关键字是已被C语言标准作为命令、数据类型或者固定函数名的字母组合。关键字不能被用做变量名或函数名。下面列举了C语言的32个关键字，它们遵循C语言的语法使用，形成了C程序设计语言。
| 序号 | 关键字 | 说明 |
| 1 | auto | |
| 2 | break | |
| 3 | case | |
| 4 | char | |
| 5 | const | |
| 6 | continue | |
| 7 | default | |
| 8 | do | |
| 9 | double | |
| 10 | else | |
| 11 | enum | |
| 12 | extern | |
| 13 | float | |
| 14 | for | |
| 15 | goto | |
| 16 | if | |
| 17 | int | |
| 18 | long | |
| 19 | register | |
| 20 | short | |
| 21 | signed | |
| 22 | sizeof | |
| 23 | static | |
| 24 | return | |
| 25 | struct | |
| 26 | switch | |
| 27 | typedef | |
| 28 | union | |
| 29 | unsigned | |
| 30 | void | |
| 31 | volatile | |
| 32 | while |