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Date: Fri, 13 Jun 2025 23:39:15 -0400
To: Jeremy Drake <cygwin AT jdrake DOT com>
Cc: cygwin AT cygwin DOT com
Subject: Re: posix_spawn difference from Linux
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From: Glenn Strauss via Cygwin <cygwin AT cygwin DOT com>
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On Fri, Jun 13, 2025 at 02:25:17PM -0700, Jeremy Drake via Cygwin wrote:
> On Fri, 13 Jun 2025, Jeremy Drake wrote:
> 
> > 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[2] 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)?
> 
> Oops, I forgot my footnote links:
> 
> [1]: https://cygwin.com/cgit/cygwin-apps/stc/
> [2]: https://pubs.opengroup.org/onlinepubs/007904975/functions/posix_spawn.html

The man pages from different OS contain something like (from Linux):

   The argument envp is an array of character pointers to null-terminated strings. These strings constitute the environment for the new
   process image. The environment array is terminated by a null pointer.

I have never interpreted NULL as a valid value for envp.
I think the behavior is unspecified, and could segfault.
If caller intends an empty environment, then caller should pass:
  char *e[] = { NULL };

Therefore, in lighttpd's portability wrapper for fork-execve,
passing NULL for envp (to my wrapper) is used to inherit default env
from current process (char **environ), whether lighttpd uses
posix_spawn() or execve().  This is the same behavior you described
for Cygwin, though lighttpd passes `environ` to posix_spawn(), not NULL.

Cheers, Glenn

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