Popen and EVFILT_WRITE question

Dag-Erling Smørgrav des at des.no
Sun Mar 30 20:49:35 PDT 2008

Mel <fbsd.hackers at rachie.is-a-geek.net> writes:

> Hi,
> from reading the manpage on EVFILT_WRITE I thought it would be an easy to use 
> interface to detect when a program wants input.
> So far, that doesn't seem to be the case. Ultimately what I want to do is pipe 
> all the popen(3)'d output to a logfile and act on any input it wants.
> Could anyone explain to me why I'm never getting the EVFILT_WRITE event in 
> below testcode?
> (It doesn't matter if I open the pipe with w+ or r+).
> test.c:
> #include <sys/types.h>
> #include <sys/event.h>
> #include <sys/time.h>
> #include <sysexits.h>
> #include <err.h>
> #include <stdio.h>
> int main(int argc, char **argv)
> {
> 	FILE *proc;
> 	int kq;
> 	struct kevent changes[2], events[2];
> 	proc = popen("./test.sh", "w+");
> 	if( -1 == (kq = kqueue()) )
> 		err(EX_OSERR, "Cannot get a kqueue");
> 	EV_SET(&changes[0], fileno(proc), EVFILT_WRITE, EV_ADD|EV_ENABLE, 0,
> 			0, 0);
> 	EV_SET(&changes[1], fileno(proc), EVFILT_READ, EV_ADD|EV_ENABLE, 0,
> 			0, 0);

This is never going to work.

First, the second kevent overrides the first, because they both have the
same ident.

Second, you're going to run into buffering issues.

Third, an EVFILT_WRITE event will trigger as long as there is space in
the pipe buffer.  There is no such thing as "when a program wants input"
in Unix; it will either read input or it won't, and what happens when it
reads depends entirely on what the fd it reads from is connected to,
whether it's a slow or fast device, blocking or non-blocking, etc.

Dag-Erling Smørgrav - des at des.no

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