cannot switch to blocking mode using fcntl in linux

Question

I have a sample program:

int main()
{
   const char* fn = "/tmp/tmpfifo";
   int i = mkfifo(fn, 0666);
   int fd = open(fn, O_RDONLY | O_NONBLOCK);
   int flags = fcntl(fd, F_GETFL);
   flags &= ~O_NONBLOCK;
   fcntl(fd, F_SETFL, flags);

   char buf[1024];
   int rd= read(fd, buf, 100);
   cout << rd << endl;
   remove(fn);
   return 0;
}

It seems that after removing the non-blocking flag from the file descriptor, the read call should block until something is written into the FIFO, but my program always runs without blocking and rd=0 result. Can you please explain this behaviour? Thanks!

Answer 1

The behavior you are seeing is expected. You've done the following:

1. Opened the read end of the FIFO using O_NONBLOCK, so that a writer need not be present on the FIFO. This guarantees that the open() will immediately succeed.
2. Disabled O_NONBLOCK before subsequent reads. You've now taken yourself back to a position that is equivalent to the standard (blocking) case where a FIFO had a reader and writer, but the writer closed the FIFO. At that point, the reader should see end-of-file, which is what you are seeing.