The declaration of a variable is different from the definition of a variable, which is often confusing.
l The declaration of a variable tells the compiler that the variable name already exists, and the compiler recognizes the name and will not cause a compilation error.
l After the variables are defined, the compiler allocates space for the variables, and the linker can find their addresses when linking.
Declaring a variable tells the compiler that the variable name already exists, but no space has been allocated for it. That is to say, a variable is declared. If the variable is referenced in the program, it can be compiled. However, if the variable is not defined in a file, the link will go wrong, because the variable is required when linking the object file. the exact address.