What are the rules for function definition and call in C language?

A C language function consists of a parameter list, function body, return value type and function name. When a function is called, the parameters are copied to the function through the value transfer mechanism, and will not affect external variables. Pointer passes directly to the memory address, modifying the pointer will affect external variables. Function prototype declaration is used to inform the compiler of function signatures to avoid compilation errors. Stack space is used to store function local variables and parameters. Too much recursion or too much space can cause stack overflow.

C language function: the mystery of definition and call

You may think that the definition and call of C functions are very simple, aren’t it just int func(int a){...} and func(5); ;? wrong! This is just a superficial phenomenon, and there are many subtle mechanisms hidden behind it. Only by understanding these can your C language skills be taken to a higher level. Let’s take a look at this layer of skin in this article.

Let’s talk about the definition first. int func(int a){/*函数体*/} is simple, but has hidden mystery. int is the return value type, telling the compiler that the function will spit out an integer after finishing work; func is the function name, you have to give it a nice name, don't use keywords; int a is the parameter list, there is only one integer type parameter a , you can have multiple parameters or no parameters (just be empty in brackets at this time), the parameter type must be clearly declared. Function body {/*函数体*/} is the specific implementation of the function, and you write your code logic here.

What about function calls? func(5); seems simple, but in fact it contains a series of operations such as parameter transfer, function jump, and return value processing. The compiler will find the entry address of the function according to the function definition, pass the value 5 to a , and then jump to the function body to execute. After execution, if the function has a return value, the return value will be assigned to the calling statement.

It looks simple, but there are many pitfalls in actual operation. For example, parameter passing. In C language, the default is value transfer, that is, copy the value of the parameter to the function. Modifying the value of the parameter inside the function will not affect the external variables. But this can cause performance issues, especially when passing large data structures. At this time, you can consider passing pointer, directly passing memory address, and modifying the content pointed to by the pointer will affect external variables. Remember, pointer delivery is powerful, but if you are not careful, it will cause memory leakage and other problems.

For example, function prototype declaration. You have to declare the function prototype before calling the function, telling the compiler the return value type, parameter type and number of functions. Otherwise, the compiler will report an error because he does not know the signature of the function. It's like knowing the other person's number before you make a call.

Another problem that is easy to ignore is stack space. Function calls use stack space to store information such as local variables, parameters, and return addresses. If the function calls too many layers recursively, or the local variables take up too much space, it may cause stack overflow and the program to crash. Therefore, when writing functions, you should control the use of stack space, avoid the recursion depth being too deep, and minimize the space occupied by local variables.

Finally, I want to end this article with an example that shows the usage of pointer passing and function prototypes, and deliberately creates some minor errors for your thinking:

 <code class="c">#include <stdio.h> // 函数原型声明，注意返回值类型和参数类型int modifyValue(int *ptr); int main() { int num = 10; // 调用函数，传递num的地址int result = modifyValue(&num); printf("The modified value is: %d\n", num); // 输出修改后的值return 0; } // 函数定义，修改指针指向的值int modifyValue(int *ptr) { *ptr = 20; // 注意这里使用指针解引用修改值return *ptr; // 返回修改后的值}</stdio.h></code>

This example seems simple, but if you carefully analyze parameter passing, pointer operation and return value, you can have a deeper understanding of the definition and calling rules of C functions. Remember, programming is a craft. Only by practicing more and thinking more can you become a real programming master. Don’t just look at it, it’s the best way to type code!

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