From 740aa8e34b8be11b3f58138b6cab6098d52050b7 Mon Sep 17 00:00:00 2001 From: Karl Williamson Date: Sat, 28 Mar 2015 20:54:49 -0600 Subject: perlhacktips: Add caution about clib ptr returns to static memory (cherry picked from commit 38f18a308b948c6eaf187519a16d060e1ec7cc20) Bug-Debian: https://bugs.debian.org/822336 Patch-Name: fixes/5.20.3/docs/clib_caution.diff --- pod/perlhacktips.pod | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+) diff --git a/pod/perlhacktips.pod b/pod/perlhacktips.pod index f2f2e6c32d..5544b5d2d2 100644 --- a/pod/perlhacktips.pod +++ b/pod/perlhacktips.pod @@ -560,6 +560,39 @@ temporarily try the following: But in any case, try to keep the features and operating systems separate. +=item * + +Assuming the contents of static memory pointed to by the return values +of Perl wrappers for C library functions doesn't change. Many C library +functions return pointers to static storage that can be overwritten by +subsequent calls to the same or related functions. Perl has +light-weight wrappers for some of these functions, and which don't make +copies of the static memory. A good example is the interface to the +environment variables that are in effect for the program. Perl has +C to get values from the environment. But the return is +a pointer to static memory in the C library. If you are using the value +to immediately test for something, that's fine, but if you save the +value and expect it to be unchanged by later processing, you would be +wrong, but perhaps you wouldn't know it because different C library +implementations behave differently, and the one on the platform you're +testing on might work for your situation. But on some platforms, a +subsequent call to C or related function WILL overwrite +the memory that your first call points to. This has led to some +hard-to-debug problems. Do a L to make a copy, thus +avoiding these problems. You will have to free the copy when you're +done to avoid memory leaks. If you don't have control over when it gets +freed, you'll need to make the copy in a mortal scalar, like so: + + if ((s = PerlEnv_getenv("foo") == NULL) { + ... /* handle NULL case */ + } + else { + s = SvPVX(sv_2mortal(newSVpv(s, 0))); + } + +The above example works only if C<"s"> is C-terminated; otherwise +you have to pass its length to C. + =back =head2 Problematic System Interfaces