Can popen() make bidirectional pipes like pipe() + fork()?

Question

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 s

Answer 1

POSIX stipulates that the popen() call is not designed to provide bi-directional communication:

The mode argument to popen() is a string that specifies I/O mode:

1. If mode is r, when the child process is started, its file descriptor STDOUT_FILENO shall be the writable end of the pipe, and the file descriptor fileno(stream) in the calling process, where stream is the stream pointer returned by popen(), shall be the readable end of the pipe.
2. If mode is w, when the child process is started its file descriptor STDIN_FILENO shall be the readable end of the pipe, and the file descriptor fileno(stream) in the calling process, where stream is the stream pointer returned by popen(), shall be the writable end of the pipe.
3. If mode is any other value, the result is unspecified.

Any portable code will make no assumptions beyond that. The BSD popen() is similar to what your question describes.

Additionally, pipes are different from sockets and each pipe file descriptor is uni-directional. You would have to create two pipes, one configured for each direction.