← board

crtl close() cannot dispatch socket vs file on ESP

What was fixed already

lib/crtl/src/netinet/in.c defined a second int close(int fd) that routed through __pxx_socket_close, on top of lib/crtl/src/unistd.c's file close. Any TU that pulled both (any program doing sockets and file I/O — e.g. test/cerrno_strings.c) sent every close through whichever module was pulled last. The duplicate was removed; unistd.c is now the single close.

On POSIX that is provably a no-op: PalBackendSocketClose calls PalBackendClose (lib/rtl/platform/posix/platform_backend.pas:847), both are SYS_close, and the errno handling is identical (__crtl_sock_fail sets errno = -rc; return -1, same as unistd's close).

What remains

On ESP-IDF the two PAL entries are genuinely different code:

lib/rtl/platform/esp/platform_backend.pas
PalBackendClose fclose(Pointer(handle)) (a FILE* cast)
PalBackendSocketClose lwip_close(handle) (an lwip fd)

One close(int fd) body cannot serve both, because crtl has no way to tell a FILE*-derived handle from an lwip fd. Before the dedup this was already broken — just broken in whichever direction the pull order happened to pick, silently. Now it is deterministically the file one, so C socket close on ESP-IDF calls fclose on an lwip fd.

Note the C socket path on ESP is not a working configuration today anyway (see the 33 refused PAL entry points in CLAUDE.md's Track S note); this ticket is about not leaving the hazard undocumented.

Options

  1. fd registry in crtl — socket() records the fd in a small table, close() consults it and picks the PAL entry. Self-contained in lib/crtl; costs a table and is wrong across dup/inherited fds.
  2. Unify in the PAL — make ESP's PalBackendClose recognise a socket handle (lwip fds and IDF FILE* values are distinguishable in practice) and dispatch there. Track A/S change, but then every language gets it right, not just C.

Option 2 is the recommendation — it puts the knowledge where the handle namespace is actually owned. Confirm the two handle spaces really are distinguishable on IDF before committing to it.

Gate

A C program that opens a file and a socket and closes both behaves correctly on POSIX (already true) and on ESP-IDF; test/cerrno_strings.c stays silent.