Read system Call

Question

Hi,

I am trying my hands with unix socket programming in C. But while reading I am getting Err No as 4. I am unable to find out the description of this error code. Does anybody have any idea?

Answer 1

If you would start with looking at ultimate source of Unix error code names (/usr/include/errno.h) you'll arrive at the file which contains your error code as

#define EINTR            4      /* Interrupted system call */

(Which is this file is left for you to find out, as an exercise ;))

Answer 2

The errno values can be different for different systems (even different Unix-like systems), so the symbolic constants should be used in code.

The perror function will print out (to stderr ) a descriptive string of the last errno value along with a string you provide.

man 3 perror

The strerror function simply returns a const char * to the string that perror prints.

If 4 is EINTR on your system then you received a signal during your call to read. There are ways to keep this from interrupting your system calls, but often you just need to:

while (1) {
   ssize_t x = read(file, buf, len);
   if (x < 0) {
       if (errno == EINTR) {
           errno = 0;
           continue;
       } else {
          // it's a real error

Answer 3

If you're getting EINTR, it probably means you've installed a signal handler improperly. Good unices will default to restartable system calls when you simply call signal, but for safety, you should either use the bsd_signal function if it's available, or call sigaction with the restartable flag to avoid the headaches of EINTR.