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documenting SVf_PROTECT

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Dave Mitchell
November 24, 2016 14:49
documenting SVf_PROTECT
Message ID:

does this commit of mine look correct?

commit cc25b9e2d291fedac80973dc493e321d31d3cb1a
Author:     David Mitchell <>
AuthorDate: Thu Nov 24 14:42:27 2016 +0000
Commit:     David Mitchell <>
CommitDate: Thu Nov 24 14:42:27 2016 +0000

    add comments explaining the point of SVf_PROTECT
    The flag was added by v5.21.4-17-ga623f89, but outside of the commit
    message itself, there didn't seem to be any explanation for this flag.

Affected files ...
    M	sv.h

Differences ...

diff --git a/sv.h b/sv.h
index 7808b89..ee65879 100644
--- a/sv.h
+++ b/sv.h
@@ -374,6 +374,15 @@ perform the upgrade if necessary.  See C<L</svtype>>.
 				       subroutine in another package. Set the
 				       GvIMPORTED_CV_on() if it needs to be
 				       expanded to a real GV */
+/* SVf_PROTECT is what SVf_READONLY should have been: i.e. modifying
+ * this SV is completely illegal. However, SVf_READONLY (via
+ * Internals::SvREADONLY()) has come to be seen as a flag that can be
+ * temporarily set and unset by the user to indicate e.g. whether a hash
+ * is "locked". Now, Hash::Util et al only set SVf_READONLY, while core
+ * sets both (SVf_READONLY|SVf_PROTECT) to indicate both to core and user
+ * code that this SV should not be messed with.
+ */
 #define SVf_PROTECT	0x00010000  /* very read-only */
 #define SVs_PADTMP	0x00020000  /* in use as tmp */
 #define SVs_PADSTALE	0x00040000  /* lexical has gone out of scope;

You live and learn (although usually you just live).

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