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377 lines
18 KiB
HTML
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<title>Boost Endian Conversion Functions</title>
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<a href="../../../index.html">
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<img src="../../../boost.png" alt="boost.png (6897 bytes)" align="middle" width="277" height="86" border="0"></a></td>
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<td width="636" align="middle">
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<font size="7">Endian Conversion Functions</font></td>
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</tr>
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</table>
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<table border="0" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" bgcolor="#D7EEFF" width="100%">
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<tr>
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<td><b><a href="../../../index.htm">Boost Home</a>
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<a href="index.html">Endian Home</a>
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<a href="conversion.html">Conversion Functions</a>
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<a href="types.html">Endian Types</a> Tutorial</b></td>
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</tr>
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</table>
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<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111" align="right">
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<td width="100%" bgcolor="#D7EEFF" align="center">
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<i><b>Contents</b></i></td>
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</tr>
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<tr>
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<td width="100%" bgcolor="#E8F5FF">
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<a href="#Introduction">Introduction</a><br>
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<a href="#FAQ">conversion.hpp FAQ</a><br>
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<a href="#Reference">Reference</a><br>
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<a href="#Synopsis">Synopsis</a><br>
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<a href="#Requirements">Requirements</a><br>
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<a href="#Functions">Functions</a><br>
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<a href="#Intrinsic">Intrinsic built-in support</a><br>
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<a href="#Acknowledgements">Acknowledgements</a></td>
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</tr>
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<tr>
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<td width="100%" bgcolor="#D7EEFF" align="center">
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<b><i>Headers</i></b></td>
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</tr>
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<tr>
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<td width="100%" bgcolor="#E8F5FF">
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<a href="../../../boost/endian/conversion.hpp"><boost/endian/conversion.hpp></a><br>
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<a href="../../../boost/endian/types.hpp"><boost/endian/types.hpp></a></td>
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</tr>
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</table>
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<h2><a name="Introduction">Introduction</a></h2>
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<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111">
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<tr>
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<td bgcolor="#FFFFCC">Please: If you haven't done so already, read
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<a href="index.html#Introduction-to-endianness">Introduction to endianness</a>!</td>
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</tr>
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</table>
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<p>Header <a href="../../../boost/endian/conversion.hpp">boost/endian/conversion.hpp</a>
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provides byte order reversal and conversion functions that convert objects of
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the multi-byte built-in
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integer types, and also types <code>float</code> and <code>double,</code>
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between native, big, or little endian byte
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ordering. User defined types are also supported.</p>
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<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111">
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<tr>
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<td bgcolor="#FFFFCC">Caution: Only big endianness and little endianness is supported;
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middle endianness is not supported.</td>
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</tr>
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</table>
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<h2>conversion.hpp <a name="FAQ">FAQ</a></h2>
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<p><b>Is the implementation header only?</b></p>
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<blockquote>
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<p>Yes.</p>
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</blockquote>
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<p><b>Does the implementation use compiler intrinsic built-in byte swapping?</b></p>
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<blockquote>
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<p>Yes, if available. See <a href="#Intrinsic">Intrinsic built-in support</a>
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below.</p>
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</blockquote>
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<p><b>Why are the template versions of <code>reverse()</code> and <code>reverse_value()</code>
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in a detail namespace?</b></p>
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<blockquote>
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<p>They are unsafe for general use. Consider reversing
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the bytes of a <code>std::pai<b>r</b></code> as a whole - the bytes from <code>first</code>
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would end up in <code>second</code> and visa versa, and this is totally
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wrong!</p>
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</blockquote>
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<p><b>Why are both value returning and modify-in-place functions provided?</b></p>
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<blockquote>
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<p>Returning the result by value is the standard C and C++ idiom for functions that compute a
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value from an argument. Modify-in-place functions allow cleaner code in many real-world
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endian use cases and are more efficient for user defined types that have
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members such as string data that do not need to be reversed. Thus both forms are
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provided.</p>
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</blockquote>
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<p><b>What are the limitations of floating point support?</b></p>
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<blockquote>
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<p>The only supported types are four byte <code>float</code> and eight byte
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<code>double</code>. Even after endianness has been accounted for, floating
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point values will not be portable between systems that use different floating
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point formats. Systems where the integer endianness and floating point
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endianness are not supported.</p>
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</blockquote>
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<p><b>What are the limitations of integer support?</b></p>
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<blockquote>
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<p>Only 16-bit, 32-bit, and 64-bit integers are supported. Tests have been
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performed on machines that use two's complement arithmetic.</p>
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</blockquote>
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<h2><a name="Reference">Reference</a></h2>
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<p>Functions are implemented <code>inline</code> if appropriate.<code> noexcept</code> is
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elided for compilers that do not support it.
