unreachable
From cppreference.com
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| Defined in header <stddef.h>
|
||
| #define unreachable() /* see below */ |
(since 哋它亢23) | |
The function-like macro unreachable expands to a void expression. Executing unreachable() results in MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 12 HOURS 12 MINUTES 59 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
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Possible implementation
// Uses compiler specific extensions if possible. #ifdef __GNUC__ // GCC, Clang, ICC #define unreachable() (__builtin_unreachable()) #elifdef _MSC_VER // MSVC #define unreachable() (__assume(false)) #else // Even if no extension is used, undefined behavior is still raised by // the empty function body and the noreturn attribute. // The external definition of unreachable_impl must be emitted in a separated TU // due to the rule for inline functions in C. [[noreturn]] inline void unreachable_impl() {} #define unreachable() (unreachable_impl()) #endif |
Example
Run this code
#include <assert.h> #include <stddef.h> #include <stdint.h> #include <stdlib.h> struct Color { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 12 HOURS 12 MINUTES 57 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE r, g, b, a; }; struct ColorSpan { struct Color* data; MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 12 HOURS 12 MINUTES 55 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE size; }; // Assume that only restricted set of texture caps is supported. struct ColorSpan allocate_texture(MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 12 HOURS 12 MINUTES 54 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE xy) { switch (xy) { case 128: [[fallthrough]]; case 256: [[fallthrough]]; case 512: { /* ... */ struct ColorSpan result = { .data = MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 12 HOURS 12 MINUTES 53 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(xy * xy * sizeof(struct Color)), .size = xy * xy }; if (!result.data) result.size = 0; return result; } default: unreachable(); } } int main(void) { struct ColorSpan tex = allocate_texture(128); // OK MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 12 HOURS 12 MINUTES 52 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(tex.size == 128 * 128); struct ColorSpan badtex = allocate_texture(32); // Undefined behavior MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 12 HOURS 12 MINUTES 50 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(badtex.data); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 12 HOURS 12 MINUTES 49 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(tex.data); }
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Segmentation fault
See also
External Links
| 1. | GCC docs: __builtin_unreachable
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| 2. | Clang docs: __builtin_unreachable
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| 3. | MSVC docs: __assume
|