The GNU C Preprocessor Internals: Hash Nodes

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## Hash Nodes

When cpplib encounters an “identifier”, it generates a hash code for it and stores it in the hash table. By “identifier” we mean tokens with type `CPP_NAME`; this includes identifiers in the usual C sense, as well as keywords, directive names, macro names and so on. For example, all of `pragma`, `int`, `foo` and `__GNUC__` are identifiers and hashed when lexed.

Each node in the hash table contain various information about the identifier it represents. For example, its length and type. At any one time, each identifier falls into exactly one of three categories:

- Macros

  These have been declared to be macros, either on the command line or with `#define`. A few, such as `__TIME__` are built-ins entered in the hash table during initialization. The hash node for a normal macro points to a structure with more information about the macro, such as whether it is function-like, how many arguments it takes, and its expansion. Built-in macros are flagged as special, and instead contain an enum indicating which of the various built-in macros it is.

- Assertions

  Assertions are in a separate namespace to macros. To enforce this, cpp actually prepends a `#` character before hashing and entering it in the hash table. An assertion’s node points to a chain of answers to that assertion.

- Void

  Everything else falls into this category—an identifier that is not currently a macro, or a macro that has since been undefined with `#undef`.

  When preprocessing C++, this category also includes the named operators, such as `xor`. In expressions these behave like the operators they represent, but in contexts where the spelling of a token matters they are spelt differently. This spelling distinction is relevant when they are operands of the stringizing and pasting macro operators `#` and `##`. Named operator hash nodes are flagged, both to catch the spelling distinction and to prevent them from being defined as macros.

The same identifiers share the same hash node. Since each identifier token, after lexing, contains a pointer to its hash node, this is used to provide rapid lookup of various information. For example, when parsing a `#define` statement, CPP flags each argument’s identifier hash node with the index of that argument. This makes duplicated argument checking an O(1) operation for each argument. Similarly, for each identifier in the macro’s expansion, lookup to see if it is an argument, and which argument it is, is also an O(1) operation. Further, each directive name, such as `endif`, has an associated directive enum stored in its hash node, so that directive lookup is also O(1).

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