Re: [9fans] trying to understand how fork/pipe a filtering program

[rog@vitanuova.com]

> Maybe it's been too long a week, but I could have sworn that a Plan 9
> pipe was a pair of unidirectional channels (queues in Streams
> terminology), so closing one end would cause a zero-byte read at the
> other, independently in each direction.  pipe(3) seems to support
> this.

the problem is that neither end was actually being closed,
as both ends of the pipe were dup'd onto stdin/stdout of the
child process. (the underlying close only happens when all references
have gone).

there is an alternative, i realised last night - you can do a zero-length
write after doing the writeimage, which will cause the child to read EOF,
which is usually fine (unless the child re-reads EOF, expecting it to remain EOF,
which can be the case with some buffered IO libraries and one-char lookahead;
depends whether it implements ungetc(EOF) - bio has this issue, for example)