[% setvar title Handling environment setting in extension modules %]

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Handling environment setting in extension modules


  Maintainer: Jonathan Leffler <jleffler@informix.com>
  Date: 25 September 2000
  Mailing List: perl6-internals@perl.org
  Number: 318
  Version: 1
  Status: Developing


The Perl 5 XS mechanism can get confused when the code called by an XS extension adds to (or alters) the environment. Perl 6 should not.


Some Perl 5 XS modules (a case in point is DBD::Informix) use C code which cannot be modified by the user (commercial libraries distributed in binary formats only). This C code contains calls to putenv() or other mechanisms which modify the environment. Since Perl also plays with the environment via %ENV, there is lots of room for confusion and corruption.

Neither POSIX.1 nor POSIX.2 defines a function for setting environment variables. In fact, there is not even a header which declares the 'extern char **environ;' variable -- the only C object mandated by POSIX without such a header. The Single Unix Specification does require the putenv() function, but POSIX-compliant (only) libraries could use some other mechanism -- a home-brew analogue of putenv() -- to modify the environment.

The Perl 6 extension mechanisms should circumvent this problem. There are at least two possible ways to do it: the magic, and the mundane.


If Perl 6 is to perform magic, it must somehow track the complete environment through environ, and ensure that the environment has not been changed since Perl last modified the environment. If the environment has changed, then Perl must resynchronize %ENV with the current contents of 'environ'. Further, Perl must ensure that 'environ' contains everything in %ENV at all times. Obviously, there are only a limited number of places where such behind-the-scenes manipulation would occur. Code that is directly written to work with XS would (presumably) be commanded to modify the %ENV hash via the equivalents to the HV-routines documented in perlguts. So, it is only when that code calls out to vendor-provided binary libraries that the problem can occur. Determining when that is what is happening is why it requires magic.


If Perl 6 is unable to perform the magic cost-effectively, Perl 6 should provide extension authors with a mechanism for letting Perl 6 know when the environment may have been modified from within extensions. One possible interface, requiring cooperation from the module author, would be a pair of functions, mark_environment() and fixup_environment() -- the names don't matter but the functionality does.

Before calling the library code that might change the environment, the extension would call mark_environment() to tell Perl 6 that the environment might be changed surreptitiously. Perl could do some sort of check-summing or other analysis of the current environment, and the result of the analysis might be returned as a value of some sort for use with fixup_environment().

The extension would then call the library code that might modify the environment. When that code returns, the extension would call fixup_environment() would use the information stashed by mark_environment() to recognize whether the environment has been changed and to reconcile the Perl internal representation of %ENV with what has been modified.

If Perl provides its own variant on putenv(), would the extensions always call it automatically?


The magical implementation, requiring no help from the module author, is unlikely to be doable at an acceptable cost. I don't have any real information on what might be required.

The mundane implementation can probably be made to work without undue hardship; the biggest problem area could be memory leaks caused by the conservative decisions the code might need to make if the called code changes the list of pointers that environ points to, because of the defined behaviour of the putenv() function).


POSIX.1 IEEE Std 1003.1, 1996 (aka ISO/IEC 9945-1:1996): Information technology - Portable Operating System Interface (POSIX) - Part 1: System Application Program Interface (API) [C Language].

POSIX.2 IEEE Std 1003.2, 199x (aka ISO/IEC 9945-2:199x): Information technology - Portable Operating System Interface (POSIX) - Part 2: ...(rest of title to be filled in)...

The Unix System Today (ISBN 1-85912-296-5, Open Group, June 2000). www.opengroup.org

Various discussions at various times in perl5-porters mailing list. ...[details can be dug out of private email archives if relevant]...