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Asked: 2026-05-10T19:20:21+00:00

I have a project that I thought was going to be relatively easy, but

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I have a project that I thought was going to be relatively easy, but is turning out to be more of a pain that I had hoped. First, most of the code I’m interacting with is legacy code that I don’t have control over, so I can’t do big paradigm changes.

Here’s a simplified explanation of what I need to do: Say I have a large number of simple programs that read from stdin and write to stdout. (These I can’t touch). Basically, input to stdin is a command like ‘Set temperature to 100’ or something like that. And the output is an event ‘Temperature has been set to 100’ or ‘Temperature has fallen below setpoint’.

What I’d like to do is write an application that can start a bunch of these simple programs, watch for events and then send commands to them as necessary. My initial plan was to something like popen, but I need a bidrectional popen to get both read and write pipes. I hacked something together that I call popen2 where I pass it the command to run and two FILE* that get filled with the read and write stream. Then all I need to do is write a simple loop that reads from each of the stdouts from each of the processes, does the logic that it needs and then writes commands back to the proper process.

Here’s some pseudocode

FILE *p1read, *p1write; FILE *p2read, *p2write; FILE *p3read, *p3write;  //start each command, attach to stdin and stdout popen2('process1',&p1read,&p1write); popen2('process2',&p2read,&p2write); popen2('process3',&p3read,&p3write);  while (1) {    //read status from each process    char status1[1024];    char status2[1024];    char status3[1024];    fread(status1,1024,p1read);    fread(status2,1024,p2read);    fread(status3,1024,p3read);     char command1[1024];    char command2[1024];    char command3[1024];    //do some logic here     //write command back to each process    fwrite(command1,p1write);    fwrite(command2,p2write);    fwrite(command3,p3write); }

The real program is more complicated where it peeks in the stream to see if anything is waiting, if not, it will skip that process, likewise if it doesn’t need to send a command to a certain process it doesn’t. But this code gives the basic idea.

Now this works great on my UNIX box and even pretty good on a Windows XP box with cygwin. However, now I need to get it to work on Win32 natively.

The hard part is that my popen2 uses fork() and execl() to start the process and assign the streams to stdin and stdout of the child processes. Is there a clean way I can do this in windows? Basically, I’d like to create a popen2 that works in windows the same way as my unix version. This way the only windows specific code would be in that function and I could get away with everything else working the same way.

Any Ideas?

Thanks!

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1 Answer

  1. On Windows, you invoke CreatePipe first (similar to pipe(2)), then CreateProcess. The trick here is that CreateProcess has a parameter where you can pass stdin, stdout, stderr of the newly-created process.

    Notice that when you use stdio, you need to do fdopen to create the file object afterwards, which expects file numbers. In the Microsoft CRT, file numbers are different from OS file handles. So to return the other end of CreatePipe to the caller, you first need _open_osfhandle to get a CRT file number, and then fdopen on that.

    If you want to see working code, check out _PyPopen in

    http://svn.python.org/view/python/trunk/Modules/posixmodule.c?view=markup

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