I'm implementing piping on a simulated file system in C++ (with mostly C). It needs to run commands in the host shell but perform the piping itself on the simulated file system.
I could achieve this with the pipe(), fork(), and system() system calls, but I'd prefer to use popen() (which handles creating a pipe, forking a process, and passing a command to the shell). This may not be possible because (I think) I need to be able to write from the parent process of the pipe, read on the child process end, write the output back from the child, and finally read that output from the parent. The man page for popen() on my system says a bidirectional pipe is possible, but my code needs to run on a system with an older version supporting only unidirectional pipes.
With the separate calls above, I can open/close pipes to achieve this. Is that possible with popen()?
For a trivial example, to run ls -l | grep .txt | grep cmds I need to:
ls -l on the host; read its output backls -l back to my simulator grep .txt on the host on the piped output of ls -lgrep cmds on the host on the piped output of grep .txtman popen
From Mac OS X:
The
popen()function 'opens' a process by creating a bidirectional pipe, forking, and invoking the shell. Any streams opened by previouspopen()calls in the parent process are closed in the new child process. Historically,popen()was implemented with a unidirectional pipe; hence, many implementations ofpopen()only allow the mode argument to specify reading or writing, not both. Becausepopen()is now implemented using a bidirectional pipe, the mode argument may request a bidirectional data flow. The mode argument is a pointer to a null-terminated string which must be 'r' for reading, 'w' for writing, or 'r+' for reading and writing.
You seem to have answered your own question. If your code needs to work on an older system that doesn't support popen opening bidirectional pipes, then you won't be able to use popen (at least not the one that's supplied).
The real question would be about the exact capabilities of the older systems in question. In particular, does their pipe support creating bidirectional pipes? If they have a pipe that can create a bidirectional pipe, but popen that doesn't, then I'd write the main stream of the code to use popen with a bidirectional pipe, and supply an implementation of popen that can use a bidirectional pipe that gets compiled in an used where needed.
If you need to support systems old enough that pipe only supports unidirectional pipes, then you're pretty much stuck with using pipe, fork, dup2, etc., on your own. I'd probably still wrap this up in a function that works almost like a modern version of popen, but instead of returning one file handle, fills in a small structure with two file handles, one for the child's stdin, the other for the child's stdout.