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POArchitectures/ABIs where sizeof(long long) != 8
 

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  1. POArchitectures/ABIs where sizeof(long long) != 8
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    <p>In x86/amd64 world <code>sizeof(long long)</code> is 8.</p> <p>Let me quote quite insightful 8 year old <a href="http://gcc.gnu.org/ml/gcc/2004-03/msg01251.html">mail by Zack Weinberg</a>:</p> <blockquote> <p>Scott Robert Ladd writes:</p> <blockquote> <p>On a 64-bit AMD64 architecture, GCC defines <code>long long</code> as 64 bits, the same as a <code>long</code>.</p> <p>Given that certain 64-bit instructions (multiply) produce 128-bit results, doesn't it seem logical the <code>long long</code> be defined as 128 bits?</p> </blockquote> <p>No, for two reasons:</p> <ol> <li><p>The choice of 64-bit '<code>long long</code>' has been written into the ABI of most LP64-model operating systems; we can't unilaterally change it.</p></li> <li><p>This is actually the correct choice, as it removes the aberration that makes '<code>long</code>' not the widest basic integral type. There is lots and lots of code in the wild written to the assumption that <code>sizeof(long) &gt;= sizeof(size_t)</code> - this is at least potentially broken by ABIs where long long is wider than long.</p> <p>(This was an extremely contentious topic during the development of C99. As best as I can tell from an outside perspective, '<code>long long</code>' was only standardized due to pressure from Microsoft who can't for some reason implement an LP64 model. Everyone else hated the idea of making '<code>long</code>' not necessarily the widest basic integral type.)</p></li> </ol> <p>Best current practice appears to be to provide an "extended integral type" <code>__int128</code>. This doesn't have the problems of '<code>long long</code>' because it's not a <em>basic</em> integral type (in particular, it cannot be used for <code>size_t</code>).</p> <p>zw</p> </blockquote> <hr> <p><code>long long</code> is widest <strong>basic</strong> integral type. It's 64-bit long on any non-dead-old architectures/ABIs I know. This allows for going with simple cross-platform (well, at least for many 32/64-bit architectures) typedefs:</p> <pre><code>typedef char s8; typedef unsigned char u8; typedef short s16; typedef unsigned short u16; typedef int s32; typedef unsigned int u32; typedef long long s64; typedef unsigned long long u64; </code></pre> <p>that are nicer than <code>intXX_t</code>, because:</p> <ul> <li>they use same underlying type for 64-bit integers on different platforms</li> <li>allows avoiding verbose <code>PRId64</code>/<code>PRIu64</code><br> (I am well aware that Visual C++ supports <code>%lld</code>/<code>%llu</code> only since 2005)</li> </ul> <p>But how portable this solution is can be expressed by answers to the following question.</p> <hr> <p><strong>What are the architectures/ABIs where <code>sizeof(long long) != 8</code>?</strong></p> <p>If you cannot provide any recent/modern ones, then go ahead with the old ones, but only if they are still in use.</p>
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    1. COBtw, `typedef int s32` fails on implementations with a 16 bit `int`, of which there are a few microcontrollers kicking about. Obviously this has nothing to do with `long long`, which means that your plans for `s32` are irrelevant to the question you're asking :-) But I think you're more likely to trip over `s32` than `s64`.
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      1. USSteve Jessop
    2. CO@SteveJessop: Sure. Question is about `long long` alone, but for usual purposes I am indeed concerned only about platforms where `char`, `short` and `int` are respectively 8-, 16- and 32-bit long.
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    3. COIs it really common to assume `sizeof(long)>=sizeof(size_t)`? I've always assumed the opposite...
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