The C Programming Language 附录 A ，A.8 Declarations 中说：

```
Declarations specify the interpretation given to each identifier; they do not necessarily reserve storage associated with the identifier. Declarations that reserve storage are called definitions.
```

所以，首先要明白，definition 是 declaration 的一种，区别在于 definition 为变量预留了内存
所以，你的问题应该修改为：int a; 有没有预留内存。

答案是：大多数情况下预留了内存。只有一种情况，就是写在函数外面

A.10.2 External Declarations 中说：

```
An external object declaration that does not have an initializer, and does not contain the extern specifier, is a tentative definition. If a definition for an object appears in a translation unit, any tentative definitions are treated merely as redundant declarations.
```

意思是，int a; 写在函数外面，编译器先会将其待定；如果发现整个编译单元（一般是一个预处理过的源文件）中有多个这样的写法，则多余的就被当作没有预留内存的 declarations 了