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180 lines
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180 lines
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<h1><a href=index.html>sjs' blog</a></h1>
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<h1><a href=working-with-c-style-structs-in-ruby.html>Working with C-style structs in Ruby</a></h1>
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<time>January 17, 2010</time>
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</header>
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<p>This is the beginning of a series on generating Mach-O object files in
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Ruby. We start small by introducing some Ruby tools that are useful when
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working with binary data. Subsequent articles will cover a subset of the
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Mach-O file format, then generating Mach object files suitable for linking
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with ld or gcc to produce working executables. A basic knowledge of Ruby and C
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are assumed. You can likely wing it on the Ruby side of things if you know any
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similar languages.</p>
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<p>First we need to read and write structured binary files with Ruby. <a
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href="http://ruby-doc.org/core/classes/Array.html#M002222">Array#pack</a> and
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<a
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href="http://ruby-doc.org/core/classes/String.html#M000760">String#unpack</a>
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get the job done at a low level, but every time I use them I have to look up
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the documentation. It would also be nice to encapsulate serializing and
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deserializing into classes describing the various binary data structures. The
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built-in <a href="http://ruby-doc.org/core/classes/Struct.html">Struct
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class</a> sounds promising but did not meet my needs, nor was it easily
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extended to meet them.</p>
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<p>Meet <a
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href="http://github.com/samsonjs/compiler/blob/master/asm/cstruct.rb#files">CStruct</a>,
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a class that you can use to describe a binary structure, somewhat similar to
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how you would do it in C. Subclassing CStruct results in a class whose
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instances can be serialized, and unserialized, with little effort. You can
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subclass descendants of CStruct to extend them with additional members.
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CStruct does not implement much more than is necessary for the compiler. For
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example there is no support for floating point. If you want to use this for
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more general purpose tasks be warned that it may require some work. Anything
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supported by Array#pack is fairly easy to add though.</p>
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<p>First a quick example and then we'll get into the CStruct class itself. In
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C you may write the following to have one struct "inherit" from another:</p>
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<p><script src="http://gist.github.com/279790.js"></script></p>
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<p>With CStruct in Ruby that translates to:</p>
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<p><script src="http://gist.github.com/279794.js"></script></p>
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<p>CStructs act like Ruby's built-in Struct to a certain extent. They are
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instantiated the same way, by passing values to #new in the same order they
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are defined in the class. You can find out the size (in bytes) of a CStruct
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instance using the #bytesize method, or of any member using #sizeof(name).</p>
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<p>The most important method (for us) is #serialize, which returns a binary
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string representing the contents of the CStruct.</p>
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<p>(I know that CStruct.new_from_bin should be called CStruct.unserialize, you
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can see where my focus was when I wrote it.)</p>
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<p>CStruct#serialize automatically creates a "pack pattern", which is an array
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of strings used to pack each member in turn. The pack pattern is mapped to the
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result of calling Array#pack on each corresponding member, and then the
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resulting strings are joined together. Serializing strings complicates matters
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so we cannot build up a pack pattern string and then serialize it in one go,
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but conceptually it's quite similar.</p>
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<p>Unserializing is the same process in reverse, and was mainly added for
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completeness and testing purposes.</p>
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<p>That's about all you need to know to use CStruct. The code needs some work
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but I decided to just go with what I have already so I can get on with the
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more interesting and fun tasks.</p>
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<p><i>Next in this series: <a
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href="basics-of-the-mach-o-file-format.html">Basics
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of the Mach-O file format</a></i><p>
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</article>
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<p class=sep>༄</p>
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<div id=around>
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<a id=next href=basics-of-the-mach-o-file-format.html>Basics of the Mach-O file format →</a>
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