);
FD_SET(socket_fd, &set);
// check if socket_fd is ready for reading
result = select(socket_fd + 1, &set, NULL, NULL, NULL);  
if (result == -1) report_error(errno);

Docs.
Windows has similar code.

I've read over the forum regarding Blocking and Non-Blocking mode of select, but couldn't understand the meaning or uses of each, nor how to implement such, would be glad if someone could explain.

A blocking operation makes your thread wait until it is done. It's 99% of functions you use. If there are sockets ready for some IO, blocking select will return something immediately. It there are no such sockets, the thread will wait for them. Non-blocking select, in the latter case, won't wait and will return -1 (error).

As an example, try to implement single threaded server that is capable of working with multiple clients, including long operations like file transfer happening simultaneously. You definitely don't want to use blocking socket operations in this case.

Last but not least (only for the time being :D ) - When binding a socketaddr_in to socket number, why does one needs to cast to socketaddr * and not leave it as sockaddr_in * ? I mean except for the fact that bind method expects this kind of pointer type ;)

Probably due to historical reasons, but I'm not sure. And there seems to be a fine answer on SO already.