diff --git a/doc/endian.adoc b/doc/endian.adoc index eeb0e86..524dede 100644 --- a/doc/endian.adoc +++ b/doc/endian.adoc @@ -18,10 +18,10 @@ Beman Dawes include::endian/overview.adoc[] include::endian/changelog.adoc[] +include::endian/choosing_approach.adoc[] include::endian/conversion.adoc[] include::endian/buffers.adoc[] include::endian/arithmetic.adoc[] -include::endian/choosing_approach.adoc[] include::endian/history.adoc[] :leveloffset: -1 diff --git a/doc/endian/choosing_approach.adoc b/doc/endian/choosing_approach.adoc index eb318ff..e1ee0e7 100644 --- a/doc/endian/choosing_approach.adoc +++ b/doc/endian/choosing_approach.adoc @@ -6,19 +6,15 @@ Distributed under the Boost Software License, Version 1.0. //// [#choosing] -# Choosing Approach +# Choosing between Conversion Functions, Buffer Types, and Arithmetic Types :idprefix: choosing_ -## Introduction - -Deciding which is the best endianness approach (conversion functions, buffer +NOTE: Deciding which is the best endianness approach (conversion functions, buffer types, or arithmetic types) for a particular application involves complex engineering trade-offs. It is hard to assess those trade-offs without some understanding of the different interfaces, so you might want to read the <>, <>, and -<> pages before diving into this page. - -## Choosing between conversion functions, buffer types, and arithmetic types +<> pages before proceeding. The best approach to endianness for a particular application depends on the interaction between the application's needs and the characteristics of each of @@ -30,7 +26,7 @@ invest the time to study engineering trade-offs, use maintain. Use the _<>_ design pattern locally around performance hot spots like lengthy loops, if needed. -### Background +## Background A dealing with endianness usually implies a program portability or a data portability requirement, and often both. That means real programs dealing with @@ -39,13 +35,13 @@ written as multiple functions spread across multiple translation units. They would involve interfaces that can not be altered as they are supplied by third-parties or the standard library. -### Characteristics +## Characteristics The characteristics that differentiate the three approaches to endianness are the endianness invariants, conversion explicitness, arithmetic operations, sizes available, and alignment requirements. -#### Endianness invariants +### Endianness invariants *Endian conversion functions* use objects of the ordinary {cpp} arithmetic types like `int` or `unsigned short` to hold values. That breaks the implicit @@ -121,7 +117,7 @@ write(data); A later maintainer can add `third_party::func(data.v3)` and it will just-work. -#### Conversion explicitness +### Conversion explicitness *Endian conversion functions* and *buffer types* never perform implicit conversions. This gives users explicit control of when conversion occurs, and @@ -131,7 +127,7 @@ may help avoid unnecessary conversions. very easy to use, but can result in unnecessary conversions. Failure to hoist conversions out of inner loops can bring a performance penalty. -#### Arithmetic operations +### Arithmetic operations *Endian conversion functions* do not supply arithmetic operations, but this is not a concern since this approach uses ordinary {cpp} arithmetic types to hold @@ -153,7 +149,7 @@ That's sufficient for many applications. integers. For an application where memory use or I/O speed is the limiting factor, using sizes tailored to application needs can be useful. -#### Alignments +### Alignments *Endianness conversion functions* only support aligned integer and floating-point types. That's sufficient for most applications. @@ -180,7 +176,7 @@ struct S { }; ``` -### Design patterns +## Design patterns Applications often traffic in endian data as records or packets containing multiple endian data elements. For simplicity, we will just call them records. @@ -189,7 +185,7 @@ If desired endianness differs from native endianness, a conversion has to be performed. When should that conversion occur? Three design patterns have evolved. -#### Convert only as needed (i.e. lazy) +### Convert only as needed (i.e. lazy) This pattern defers conversion to the point in the code where the data element is actually used. @@ -198,7 +194,7 @@ This pattern is appropriate when which endian element is actually used varies greatly according to record content or other circumstances [#choosing_anticipating_need] -#### Convert in anticipation of need +### Convert in anticipation of need This pattern performs conversion to native endianness in anticipation of use, such as immediately after reading records. If needed, conversion to the output @@ -213,7 +209,7 @@ from native to the desired output endianness. This pattern is appropriate when all endian elements in a record are typically used regardless of record content or other circumstances. -#### Convert only as needed, except locally in anticipation of need +### Convert only as needed, except locally in anticipation of need This pattern in general defers conversion but for specific local needs does anticipatory conversion. Although particularly appropriate when coupled with the @@ -264,9 +260,9 @@ cost might be significant if the loop is repeated enough times. On the other hand, the program may be so dominated by I/O time that even a lengthy loop will be immaterial. -### Use case examples +## Use case examples -#### Porting endian unaware codebase +### Porting endian unaware codebase An existing codebase runs on big endian systems. It does not currently deal with endianness. The codebase needs to be modified so it can run on little @@ -279,7 +275,7 @@ needs. A relatively small number of header files dealing with binary I/O layouts need to change types. For example, `short` or `int16_t` would change to `big_int16_t`. No changes are required for `.cpp` files. -#### Porting endian aware codebase +### Porting endian aware codebase An existing codebase runs on little-endian Linux systems. It already deals with endianness via @@ -293,7 +289,7 @@ just mechanically changes the calls to `htobe32`, etc. to `boost::endian::native_to_big`, etc. and replaces `` with ``. -#### Reliability and arithmetic-speed +### Reliability and arithmetic-speed A new, complex, multi-threaded application is to be developed that must run on little endian machines, but do big endian network I/O. The developers believe @@ -304,7 +300,7 @@ slow conversions if full-blown endian arithmetic types are used. The <> approach is made-to-order for this use case. -#### Reliability and ease-of-use +### Reliability and ease-of-use A new, complex, multi-threaded application is to be developed that must run on little endian machines, but do big endian network I/O. The developers believe diff --git a/doc/endian/overview.adoc b/doc/endian/overview.adoc index 9a3937e..6086db3 100644 --- a/doc/endian/overview.adoc +++ b/doc/endian/overview.adoc @@ -83,7 +83,8 @@ Boost.Endian provides three different approaches to dealing with endianness. All three approaches support integers and user-define types (UDTs). Each approach has a long history of successful use, and each approach has use -cases where it is preferred to the other approaches. +cases where it is preferred to the other approaches. See +<>. <>:: The application uses the built-in integer types to hold values, and calls the @@ -113,10 +114,6 @@ Boost Endian is a header-only library. {cpp}11 features affecting interfaces, such as `noexcept`, are used only if available. See <> for details. -## Choosing between conversion functions, buffer types, and arithmetic types - -This section has been moved to its own <> page. - [#overview_intrinsics] ## Built-in support for Intrinsics @@ -260,6 +257,28 @@ Iterations: 10'000'000'000, Intrinsics: `` `_byteswap_ushort`, etc. |64-bit aligned little endian |3.35 s |2.73 s |=== +[#overview_cpp03_support] +## {cpp}03 support for {cpp}11 features + +[%header,cols=2*] +|=== +|{cpp}11 Feature +|Action with {cpp}03 Compilers +|Scoped enums +|Uses header +http://www.boost.org/libs/core/doc/html/core/scoped_enum.html[boost/core/scoped_enum.hpp] +to emulate {cpp}11 scoped enums. +|`noexcept` +|Uses `BOOST_NOEXCEPT` macro, which is defined as null for compilers not +supporting this {cpp}11 feature. +|{cpp}11 PODs +(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2342.htm[N2342]) +|Takes advantage of {cpp}03 compilers that relax {cpp}03 POD rules, but see +Limitations <> and <>. +Also see macros for explicit POD control <> and +<> +|=== + [#overview_faq] ## Overall FAQ @@ -326,11 +345,11 @@ the same code being generated for either types. What are the limitations of integer support?:: Tests have only been performed on machines that use two's complement -arithmetic. The Endian conversion functions only support 16, 32, and 64-bit +arithmetic. The Endian conversion functions only support 8, 16, 32, and 64-bit aligned integers. The endian types only support 8, 16, 24, 32, 40, 48, 56, and 64-bit unaligned integers, and 8, 16, 32, and 64-bit aligned integers. -Why is there no floating point support?:: +Is there floating point support?:: An attempt was made to support four-byte ``float``s and eight-byte ``double``s, limited to http://en.wikipedia.org/wiki/IEEE_floating_point[IEEE 754] (also known as @@ -338,29 +357,9 @@ ISO/IEC/IEEE 60559) floating point and further limited to systems where floating point endianness does not differ from integer endianness. Even with those limitations, support for floating point types was not reliable and was removed. For example, simply reversing the endianness of a floating point number can -result in a signaling-NAN. For all practical purposes, binary serialization and -endianness for integers are one and the same problem. That is not true for -floating point numbers, so binary serialization interfaces and formats for -floating point does not fit well in an endian-based library. - -[#overview_cpp03_support] -## {cpp}03 support for {cpp}11 features - -[%header,cols=2*] -|=== -|{cpp}11 Feature -|Action with {cpp}03 Compilers -|Scoped enums -|Uses header -http://www.boost.org/libs/core/doc/html/core/scoped_enum.html[boost/core/scoped_enum.hpp] -to emulate {cpp}11 scoped enums. -|`noexcept` -|Uses `BOOST_NOEXCEPT` macro, which is defined as null for compilers not -supporting this {cpp}11 feature. -|{cpp}11 PODs -(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2342.htm[N2342]) -|Takes advantage of {cpp}03 compilers that relax {cpp}03 POD rules, but see -Limitations <> and <>. -Also see macros for explicit POD control <> and -<> -|=== +result in a signaling-NAN. ++ +Support for `float` and `double` has since been reinstated for `endian_buffer` +and `endian_arithmetic`. The conversion functions still do not support floating +point due to the above issues; reversing the bytes of a floating point number +does not necessarily produce another valid floating point number.