posix_spawn difference from Linux

Fri Jun 13 21:23:11 GMT 2025

I am working on some posix_spawn tests for the new stc repository [1], and
making sure they behave the same between Cygwin and Linux. I found one
case (so far) which does not: passing NULL for argument "envp" to
posix_spawn.
In Cygwin, this results in the child inheriting the environment from the
caller (same as passing "environ"), but on Linux this results in the child
getting an empty environment (same as passing a char *envp[] = {NULL}).
The Open Group doc on posix_spawn doesn't seem to say anything about the
potential for envp being NULL, but does mention
> For the Ada language binding for Start_Process to be implemented with
> posix_spawn(), that binding would need to explicitly pass an empty
> signal mask and the parent's environment to posix_spawn() whenever the
> caller of Start_Process allowed these arguments to default, since
> posix_spawn() does not provide such defaults.

That at least implies that passing NULL does not default to using the
parent's environment.
Thoughts? Is this a bug in Cygwin, or "undefined behavior" that it's
perfectly within its rights to do whatever it feels like in response
(empty environment or inherited environment, or crash every second
Tuesday)?