What is "socket + 1" achieving when making a select call? unix/c++

Question

I am implementing a udp listen server based on this https://linux.m2osw.com/c-implementation-udp-clientserver. I noticed when establishing a timeout receiver the author included "f_socket+1" when making the select call. I am wondering what exactly this is doing? Any explanation helpful, thank you!

excerpt of function from above link:

    FD_ZERO(&s);
    FD_SET(f_socket, &s);
    struct timeval timeout;
    timeout.tv_sec = max_wait_ms / 1000;
    timeout.tv_usec = (max_wait_ms % 1000) * 1000;
    int retval = select(f_socket + 1, &s, &s, &s, &timeout);

Probably an array of sockets

Michael Chourdakis
– Michael Chourdakis

2020-04-22 16:58:12 +00:00
Commented Apr 22, 2020 at 16:58 — Michael Chourdakis
– Michael Chourdakis, Commented Apr 22, 2020 at 16:58

Brian Bi · Accepted Answer · 2020-04-22 16:59:47Z

5

See https://pubs.opengroup.org/onlinepubs/007908799/xsh/select.html

The nfds argument specifies the range of file descriptors to be tested. The select() function tests file descriptors in the range of 0 to nfds-1.

Thus, that argument should be set to 1 greater than the maximum file descriptor you want to monitor.

answered Apr 22, 2020 at 16:59

Brian Bi

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1 Comment

Remy Lebeau Over a year ago

FYI, (e)poll() is generally preferred over select(), then you don't have to worry about this, since (e)poll() is given an array of specific socket descriptors to work with, it doesn't have to deal with ranges of descriptors.

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