How to describe variables: storage class, life cycle, scope, link attribute

According to the storage class of a variable 、 Scope 、 Link properties 、 Life cycle to describe the variable .

among , Storage classes determine the lifecycle , The scope determines the link properties .

Storage class

Storage classes indicate where variables are stored . See Bowen Linux Next C Memory layout of language programs _ Tiantutu's blog -CSDN Blog

Scope

Scope indicates the scope of the variable . Include ：

（1） Code block scope , That is, a piece of code within curly brackets .

（2） Function prototype scope .

（3） File scope , That is, variables defined outside all functions , It is visible from its definition to the end of the file , Also called global variables .

Link properties

Link properties , It refers to whether variables are shared among multiple files .

Link properties , Include empty links 、 External links 、 internal link .

Empty connection means that only the function prototype or code block where the variable is located is allowed to use .

Variables in the file scope may have external links or internal links , Variables with external links can be used anywhere in a multi file program （ use extern modification ）, Variables with internal links can be used anywhere in a file （ use static modification ）.

Life cycle

The life cycle indicates when to allocate and reclaim memory for variables .

The lifecycle includes static storage periods 、 Automatic storage period ：

Static storage period , Indicates that this variable has been present during program execution .

Automatic storage period , When the program enters the code block that defines these variables , Will allocate memory for these variables , When exiting this code block , The allocated memory will be freed ;