How do I do a non-blocking IPC read on Windows?
ipc, perl, windows
Solution
`select` only works on sockets in Windows. It looks like IPC::OpenX uses normal filehandles, so you won't be able to use `select` with the handles it creates.
If you don't need the timeout/detection of activity that select provides, you can set the handles to be non-blocking and just read or write as per normal.
If you need more nuanced control, IPC::Run may work well for you.
You could also look at creating a `socketpair` and use those handles with your child processes. Newer perls (5.8 and up) support `socketpair` emulation on Windows using TCP sockets.
If you try to clone `STDOUT` and `STDERR` for a program that runs without a console (ie it is started with wperl, instead of perl), you won't be able to get data through STDIO.
In practice this has been a huge pain for me on several projects. What I found worked best was to write the child process to connect to the parent server via TCP. If you don't control the child processes, look at `IPC::Run` or `socketpair`.
Problem
I have a Perl script that uses an external tool (cleartool) to gather information about a list of files. I want to use IPC to avoid spawning a new process for each file: ``` use IPC::Open2; my ($cin, $cout); my $child = open2($cout, $cin, 'cleartool'); ``` Commands that return single-lines work well. e.g. ``` print $cin "describe -short $file\n"; my $description = <$cout>; ``` Commands that return multiple lines have me at a dead end for how to consume the entire response without getting hung up by a blocking read: ``` print $cin "lshistory $file\n"; # read and process $cout... ``` I've tried to set the filehandle for non-blocking reads via `fcntl`: ``` use Fcntl; my $flags = ''; fcntl($cout, F_GETFL, $flags); $flags |= O_NONBLOCK; fcntl($cout, F_SETFL, $flags); ``` but Fcntl dies with the message "Your vendor has not defined Fcntl macro F_GETFL." I've tried using IO::Handle to set `$cout->blocking(0)` but that fails (it returns `undef` and sets `$!` to "Unknown error"). I've tried to use `select` to determine if there's data available before attempting to read: ``` my $rfd = ''; vec($rfd, fileno($cout), 1) = 1; while (select($rfd, undef, undef, 0) >= 0) { my $n = read($cout, $buffer, 1024); print "Read $n bytes\n"; # do something with $buffer... } ``` but that hangs without ever reading anything. Does anyone know how to make this work (on Windows)?