getting a POSIX file descriptor from a GNU libstdc++ stream

Assuming (on GNU linux, with GNU libstdc++ and C++17 -not C++23- compiled by [GCC](https://gcc.gnu.org/) version 15 or later) I have a std::ifstream* f; pointer to a valid input stream (presumably opened). How can I get the POSIX input file descriptor? (e.g. to use [poll(2)](https://man7.org/linux/man-pages/man2/poll.2.html) or [fstat(2)](https://man7.org/linux/man-pages/man2/stat.2.html) ...) I tried f->get_fd() and it does not compile. Same question for std::ofstream and the POSIX output file descriptor. Thanks.