# The goal being to be able to understand C code, rather than writing it effectively.
## C preprocessor
Produces a new file that compiler will process. Command to preprocessor start line with a # symbol as the first non-whitespace character.

\#include : use to pull the contents of one file into the current source file.

For ex to pull the entire content of `multiprocessing.h` :

```
#include "multiprocessing.h"
```

\#define : lets you define a new symbol that gets replaced with a text string .

For ex. `#define SEM_FAILED NULL` replace every instance of `SEM_FAILED` with the literal string `NULL` before the code is sent to the compiler.

\#under : erases any previous preprocessor definition from \#define. You can have \#define effect only a section of the file.

\#if : conditional statements, which allow to include/exclude certain sections of the text based on certain conditions.

There are three basic forms of \#if in CPython source:

1. \#ifdef <macro\> includes the subsequent text-block if the macro is defined.
2. \#ifndef <macro\> includes the text-block if the specified macro is not defined.
3. \#if <macro\> includes the text-block if macro defined and True.

\#pragma : instructions or hint to the compiler. Usually deals with how code is compiled not how it runs.

\#error : displays a message and cause the preprocessor to stop executing.

### Preprocessor directives

### C syntax compared to python.

### Loops, functions and strings.