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In more detail, in a compiler, the lexer performs one of the earliest stages of converting the source code to a program. It scans the text to extract meaningful tokens, such as words, numbers, and strings. The parser analyzes sequences of tokens attempting to match them to syntax rules representing language structures, such as loops and ...
The CPP is run during the compilation of each translation unit before the C compiler. The build process (e.g. make) can be implemented in a conditional way such that just different versions of a symbol are created or different (specialized) libraries are used and linked depending on the target.
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A more complex example is the lexer hack in C, where the token class of a sequence of characters cannot be determined until the semantic analysis phase since typedef names and variable names are lexically identical but constitute different token classes. Thus in the hack, the lexer calls the semantic analyzer (say, symbol table) and checks if ...
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Any object code produced by compilers is usually linked with other pieces of object code (produced by the same or another compiler) by a type of program called a linker. The linker needs a great deal of information on each program entity. For example, to correctly link a function it needs its name, the number of arguments and their types, and ...
an atom is either a number or a symbol; a number is an unbroken sequence of one or more decimal digits, optionally preceded by a plus or minus sign; a symbol is a letter followed by zero or more of any characters (excluding whitespace); and; a list is a matched pair of parentheses, with zero or more expressions inside it.
A compiler may use one large symbol table for all symbols or use separated, or hierarchical symbol tables for different scopes. For example, in a scoped language such as Algol or PL/I a symbol "p" can be declared separately in several procedures, perhaps with different attributes. The scope of each declaration is the section of the program in ...