Question

At some point during my C programming adventures on Linux, I encountered flags (possibly ioctl/fcntl?), that make reads and writes on a file descriptor uninterruptible.

Unfortunately I cannot recall how to do this, or where I read it. Can anyone shed some light?

Update0

To refine my query, I’m after the same blocking and guarantees that fwrite() and fread() provide, sans userspace buffering.

Answer 1

You can avoid EINTR from read() and write() by ensuring all your signal handlers are installed with the SA_RESTART flag of sigaction().

However this does not protect you from short reads / writes. This is only possible by putting the read() / write() into a loop (it does not require an additional buffer beyond the one that must already be supplied to the read() / write() call.)

Such a loop would look like:

/* If return value is less than `count', then errno == 0 indicates end of file,
 * otherwise errno indicates the error that occurred. */
ssize_t hard_read(int fd, void *buf, size_t count)
{
    ssize_t rv;
    ssize_t total_read = 0;

    while (total_read < count)
    {
        rv = read(fd, (char *)buf + total_read, count - total_read);

        if (rv == 0)
            errno = 0;

        if (rv < 1)
            if (errno == EINTR)
                continue;
            else
                break;

        total_read += rv;
    }

    return rv;
}