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unreachable

From cppreference.net
C
Program support utilities
Program termination
Unreachable control flow
unreachable
(C23)
Communicating with the environment
Memory alignment query
(C23)
Signals
Signal types
Non-local jumps
Types
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).

Contents

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 { 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

External Links

1. GCC docs: __builtin_unreachable
2. Clang docs: __builtin_unreachable
3. MSVC docs: __assume