C-language

1. Syntax
2. Preprocessor
- 2.1. list of important directives
- 2.2. macro
3. Library management
- 3.1. Using header file
  - 3.1.1. avoid including twice
  - 3.1.2. computed include
4. variable
5. File manipulation
6. Memory management
7. Compiling and Execution
8. Usage
9. resources
Backlinks
- programming languages

The C programming language

1. Syntax

1.1. Functions

1.2. Control Flow

1.3. Arithmatics

1.4. Data structures

2. Preprocessor

C Preprocessor is a text substitution tool that work with directives. Before feeding source code to compiler, Preprocessor would scan the source code file and substitue all directives according to their meanings.

2.1. list of important directives

directive	meaning
`#define`	macro def
`#include`	include header file
`#undef`	undefine a macro
`#ifdef`	return true if macro is defined
`#ifndef`	return ture if macro is undefined
`#if`
`#else`
`#elif`
`#endif`
`#error`	print error message on stderr
`#pragma`

2.2. macro

this link

3. Library management

use and create library in c

3.1. Using header file

header file: file.h containing C functions and macros used by multiple source files.

#include directive

header file could be included with #include directive.

#include <stdio.h> search a standard list of system directories for "stdio.h" (like /usr/include/ on ubantu). This list could be prepended with -I compiler option
#include "file.h" search the current directory for file.h

what #include does

when #include directive is scanned, the preprocessor would go to find and scan the given header file.

as result, the compiled program would be equivalent to replace #include "file.h" with content of file.h. this link

3.1.1. avoid including twice

to avoid a header file being included twice, could use:

#ifndef HEADER_FILE
#define HEADER_FILE

/* the entire header file file */

#endif

#ifndef directive would ignore stuff until #endif if HEADER_FILE is defined

3.1.2. computed include

you can select which header file to include with

#define SYSTEM_H "system_1.h"
...
#include SYSTEM_H

SYSTEM_H is a macro
SYSTEM_H could be given to compiler with "-D" option, and thus specified in makefile

4. variable

4.1. constant

4.2. enum

4.3. variable

5. File manipulation

6. Memory management

7. Compiling and Execution

8. Usage

C can be used to manipulate OS concepts like process directly(and of course used to write OS itself).