What is the difference between declaration and definition of C user identifiers?

In C language, the declaration of an identifier declares its existence in the compiler, while the definition gives it a specific value based on the declaration. Declarations can be repeated, while definitions can only occur once and must be in appropriate .c files, usually used in header files, and definitions in .c files.

In C language, the declaration and definition of identifiers are to put it bluntly to tell the compiler the difference between "I know there is such a thing" and "what is this thing". These two sound like each other, but they are actually far apart. If you can't figure this out, no matter how fancy your code is, it is easy to cause trouble.

You think, you have to tell the compiler what tools you plan to use, right? That's what the statement means. It tells the compiler: "Hey, I plan to use a variable called myVariable , it is an integer." But it doesn't say what the specific value of myVariable is. It's like you booked a table but haven't sat down yet.

What is the definition? Only by defining things truly “create”. It not only tells the compiler that myVariable is an integer, but also assigns a value to it, such as myVariable = 10; . This is equivalent to not only booking a table, but also sitting down, and this table is yours now.

Therefore, only the declaration is of type and the definition is of value. Declaration can be made multiple times, and definition can only be made once. You can declare the same variable in multiple places and tell the compiler "I want to use it here too", but you can only define it in one place and assign it the initial value.

Look at the code and you will understand:

 <code class="c">// 声明：告诉编译器有个叫myVar的整型变量，但还没赋值extern int myVar; // 定义：创建myVar这个变量，并赋值为5 int myVar = 5; void myFunction() { // 声明：在这个函数里，我也要使用myVar extern int myVar; // 使用myVar myVar = 10; } int main() { // 使用myVar printf("myVar = %d\n", myVar); myFunction(); printf("myVar = %d\n", myVar); return 0; }</code>

In this code, extern int myVar; is a declaration, which tells the compiler that the variable myVar is defined elsewhere, and I want to use it here. And int myVar = 5; is the definition, it creates myVar and assigns values. You can declare the same global variable in multiple .c files, but you can only define it in one .c file. Otherwise, the compiler will go crazy because it doesn't know what myVar is.

Let me remind you again: header files ( .h files) are usually used to declare, not define. Definitions are usually placed in .c files. If you define global variables in a header file and multiple files contain this header file, it will lead to repeated definition errors and the compiler will report errors without mercy. This is a common pitfall! Remember, declarations can be made multiple times, definitions can only be made once, and they should be placed in a suitable .c file. This is the essence of C language and it is also a place where many beginners are prone to make mistakes. You must understand thoroughly in order to write robust and efficient C code.

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