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Improve unconstify() documentation

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Refer to expression instead of variable when appropriate.

Discussion: https://www.postgresql.org/message-id/08adbe4e-38f8-2c73-55f0-591392371687%402ndquadrant.com
This commit is contained in:
Peter Eisentraut 2018-10-25 00:47:59 +01:00
parent 4beea5508e
commit f2898de98a
1 changed files with 8 additions and 8 deletions
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16
src/include/c.h
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@ -1122,14 +1122,14 @@ typedef union PGAlignedXLogBlock
#endif
/*
* Macro that allows to cast constness away from a variable, but doesn't
* Macro that allows to cast constness away from an expression, but doesn't
* allow changing the underlying type. Enforcement of the latter
* currently only works for gcc like compilers.
*
* Please note IT IS NOT SAFE to cast constness away if the variable will ever
* Please note IT IS NOT SAFE to cast constness away if the result will ever
* be modified (it would be undefined behaviour). Doing so anyway can cause
* compiler misoptimizations or runtime crashes (modifying readonly memory).
* It is only safe to use when the the variable will not be modified, but API
* It is only safe to use when the the result will not be modified, but API
* design or language restrictions prevent you from declaring that
* (e.g. because a function returns both const and non-const variables).
*
@ -1137,13 +1137,13 @@ typedef union PGAlignedXLogBlock
* be nice, but not trivial, to improve that).
*/
#if defined(HAVE__BUILTIN_TYPES_COMPATIBLE_P)
#define unconstify(underlying_type, var) \
(StaticAssertExpr(__builtin_types_compatible_p(__typeof(var), const underlying_type), \
#define unconstify(underlying_type, expr) \
(StaticAssertExpr(__builtin_types_compatible_p(__typeof(expr), const underlying_type), \
"wrong cast"), \
(underlying_type) (var))
(underlying_type) (expr))
#else
#define unconstify(underlying_type, var) \
((underlying_type) (var))
#define unconstify(underlying_type, expr) \
((underlying_type) (expr))
#endif
/* ----------------------------------------------------------------