Previous C compilers did not follow such a simple sequence of phases, and the order in which these steps were applied was not predictable. A separate preprocessor recognized tokens and white space at essentially the same time as it replaced macros and handled directive lines. The output was then completely retokenized by the compiler proper, which then parsed the language and generated code.
The tokenization process within the preprocessor was a moment-by-moment operation and macro replacement was done as a character-based, not token-based, operation. Therefore, the tokens and white space could greatly vary during preprocessing.
A number of differences arise from these two approaches. The rest of this section discusses how code behavior can change due to line splicing, macro replacement, stringizing, and token pasting, which occur during macro replacement.