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The GNU C preprocessor is implemented as a library, cpplib, so it can be easily shared between a stand-alone preprocessor, and a preprocessor integrated with the C, C++ and Objective-C front ends. It is also available for use by other programs, though this is not recommended as its exposed interface has not yet reached a point of reasonable stability.
The library has been written to be re-entrant, so that it can be used to preprocess many files simultaneously if necessary. It has also been written with the preprocessing token as the fundamental unit; the preprocessor in previous versions of GCC would operate on text strings as the fundamental unit.
This brief manual documents the internals of cpplib, and explains some of the tricky issues. It is intended that, along with the comments in the source code, a reasonably competent C programmer should be able to figure out what the code is doing, and why things have been implemented the way they have.