From d1211c63f010c4e0c22d6070872e80b8c8b4df82 Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Tue, 16 Oct 2018 12:05:50 -0700 Subject: [PATCH] Add macro to cast away const without allowing changes to underlying type. The new unconsitify(underlying_type, var) macro 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). This'll be used in an upcoming change, but seems like it's independent infrastructure. Author: Andres Freund Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de --- src/include/c.h | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) diff --git a/src/include/c.h b/src/include/c.h index 86fbb630d4..4a757bc8ea 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