Is a return value of 0 from write(2) in C an error?

Stevens probably does this to catch old implementations ofwrite() that behaved differently. For instance, the Single Unix Specsays (http://www.opengroup.org/onlinepubs/000095399/functions/write.html)

Where this volume of IEEE Std 1003.1-2001 requires -1 to be returned and errno set to [EAGAIN], most historical implementations return zero

This will ensure that the code does not spin indefinitely, even if the file descriptor is not a TCP socket or unexpected non-blocking flags are in effect. On some systems, certain legacy non-blocking modes (e.g. O_NDELAY) cause write() to return 0 (without setting errno) if no data can be written without blocking, at least for certain types of file descriptors. (The POSIX standard O_NONBLOCK uses an error return for this case.) And some of the non-blocking modes on some systems apply to the underlying object (e.g. socket, fifo) rather than the file descriptor, and so could even have been enabled by another process having an open file descriptor for the same object. The code protects itself from spinning in such a situation by simply treating it as an error, since it is not intended for use with non-blocking modes.

Also, and just to be somewhat pedantic here, if you are not writing to a socket, i would check to make sure that the buffer length ("count" in the first example) is actually being calculated correctly. In the Stevens example, you wouldn't even execute the write() call if the buffer length was 0.