Bug report

There are a a few non thread-safe libc functions used in Python that can be an issue for free threading, isolated subinterpreters, or sometimes even with the GIL.

In #126316, @vstinner fixed the use setgrent / getgrent. It's probably a good time to look for other similar issues.

clang-tidy

clang-tidy has a concurrency-mt-unsafe check that looks for "known-to-be-unsafe functions".

./configure -C --with-pydebug --disable-gil

# generate compile_commands.json
bear -- make -j 

run-clang-tidy-18 -checks='-*,concurrency-mt-unsafe' -p .

Unsafe libc functions

• localeconv(): not thread-safe, see glibc's manual. Is nl_langinfo a substitue?
• setlocale
• setpwent, getpwent, and endpwent in pwdmodule.c. These are similar to grpmodule.c and can likely be addressed the same way.
• getservbyname, getservbyport, getprotobyname in Modules/socketmodule.c: use getservbyname_r, etc.? Note these thread-safety issues affect the default build because we release the GIL around the relevant calls.
• dbm_open, dbm_close, etc. in Modules/_dbmmodule.c
• getlogin: use getlogin_r if available?

Unfixable by us?

• getenv, setenv, unsetenv, putenv: environment modification isn't thread-safe and getenv is used extensively in other C libraries, so putting a lock around our accesses doesn't do much. (Might finally be fixed in glibc, see https://inbox.sourceware.org/libc-alpha/cover.1722193092.git.fweimer@redhat.com/).
• login_tty: not sure it matters as login_tty is usually called after a fork()

Safe due to our usage

• getc_unlocked, safe because we use it within a flockfile() call.
• mbrtowc() - safe as long as the passed in mbstate_t * is non-NULL, which is the case in CPython.

Safe in glibc

These functions are flagged by clang-tidy because they are not guaranteed to be safe by POSIX, but they are safe in glibc. It'd be nice to verify that they are safe in other libc implementations. I don't think it's worth changing them:

• dlerror()
• strerror(): see https://man7.org/linux/man-pages/man3/strerror.3.html
• system(): see https://man7.org/linux/man-pages/man3/system.3.html
• readdir(DIR *dirp): see https://man7.org/linux/man-pages/man3/readdir.3.html: "...in modern implementations (including the glibc implementation), concurrent calls to readdir() that specify different directory streams are thread-safe."
• wcstombs: see https://man7.org/linux/man-pages/man3/wcstombs.3.html
• nl_langinfo: https://www.man7.org/linux/man-pages/man3/nl_langinfo.3.html (as long as locale doesn't change)
• tcsendbreak, tcflow: see https://www.man7.org/linux/man-pages/man3/termios.3.html#ATTRIBUTES
• setlogmask: safe since glibc 2.33
• strsignal: not documented as thread-safe, but uses glibc uses TLS internally since 2.32

Other

• exit(): apparently concurrent calls to exit() are not thread-safe, but I don't think it matters for our usages.
• ptsname(): we already use ptsname_r(), but the static analyzer gets confused

Linked PRs

• gh-127081: fix some un-thread-safe use of libc functions #132591
• gh-127081: lock non-re-entrant *pwent calls #132748
• gh-127081: add critical sections to dbm objects #132749
• gh-127081: use re-entrant variants of get{proto,serv}by{name,port} #132750
• gh-127081: use getlogin_r if available #132751
• [3.14] gh-127081: use getlogin_r if available (gh-132751) #135097
• [3.13] gh-127081: use getlogin_r if available (gh-132751) #135098