Checking if errno != EINTR: what does it mean?

Question

I've found this piece of code used several times (also a similar one where it's used open() instead of write()).

int c = write(fd, &v, sizeof(v));
if (c == -1 && errno != EINTR) {
    perror("Write to output file");
    exit(EXIT_FAILURE);
}

Why it is checked if && errno != EINTR here ?

Looking for errno on man I found the following text about EINTR, but even if I visited man 7 signal that doesn't enlighten me.

EINTR Interrupted function call (POSIX.1); see signal(7).

Check the man page for write instead. It should tell you what errno is set to on certain errors. — Kevin
Note also that write() returns ssize_t, and NOT int. They are not the same. — Andrew Henle
Thank you both! @AndrewHenle I know it, I just decided to copy the piece of code as I found it :) do you think I should edit the question all the same with ssize_t instead of int? — Robb1

Barmar Barmar · Accepted Answer · 2017-01-04T23:02:23

Many system calls will report the EINTR error code if a signal occurred while the system call was in progress. No error actually occurred, it's just reported that way because the system isn't able to resume the system call automatically. This coding pattern simply retries the system call when this happens, to ignore the interrupt.

For instance, this might happen if the program makes use of alarm() to run some code asynchronously when a timer runs out. If the timeout occurs while the program is calling write(), we just want to retry the system call (aka read/write, etc).

Checking if errno != EINTR: what does it mean?

3 Answers