What does a C preprocessor do?

The role of the C preprocessor is to handle preprocessor directives before the actual compilation of the code begins. This includes tasks such as including header files, defining macros, and conditional compilation. The preprocessor scans the source code for these directives, processes them, and produces a modified version of the code that serves as the input for the compiler. As a result, any replacements or file inclusions defined by the preprocessor take effect before the compiler turns the code into machine language.

In contrast, compiling code into machine language is the primary function of the compiler, not the preprocessor. Code optimization is typically conducted by the compiler or linker, which adjusts the generated machine code to improve performance after preprocessing. Lastly, executing functions during runtime pertains to how C programs operate after being compiled and linked, and is not within the preprocessor's duties. Thus, the key function of the preprocessor is to manage directives and prepare the source code for compilation.