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| -rw-r--r-- | src/include/c.h | 24 |
1 files changed, 24 insertions, 0 deletions
diff --git a/src/include/c.h b/src/include/c.h index 86fbb630d4a..4a757bc8ea1 100644 --- a/src/include/c.h +++ b/src/include/c.h @@ -1121,6 +1121,30 @@ typedef union PGAlignedXLogBlock #define PG_TEXTDOMAIN(domain) (domain "-" PG_MAJORVERSION) #endif +/* + * Macro that allows to cast constness away from a variable, 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 + * 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 + * design or language restrictions prevent you from declaring that + * (e.g. because a function returns both const and non-const variables). + * + * Note that this only works in function scope, not for global variables (it'd + * 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), \ + "wrong cast"), \ + (underlying_type) (var)) +#else +#define unconstify(underlying_type, var) \ + ((underlying_type) (var)) +#endif /* ---------------------------------------------------------------- * Section 9: system-specific hacks |