There is a regression between Python 3.7 and 3.8 when using PySys_SetArgvEx(0, NULL, 0). Consider this example: #include <Python.h> int main() { Py_Initialize(); PySys_SetArgvEx(0, NULL, 0); /* HERE */ PyRun_SimpleString("from time import time,ctime\n" "print('Today is', ctime(time()))\n"); Py_FinalizeEx(); return 0; } This works in 3.7 but no longer does in 3.8: $ gcc $(python3.7-config --cflags --ldflags) example.c $ ./a.out Today is Fri Aug 23 07:59:52 2019 $ gcc $(python3.8-config --cflags --ldflags --embed) example.c $ ./a.out Fatal Python error: no mem for sys.argv SystemError: /builddir/build/BUILD/Python-3.8.0b3/Objects/unicodeobject.c:2089: bad argument to internal function Current thread 0x00007f12c78b9740 (most recent call first): Aborted (core dumped) The documentation https://docs.python.org/3/c-api/init.html#c.PySys_SetArgvEx explicitly mentions passing 0 to PySys_SetArgvEx: "if argc is 0..." So I guess this is not something you shouldn't do.

Oops, it's my fault! PR 15415 fix the crash. The regression was introduced by: commit 74f6568bbd3e70806ea3219e8bacb386ad802ccf Author: Victor Stinner <vstinner@redhat.com> Date: Fri Mar 15 15:08:05 2019 +0100 bpo-36301: Add _PyWstrList structure (GH-12343) Simplified change: - static wchar_t *empty_argv[1] = {L""}; + wchar_t* empty_argv[1] = {L""}; It's a deliberate change to not waste memory: PySys_SetArgvEx() (make_sys_argv()) is only called once, whereas static keeps the memory for the whole lifecycle of the process. But I didn't notice a subtle issue with memory allocated on the stack: the code works well with gcc -O0! The bug only triggers with gcc -O3.

 New changeset c48682509dc49b43fe914fe6c502bc390345d1c2 by Victor Stinner in branch 'master': bpo-37926: Fix PySys_SetArgvEx(0, NULL, 0) crash (GH-15415) https://github.com/python/cpython/commit/c48682509dc49b43fe914fe6c502bc390345d1c2 

 New changeset ca9ae94a2aba35d94ac1ec081f9bcac3a13aebd3 by Victor Stinner in branch '3.8': bpo-37926: Fix PySys_SetArgvEx(0, NULL, 0) crash (GH-15415) (GH-15420) https://github.com/python/cpython/commit/ca9ae94a2aba35d94ac1ec081f9bcac3a13aebd3 

I tested my fix manually using example from msg350262 and gcc -O3 (./configure && make): I can reproduce the crash without the change, and I confirm that my change fix the crash. Thanks Miro for the bug report: the fix will be included in next Python 3.8 beta release. I fixed the bug in 3.8 and master branches (3.7 is not affected).

FYI this bug was found in paraview in Fedora Rawhide: https://bugzilla.redhat.com/show_bug.cgi?id=1743896