std-unix@ut-sally.UUCP (12/04/86)
References: From: scgvaxd!stb!michael@seismo.UUCP Date: Tue Nov 25 17:39:33 1986 Ok, lets look at read() and write(). 1. There is no requirement that anything written will be available for a read(). 2. There is no requirement that read/write return everything that they can. In general, you can't require this. The terminal lines are a good example; writting to a terminal will not result in it being readable; the terminal drivers only return a line at a time no matter how much is requested. Or at least, thats what the docs say (I've never actually tested it, but it seems that if it were false, then type ahead would not work as well.) In general, it is probably safe to require that anything written to a file should be available to a subsequent read provided that the read is done on a file descriptor corresponding to the same name, or a link to the same named file that was written to, all providing that it is a regular file. Certainly not for device or special files. Incidently, don't think that 2 is obvious; my first unix programs assumed that the O/S would return a number of bytes so that the reads would be re-aligned on a 512 byte boundary, and that I had to call read() multiple times until I had gotten everything. I was quite suprised to find that other people had written stuff that did not do this, and even more suprised to find that it actually worked. No :-) Michael Gersten Volume-Number: Volume 8, Number 63