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Commitd1211c6

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Add macro to cast away const without allowing changes to underlying type.
The new unconsitify(underlying_type, var) macro allows to castconstness away from a variable, but doesn't allow changing theunderlying type. Enforcement of the latter currently only works forgcc like compilers.Please note IT IS NOT SAFE to cast constness away if the variable will everbe modified (it would be undefined behaviour). Doing so anyway can causecompiler misoptimizations or runtime crashes (modifying readonly memory).It is only safe to use when the the variable will not be modified, but APIdesign or language restrictions prevent you from declaring that(e.g. because a function returns both const and non-const variables).This'll be used in an upcoming change, but seems like it's independentinfrastructure.Author: Andres FreundDiscussion:https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de
1 parent2c300c6 commitd1211c6

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‎src/include/c.h

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@@ -1121,6 +1121,30 @@ typedef union PGAlignedXLogBlock
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#definePG_TEXTDOMAIN(domain) (domain "-" PG_MAJORVERSION)
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#endif
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/*
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* Macro that allows to cast constness away from a variable, but doesn't
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* allow changing the underlying type. Enforcement of the latter
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* currently only works for gcc like compilers.
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*
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* Please note IT IS NOT SAFE to cast constness away if the variable will ever
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* be modified (it would be undefined behaviour). Doing so anyway can cause
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* compiler misoptimizations or runtime crashes (modifying readonly memory).
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* It is only safe to use when the the variable will not be modified, but API
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* design or language restrictions prevent you from declaring that
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* (e.g. because a function returns both const and non-const variables).
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*
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* Note that this only works in function scope, not for global variables (it'd
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* be nice, but not trivial, to improve that).
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*/
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#if defined(HAVE__BUILTIN_TYPES_COMPATIBLE_P)
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#defineunconstify(underlying_type,var) \
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(StaticAssertExpr(__builtin_types_compatible_p(__typeof(var), const underlying_type), \
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"wrong cast"), \
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(underlying_type) (var))
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#else
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#defineunconstify(underlying_type,var) \
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((underlying_type) (var))
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#endif
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/* ----------------------------------------------------------------
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*Section 9: system-specific hacks

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