[perl #18256] xsubpp can make nested comments in C code

From: Nicholas Clark (via RT) <perlbug@perl.org>
Message-Id: <rt-18256-41218.19.7625495305233@bugs6.perl.org>

p4raw-id: //depot/perl@18270

1 files changed, 13 insertions, 1 deletions

diff --git a/lib/ExtUtils/xsubpp b/lib/ExtUtils/xsubpp
index 08df7e3446..647ffd78b5 100755
--- a/lib/ExtUtils/xsubpp
+++ b/lib/ExtUtils/xsubpp
@@ -907,7 +907,19 @@ while (<$FH>) {
         my $podstartline = $.;
     	do {
 	    if (/^=cut\s*$/) {
-		print("/* Skipped embedded POD. */\n");
+		# We can't just write out a /* */ comment, as our embedded
+		# POD might itself be in a comment. We can't put a /**/
+		# comment inside #if 0, as the C standard says that the source
+		# file is decomposed into preprocessing characters in the stage
+		# before preprocessing commands are executed.
+		# I don't want to leave the text as barewords, because the spec
+		# isn't clear whether macros are expanded before or after
+		# preprocessing commands are executed, and someone pathological
+		# may just have defined one of the 3 words as a macro that does
+		# something strange. Multiline strings are illegal in C, so
+		# the "" we write must be a string literal. And they aren't
+		# concatenated until 2 steps later, so we are safe.
+		print("#if 0\n  \"Skipped embedded POD.\"\n#endif\n");
 		printf("#line %d \"$filename\"\n", $. + 1)
 		  if $WantLineNumbers;
 		next firstmodule