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Boost scoped enum emulation is used so that the library still works for compilers that do not support scoped enums. </p>
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<h3>
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<a name="Synopsis">Synopsis</a></h3>
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<pre>#define BOOST_ENDIAN_INTRINSIC_MSG "message describing presence or absence of intrinsics"
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namespace boost
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{
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namespace endian
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{
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enum class <a name="order">order</a> {big, little, native};
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// reverse byte order (i.e. endianness)
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int16_t <a href="#reverse_value">reverse_value</a>(int16_t x) noexcept;
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int32_t <a href="#reverse_value">reverse_value</a>(int32_t x) noexcept;
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int64_t <a href="#reverse_value">reverse_value</a>(int64_t x) noexcept;
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uint16_t <a href="#reverse_value">reverse_value</a>(uint16_t x) noexcept;
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uint32_t <a href="#reverse_value">reverse_value</a>(uint32_t x) noexcept;
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uint64_t <a href="#reverse_value">reverse_value</a>(uint64_t x) noexcept;
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float <a href="#reverse_value">reverse_value</a>(float x) noexcept;
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double <a href="#reverse_value">reverse_value</a>(double x) noexcept;
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void <a href="#reverse">reverse</a>(int16_t& x) noexcept;
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void <a href="#reverse">reverse</a>(int32_t& x) noexcept;
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void <a href="#reverse">reverse</a>(int64_t& x) noexcept;
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void <a href="#reverse">reverse</a>(uint16_t& x) noexcept;
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void <a href="#reverse">reverse</a>(uint32_t& x) noexcept;
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void <a href="#reverse">reverse</a>(uint64_t& x) noexcept;
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void <a href="#reverse">reverse</a>(float& x) noexcept;
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void <a href="#reverse">reverse</a>(double& x) noexcept;
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// reverse byte order unless native endianness is big
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template <class ReversibleValue >
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ReversibleValue <a href="#big_endian_value">big_endian_value</a>(ReversibleValue x) noexcept;
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template <class Reversible>
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void <a href="#big_endian">big_endian</a>(Reversible& x) noexcept;
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// reverse byte order unless native endianness is little
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template <class ReversibleValue >
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ReversibleValue <a href="#little_endian_value">little_endian_value</a>(ReversibleValue x) noexcept;
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template <class Reversible>
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void <a href="#little_endian">little_endian</a>(Reversible& x) noexcept;
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// synonyms, based on names popularized by BSD (e.g. OS X, Linux)
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// "h" for "host" (i.e. native), "be" for "big endian",
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// "le" for "little endian", "m" for "modify" in place
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template <class T> T bswap(T x) noexcept {return reverse_value(x);}
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template <class T> T htobe(T host) noexcept {return big_endian_value(host);}
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template <class T> T htole(T host) noexcept {return little_endian_value(host);}
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template <class T> T betoh(T big) noexcept {return big_endian_value(big);}
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template <class T> T letoh(T little) noexcept {return little_endian_value(little);}
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template <class T> void bswapm(T& x) noexcept {reverse(x);}
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template <class T> void htobem(T& host) noexcept {big_endian(host);}
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template <class T> void htole(mT& host noexcept) {little_endian(host);}
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template <class T> void betohm(T& big) noexcept {big_endian(big);}
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template <class T> void letohm(T& little) noexcept {little_endian(little);}
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// generic byte order conversion
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template <order From, order To, class ReversibleValue>
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ReversibleValue <a href="#convert_value_generic">convert_value</a>(ReversibleValue from) noexcept;
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template <order From, order To, class Reversible>
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void <a href="#convert_generic">convert</a>(Reversible& x) noexcept;
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// runtime effective byte order determination
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order <a href="#effective_order">effective_order</a>(order x) noexcept;
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// runtime byte-order conversion
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template <class ReversibleValue>
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ReversibleValue <a href="#convert_value_runtime">convert_value</a>(ReversibleValue from,
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order from_order, order to_order) noexcept;
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template <class Reversible>
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void <a href="#convert_runtime">convert</a>(Reversible& x,
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order from_order, order to_order) noexcept;
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} // namespace endian
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} // namespace boost</pre>
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<h3><a name="Requirements">Requirements</a></h3>
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<p>The template definitions in this header refer to named
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requirements whose details are set out in this section. User defined types may
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be used in the function templates in this header only if they meet the
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function's template parameter requirements.</p>
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<h4><a name="ReversibleValue">ReversibleValue</a> requirements</h4>
