take_fd

Transfer an arbitrary file descriptor from parent process to the child.

This function takes ownership of the source_fd; it will be closed in the parent when this is freed.

By default, all file descriptors from the parent will be closed. This function allows you to create (for example) a custom `pipe()` or `socketpair()` before launching the process, and choose the target descriptor in the child.

An example use case is GNUPG, which has a command line argument `--passphrase-fd` providing a file descriptor number where it expects the passphrase to be written.

Parameters:

this	a SubprocessLauncher
source_fd	File descriptor in parent process
target_fd	Target descriptor for child process