Help with C clearenv and setenv

[Eric Blake]

On 05/30/2018 09:48 AM, Sam Habiel wrote:
> I have code for a database I am porting to Cygwin.
> 
> Part of that code is a clearenv() then a couple of setenvs. There is
> an ifdef for Cygwin, as it doesn't implement clearenv.

It wouldn't be hard to implement clearenv() for a future release of 
Cygwin, at least for a version that leaves environ pointing to a 
1-element array containing NULL.  A bit more effort and we could 
probably also support glibc's notion of environ==NULL being shorthand 
for an empty environment; in fact, POSIX states:

"After assigning a new value to environ, applications should not rely on 
the new environment strings remaining part of the environment, as a call 
to getenv(), [XSI] [Option Start] putenv(), [Option End] setenv(), 
unsetenv(), or any function that is dependent on an environment variable 
may, on noticing that environ has changed, copy the environment strings 
to a new array and assign environ to point to it."

where we could check for environ==NULL on entry to any of those 
functions as one of the times that we reassign environ back to a 
non-NULL array with one NULL entry.

Note, however, that POSIX says that it is not portable to exec processes 
with a completely empty environment.  In particular, Cygwin is rather 
fragile if some things like PATH are not defined in the environment. 
Having a temporarily empty environ may work if you then add back enough 
state before Cygwin has to look something up from the environment.  The 
POSIX recommendation is that you should always have at least the 
equivalent of:

env -i $(getconf V7_ENV) PATH="$(getconf PATH)" command

in your environment, at least when exec'ing processes.

> It just sets
> environ = NULL. Well--that really breaks setenv! It returns a "Bad
> Poniter" error (-1).

What it SHOULD do is set environ to a one-element array containing NULL, 
at least until someone submits a patch adding the glibc extension of 
clearenv() to Cygwin.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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