I remember once, long ago--probably in the early 1980s--writing a program that expected fstat on a pipe to return the amount of data buffered in the pipe. It worked on the system on which I wrote the code. Then I tried it on another, related but different UNIX, and it didn't work. So if POSIX/SUS don't prescribe a standard, I don't think one should pretend there is one, and (as I learned back then) it's unwise to depend on the result, except I think it's fair not to expect fstat to fail on any valid file descriptor. I'm pretty sure that in 7/e and earlier, fstat on a pipe reported a regular file with zero links. There was a reason for this: the kernel in fact allocated an i-node from a designated pipe device (pipedev) file system, usually the root. So the excuse that `there's no i-node' was just wrong. In last-generation Research systems, when pipes were streams (and en passant became full duplex, which caused no trouble at all but simplified life elsewhere--I think I was the one who realized that meant we didn't need pseudo-ttys any more), the system allocated a pair of in-core i-nodes when a pipe was created. As long as such an i-node cannot be accidentally confused with one belonging to any disk file system, this causes no trouble at all, and since it is possible to have more than one disk file system this is trivially possible just by reserving a device number. (In fact by then our in-core i-nodes were marked with a file system type as well, and pipes just became their own file system.) stat always returned size 0 for (Research) stream pipes, partly because nobody cared enough, partly because the implementation of streams didn't keep an exact count of all the buffered data all along the stream, just a rough one sufficient for flow control. Besides, with a full-duplex pipe, which direction's data should be counted? Returning to the original question, I'd suggest that: -- fstat(fd) where fd is a pipe should succeed -- the file should be reported to have zero links, since that is the case for a pipe (unless a named pipe, but if you support those you probably have something else to stat anyway) -- the file type should be IFIFO if that type exists in xv6 (which it wouldn't were it a real emulation of 6/e, but I gather that's not the goal), IFREG otherwise -- permissions probably don't matter much, but for sanity's sake should be some reasonable constant. Norman Wilson Toronto ON