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<p><code>ReversibleValue</code> is an object type to be
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supplied by a C++ program instantiating a template; <code>x</code> is a value of
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type (possibly <code>const</code>) <code>ReversibleValue</code>.</p>
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<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111">
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<tr>
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<td width="160"><b>Expression</b></td>
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<td width="150"><b>Return type</b></td>
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<td width="347"><b>Requirement</b></td>
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</tr>
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<tr>
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<td valign="top">
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<p><code>reverse_value(x)</code></td>
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<td valign="top">
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<p><a name="ReversibleValue"><code>ReversibleValue</code></a></td>
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<td>
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<p>The returned value is the value of <code>x</code> with the
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order of its constituent bytes reversed.</td>
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</tr>
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</table>
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<h4><a name="Reversible">Reversible</a> requirements</h4>
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<p><code>Reversible</code> is an object type to be
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supplied by a C++ program instantiating a template; <code>x</code> is a
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modifiable lvalue of type <code>Reversible</code>.</p>
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<table border="1" cellpadding="5" cellspacing="0" style="border-collapse: collapse" bordercolor="#111111">
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<tr>
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<td width="160"><b>Expression</b></td>
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<td width="347"><b>Post-condition</b></td>
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</tr>
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<tr>
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<td valign="top">
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<p><code>reverse(x)</code></td>
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<td>
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<p>The order of the constituent bytes of <code>x</code> are
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reversed.</td>
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</tr>
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</table>
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<h3><a name="Functions">Functions</a></h3>
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<pre><a name="reverse_value"></a>int16_t reverse_value(int16_t x) noexcept;
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int32_t reverse_value(int32_t x) noexcept;
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int64_t reverse_value(int64_t x) noexcept;
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uint16_t reverse_value(uint16_t x) noexcept;
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uint32_t reverse_value(uint32_t x) noexcept;
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uint64_t reverse_value(uint64_t x) noexcept;
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float reverse_value(float x) noexcept;
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double reverse_value(double x) noexcept;</pre>
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<blockquote>
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<p><i>Returns:</i> <i><code>x</code></i>, with the order of its
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constituent bytes reversed.</p>
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</blockquote>
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<pre><a name="reverse"></a>void reverse(int16_t& x) noexcept;
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void reverse(int32_t& x) noexcept;
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void reverse(int64_t& x) noexcept;
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void reverse(uint16_t& x) noexcept;
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void reverse(uint32_t& x) noexcept;
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void reverse(uint64_t& x) noexcept;
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void reverse(float& x) noexcept;
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void reverse(double& x) noexcept;</pre>
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<blockquote>
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<p><i>Postconditions:</i> The order of the constituent bytes of
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<code>x</code> are reversed.</p>
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</blockquote>
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<pre><a name="big_endian_value"></a>template <class ReversibleValue >
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ReversibleValue big_endian_value(ReversibleValue x) noexcept;
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<a name="big_endian"></a>template <class Reversible>
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void big_endian(Reversible& x) noexcept;</pre>
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<blockquote>
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<p dir="ltr"><i>Returns (first form)</i>: <code>x</code> if the native byte order is big
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endian, otherwise <code>reverse_value(x)</code>.</p>
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<p dir="ltr"><i>Effects (second form):</i> None if the native byte order is big
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endian, otherwise <code>reverse(x)</code>.</p>
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<p dir="ltr"><i>Example:</i></p>
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<blockquote>
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<pre>int32_t x = <b><i>some-value</i></b>;
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big_endian(x); // reverses the byte order of x, unless
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// the native byte order is big-endian</pre>
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</blockquote>
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</blockquote>
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<pre dir="ltr"><a name="little_endian_value"></a>template <class ReversibleValue >
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ReversibleValue little_endian_value(ReversibleValue x) noexcept;
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<a name="little_endian"></a>template <class Reversible>
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void little_endian(Reversible& x) noexcept;</pre>
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<blockquote>
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<p dir="ltr"><i>Returns (first form)</i>: <code>x</code> if the native byte order is little
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endian, otherwise <code>reverse_value(x)</code>.</p>
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<p dir="ltr"><i>Effects (second form):</i> None if the native byte order is little
