Force blocking read after EAGAIN?

Question

I have a file descriptor that is open for reading which may be non-blocking. What I need to do is simply read all data until reaching EOF and write that data to a writable file descriptor. As the process that performs this copying is not "aware" of anything that is going on around it, I don't think that I can do something useful while waiting for data, and I don't want to use a while loop (while errno is not EAGAIN) because I think that it would be wasteful. Is there a way to block or otherwise suspend execution of the copying process until data becomes available?

Answer 1

Chapter 7 of the Linux SCSI Generic (sg) HOWTO gives an example of how to do this:

int flags = fcntl(fd, F_GETFL);
fcntl(fd, F_SETFL, flags & (~O_NONBLOCK));

Answer 2

Your other answer simply unsets O_NONBLOCK, which sets the file back to blocking. That's perfectly fine, if that works in your situation; but if not, you can use select() to block until your non-blocking file descriptor is readable.