From 82ba818ff456bcd6d56a06226e3f27e98fbb55c3 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Thu, 14 Aug 2025 22:58:58 -0500 Subject: removing all downloaded devdocs files --- devdocs/c/program%2Funreachable.html | 79 ------------------------------------ 1 file changed, 79 deletions(-) delete mode 100644 devdocs/c/program%2Funreachable.html (limited to 'devdocs/c/program%2Funreachable.html') diff --git a/devdocs/c/program%2Funreachable.html b/devdocs/c/program%2Funreachable.html deleted file mode 100644 index 09e33e03..00000000 --- a/devdocs/c/program%2Funreachable.html +++ /dev/null @@ -1,79 +0,0 @@ -

unreachable

Defined in header <stddef.h> 
#define unreachable() /* see below */
-		 (since C23)

The function-like macro unreachable expands to a void expression. Executing unreachable() results in undefined behavior.

-

An implementation may use this to optimize impossible code branches away (typically, in optimized builds) or to trap them to prevent further execution (typically, in debug builds).

-

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

#include <assert.h>
-#include <stddef.h>
-#include <stdint.h>
-#include <stdlib.h>
- 
-struct Color { uint8_t r, g, b, a; };
-struct ColorSpan { struct Color* data; size_t size; };
- 
-// Assume that only restricted set of texture caps is supported.
-struct ColorSpan allocate_texture(size_t xy)
-{
-    switch (xy)
-    {
-    case 128: [[fallthrough]];
-    case 256: [[fallthrough]];
-    case 512:
-    {
-        /* ... */
-        struct ColorSpan result = {
-            .data = malloc(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
-    assert(tex.size == 128 * 128);
- 
-    struct ColorSpan badtex = allocate_texture(32);  // Undefined behavior
- 
-    free(badtex.data);
-    free(tex.data);
-}

Possible output:

-
Segmentation fault

See also

-
C++ documentation for unreachable
- - - -
1. -GCC docs: __builtin_unreachable
2. -Clang docs: __builtin_unreachable
3. -MSVC docs: __assume
-

- © cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
- https://en.cppreference.com/w/c/program/unreachable -

-
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