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endian, otherwise <code>reverse(x)</code>.</p>
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<p dir="ltr"><i>Example:</i></p>
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<blockquote>
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<pre>int32_t x = <b><i>some-value</i></b>;
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int32_t y(little_endian(x));
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// y has been set to x; the byte order is reversed unless
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// the native byte order is little-endian.</pre>
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</blockquote>
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</blockquote>
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<pre dir="ltr"><a name="convert_value_generic"></a>template <order From, order To, class ReversibleValue>
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ReversibleValue convert_value(ReversibleValue from) noexcept;
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<a name="convert_generic"></a>template <order From, order To, class Reversible>
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void convert(Reversible& x) noexcept;
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</pre>
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<blockquote>
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<p dir="ltr">The <b><i>effective order</i></b> of an order template parameter
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is the same as the order template parameter if the parameter is not <code>
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order::native</code>, otherwise it is the constant <code>order::big</code> or
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<code>order::little</code> that represents the actual native byte order.</p>
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<p dir="ltr"><i>Returns (first form)</i>: <code>from</code> if <code>From</code>
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and <code>To</code> have the same effective order, otherwise <code>
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reverse_value(from)</code>.</p>
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<p dir="ltr"><i>Effects (second form):</i> None if <code>From</code> and <code>
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To</code> have the same effective order, otherwise <code>reverse(x)</code>.</p>
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<p dir="ltr"><i>Example:</i></p>
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<blockquote>
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<pre>int32_t x;
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<i>... read an external big-endian value into x</i>
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convert<order::big, order::native>(x); // more generic equivalent of big_endian(x);</pre>
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</blockquote>
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</blockquote>
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<pre><a name="effective_order"></a>order effective_order(order x) noexcept;<blockquote><p dir="ltr"><i>Returns:</i> <code>x</code> if <code>x != order::native</code>, otherwise the <code>order</code> constant for the actual native byte order.</p><p dir="ltr"><i>Example:</i></p><blockquote><pre dir="ltr">effective_order(order::big); // returns order::big
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effective_order(order::little); // returns order::little
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effective_order(order::native); // returns order::big if the native order
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// is big-endian, otherwise order::little</pre></blockquote></blockquote><pre dir="ltr"><a name="convert_value_runtime"></a>template <class ReversibleValue>
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ReversibleValue convert_value(ReversibleValue from,
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order from_order, order to_order) noexcept;
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<a name="convert_runtime"></a>template <class Reversible>
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void convert(Reversible& x,
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order from_order, order to_order) noexcept;</pre><blockquote><p dir="ltr"><i>Returns (first form)</i>: <code>from</code> if <code>effect_order(from_order) == effective_order(to_order)</code>, otherwise <code>reverse_value(from)</code>.</p>
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<p dir="ltr"><i>Effects (second form):</i> None if <code>effect_order(from_order) == effective_order(to_order)</code>, otherwise <code>reverse(x)</code>.</p>
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<p dir="ltr"><i>Example:</i></p>
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<blockquote>
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<pre>int32_t x;
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<i>... read an external value of an endianness know only at runtime into x</i>
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convert(x, some_order, order::native); // convert to native byte order if needed</pre>
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</blockquote>
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</blockquote><h2><a name="Intrinsic">Intrinsic</a> built-in support</h2>
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<p>Recent compilers, including GCC, Clang, and Microsoft, supply intrinsic built-in support for byte swapping. Such support is automatically detected and used since it results in smaller and much faster generated code for release builds.</p><p>Defining BOOST_ENDIAN_NO_INTRINSICS will suppress use of the intrinsics. Please try defining it if you get compiler errors, such as header byteswap.h not being found.</p><p>The macro BOOST_ENDIAN_INTRINSIC_MSG is defined as either <code>"no byte swap intrinsics"</code> or a string describing the particular set of intrinsics being used.</p><h2><a name="Acknowledgements">Acknowledgements</a></h2><p>Tomas Puverle was instrumental in identifying and articulating the need to
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support endian conversion as separate from endian integer types. Phil Endecott suggested the form of the value returning signatures. Vicente Botet and other reviewers suggested supporting floating point types and user defined types. General reverse template implementation approach using std::reverse suggested by Mathias Gaunard. Portable implementation approach for 16, 32, and 64-bit integers suggested by tymofey, with avoidance of undefined behavior as suggested by Giovanni Piero Deretta, and a further refinement suggested by Pyry Jahkola. Intrinsic builtins implementation approach for 16, 32, and 64-bit integers suggested by several reviewers, and by David Stone, who provided his Boost licensed macro implementation that became the starting point for <a href="../include/boost/endian/detail/intrinsic.hpp">boost/endian/detail/intrinsic.hpp</a>.</p>
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<hr>
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<p>Last revised: <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B, %Y" startspan -->20 May, 2013<!--webbot bot="Timestamp" endspan i-checksum="13976" --></p>
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<p>� Copyright Beman Dawes, 2011</p>
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<p>Distributed under the Boost Software License, Version 1.0. See <a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/ LICENSE_1_0.txt</a></p>
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