9.10. errno

Holds the error code for the last system call

#include <errno.h>

int errno;
This is the variable that holds error information for a lot of system calls. If you'll recall, things like socket() and listen() return -1 on error, and they set the exact value of errno to let you know specifically which error occurred.

The header file errno.h lists a bunch of constant symbolic names for errors, such as EADDRINUSE, EPIPE, ECONNREFUSED, etc. Your local man pages will tell you what codes can be returned as an error, and you can use these at run time to handle different errors in different ways.

Or, more commonly, you can call perror() or strerror() to get a human-readable version of the error.

One thing to note, for you multithreading enthusiasts, is that on most systems errno is defined in a threadsafe manner. (That is, it's not actually a global variable, but it behaves just like a global variable would in a single-threaded environment.)

Return Value
The value of the variable is the latest error to have transpired, which might be the code for "success" if the last action succeeded.

s = socket(PF_INET, SOCK_STREAM, 0);
if (s == -1) {
    perror("socket"); // or use strerror()

if (select(n, &readfds, NULL, NULL) == -1) {
    // an error has occurred!!

    // if we were only interrupted, just restart the select() call:
    if (errno == EINTR) goto tryagain;  // AAAA!  goto!!!

    // otherwise it's a more serious error:
See Also
perror(), strerror()