stdio/sh behaviour guaranteed?

[Jan Grant]

Consider the following snippet.

[[[
#!/bin/sh

echo one > t
while read line
do
        echo $line
        case $line in
        one) echo two >> t
                ;;
        two) echo three >> t
                ;;
        esac
done <t
]]]

This produces three lines of output on FreeBSD: which is what I'd 
intuitively expect and it's certainly useful behaviour.

I'm just trying to determine if that behaviour is one that I can rely on 
- in other words, I guess, if stdio performs a "short read" that fails 
to fill a buffer, and the underlying file is then extended outside the 
process, will another attempt to read from the FILE* (or a test of feof, 
say) honour the new, longer file contents? And in particular, is the 
idiom above blessed by appropriate posix standards?

I'm having a hard time pinning down any appropriate standards that deal 
with this.

Cheers,
jan

PS. Would appreciate a CC: on any responses.

-- 
jan grant, ISYS, University of Bristol. http://www.bris.ac.uk/
Tel +44 (0)117 3317661   http://ioctl.org/jan/
Not as randy or clumsom as a blaster.