Interrupt handlers should care about errno

[franzhollerer]

When using signal handlers it is essential to care that called functions are async-signal-safe. Beside that, if the called function may update errno, the signal handler should save and restore the value of errno.

To cite "The Linux Programming Interface" by Michael Kerrisk, ISBN-13: 978-1-59327-220-3, Chapter 21, page 427:

Use of errno inside signal handlers

Because they may update errno, use of function listed in Table 21-1 can never-
theless render a signal handler nonreentrant, since they may overwrite the errno
value that was set by a function called from the main program. The workaround is
to save the value of errno on entry to a signal handler that uses any of the function
in Table 21-1 and restore the errno value on exit from the handler, as in the following-
ing example:

void
handler(int sig)
{
        int savedErrno;

        savedErrno = errno;

        /* Now we can execute a function that might modify errno */

        errno = savedErrno;
}

The referenced Table 21-1 is a list of async-signal-safe functions as specified by POSIX.

This problem affects (probably among others) the example interrupt.c:

static void s_signal_handler (int signal_value)
{
    int rc = write (s_fd, S_NOTIFY_MSG, sizeof(S_NOTIFY_MSG));
    if (rc != sizeof(S_NOTIFY_MSG)) {
        write (STDOUT_FILENO, S_ERROR_MSG, sizeof(S_ERROR_MSG)-1);
        exit(1);
    }
}

write() may update errno. Thus the signal handler should save and restore the errno value.