This might precisely be the reason that the unix port does mp_stack_ctrl_init as the very first thing.

Remove mp_stack_ctrl_init() from mp_embed_init() and require the application itself to call it

Probably the way to go, though should be configurable perhaps, and in any case an error/exit should be generated immediately if someone forgets to call it since otherwise the resulting errors might be quite obscure.

FWIW I've been using a function similar to mp_embed_init for years and never had stack issues. Actually, also in light of #11430, looking at the current state where there's a myriad of variations on init/exec_str/exec_mpy/deinit/... spread over ports and utility functions I think it would be good to try and unify this into common functions which are either configurable at compiletime or else at runtime. Also by means of documentation. Take mp_embed_exec_str for example: it's there, it's an example, but once you actually want to use it it's going to take a couple of minutes and then you realize you need the return value and you're going to turn to one of th 5 or 10 other implementations which are all tiny variations on the same thing. Next you want to use MICROPYPATH and have to copy that from the unix port (or so), and so on.

I have a reproducer: cwalther@b404d15. It’s not a segfault but an assert this time (apparently the thing the pointer is overwritten with isn’t that outlandish, but still sufficiently wrong), but that should be enough to illustrate the problem. The added printf()s are not important, they were just useful to nudge the garbage collections into the right spot.

Build the embedding example in the usual way with make -f micropython_embed.mk && make and run it.

Actual result:

cwalther@tartan embedding % ./embed                                         
PARSE
COMPILE
RUN
hello world! [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]eol
DONE
PARSE
COMPILE
COLLECT
Assertion failed: (ATB_GET_KIND(area, block) == AT_HEAD), function gc_free, file gc.c, line 776.
zsh: abort      ./embed

Expected result:

cwalther@tartan embedding % ./embed                                         
PARSE
COMPILE
RUN
hello world! [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32]eol
DONE
PARSE
COMPILE
COLLECT
RUN
iter 00000000
iter 00000001
iter 00000002
iter 00000003
iter 00000004
iter 00000005
iter 00000006
iter 00000007
iter 00000008
iter 00000009
caught exception ZeroDivisionError('divide by zero',)
run GC collect
COLLECT
hello world! f 1
hello world! f 2
hello world! f 3
hello world! f 4
hello world! f 5
hello world! f 6
hello world! f 7
hello world! f 8
hello world! f 9
hello world! f 10
hello world! f 11
hello world! f 12
hello world! f 13
hello world! f 14
hello world! f 15
hello world! f 16
hello world! f 17
hello world! f 18
hello world! f 19
hello world! f 20
finish
DONE

The expected result can be achieved by moving the call of mp_stack_ctrl_init() as proposed: cwalther@60a889e