Is there a cleaner way to use the write() function reliably?

Question

I read the man pages, and my understanding is that if write() fails and sets the errno to EAGAIN or EINTR, I may

Answer 1

You have a bit of a "don't repeat yourself" problem there - there's no need for two separate calls to write, nor for two nested loops.

My normal loop would look something like this:

for (int n = 0; n < count; ) {
    int ret = write(fd, (char *)buf + n, count - n);
    if (ret < 0) {
         if (errno == EINTR || errno == EAGAIN) continue; // try again
         perror("write");
         break;
    } else {
        n += ret;
    }
}

// if (n < count) here some error occurred