devdocs/c/program%2Funreachable.html


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    <h1 id="firstHeading" class="firstHeading">unreachable</h1>            <table class="t-dcl-begin"> <tr class="t-dsc-header"> <th> Defined in header <code>&lt;stddef.h&gt;</code> </th> <th> </th> <th> </th> </tr> <tr class="t-dcl t-since-c23"> <td> <pre data-language="c">#define unreachable() /* see below */</pre>
</td> <td class="t-dcl-nopad"> </td> <td> <span class="t-mark-rev t-since-c23">(since C23)</span> </td> </tr>  </table> <p>The function-like macro <code>unreachable</code> expands to a <code>void</code> expression. Executing <code>unreachable()</code> results in <a href="../language/behavior" title="c/language/behavior">undefined behavior</a>.</p>
<p>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).</p>
<h3 id="Possible_implementation"> Possible implementation</h3> <table class="eq-fun-cpp-table"> <tr> <td> <div class="c source-c"><pre data-language="c">// 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</pre></div> </td>
</tr>
</table> <h3 id="Example"> Example</h3> <div class="t-example"> <div class="c source-c"><pre data-language="c">#include &lt;assert.h&gt;
#include &lt;stddef.h&gt;
#include &lt;stdint.h&gt;
#include &lt;stdlib.h&gt;
 
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);
}</pre></div> <p>Possible output:</p>
<div class="text source-text"><pre data-language="c">Segmentation fault</pre></div> </div> <h3 id="See_also"> See also</h3> <table class="t-dsc-begin"> <tr class="t-dsc"> <td colspan="2"> <span><a href="https://en.cppreference.com/w/cpp/utility/unreachable" title="cpp/utility/unreachable">C++ documentation</a></span> for <code>unreachable</code> </td>
</tr> </table> <h3 id="External_Links"> External Links</h3> <table> <tr style="vertical-align:top;"> <td>1. </td> <td>
<a rel="nofollow" class="external text" href="https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005funreachable">GCC docs: <code>__builtin_unreachable</code></a> </td>
</tr> <tr style="vertical-align:top;"> <td>2. </td> <td>
<a rel="nofollow" class="external text" href="https://clang.llvm.org/docs/LanguageExtensions.html#builtin-unreachable">Clang docs: <code>__builtin_unreachable</code></a> </td>
</tr> <tr style="vertical-align:top;"> <td>3. </td> <td>
<a rel="nofollow" class="external text" href="https://docs.microsoft.com/en-us/cpp/intrinsics/assume">MSVC docs: <code>__assume</code></a> </td>
</tr>
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