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The Art of Unix Programming
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Unix Programming - Studying Cases - Case Study: GCC

Case Study: GCC

GCC, the GNU C compiler used on most modern Unixes, is perhaps an even better example of engineering for transparency. GCC is organized as a sequence of processing stages knit together by a driver program. The stages are: preprocessor, parser, code generator, assembler, and linker.

Each of the first three stages takes in a readable textual format and emits a readable textual format (the assembler has to emit and the linker to accept binary formats, pretty much by definition). With various command-line options of the gcc(1) driver, you can see not just the results after C preprocessing, after assembly generation, and after object code generation — but you can also monitor the results of many intermediate steps in parsing and code generation.

This is exactly the structure of cc, the first (PDP-11) C compiler.

-- Ken Thompson

There are many benefits of this organization. One that is particularly important for GCC is regression testing.[60] Because most of the various intermediate formats are textual, deviations from expected results in a regression test are easily spotted and analyzed using simple textual diff operations on the intermediate results; there is no need for specialist dump-analysis tools that may well harbor their own bugs, and in any case would represent an additional maintenance burden.

The design pattern to extract from this example is that the driver program has monitoring switches that merely (but sufficiently) expose the textual data flows among the components. As with fetchmail's -v option, these options are not afterthoughts; they are designed in for discoverability.


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The Art of Unix Programming
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