C language functions have two basic requirements: declaration and definition. The declaration tells the compiler the function name, parameter type and return value type; the definition contains the specific implementation of the function. The parameter transfer method determines the way the function processes data (value transfer or pointer transfer), and the return value determines the execution result of the function. Common errors include forgetting declarations, mismatch of parameter types, and memory leaks. Performance optimization techniques include inline functions, and best practices recommend modular design.

C language functions: those underlying secrets you must know

Many people think that C language functions are very simple, aren’t they just code blocks with names? wrong! This is just a superficial phenomenon. To truly master C language functions, you need to understand its deep operating mechanism in order to write efficient, reliable and easy-to-maintain code. In this article, let’s take a look at the things about C language functions, so that you can advance from a novices to an expert.

Let’s talk about the conclusion first: The basic requirements of C language functions are actually two points: declaration and definition . But behind these two points, there are many details hidden, which will directly affect your program performance and stability. Ignore these details, your code may hide various bugs, making you suspicious of your life.

Declaration: This is like the ID card of a function, telling the compiler what name the function is, what type of parameters to accept, and what type of result to return. Without a declaration, the compiler will not know which function you plan to use, so naturally it will not be able to compile your code. Declarations are generally placed in header files (.h), so that multiple files can share the declaration of the same function.

 <code class="c">// 函數(shù)聲明，告訴編譯器函數(shù)的接口int add(int a, int b); //聲明一個名為add的函數(shù)，接收兩個int型參數(shù)，返回一個int型結(jié)果</code>

Definition: This is the true body of a function, which contains the specific implementation of the function. Definitions describe how the function works inside. A function can only be defined once, otherwise the compiler will report an error.

 <code class="c">// 函數(shù)定義，包含函數(shù)的具體實現(xiàn)int add(int a, int b) { return ab; // 函數(shù)體，執(zhí)行加法運(yùn)算并返回結(jié)果}</code>

What is the difference between a declaration and a definition? The declaration only tells the compiler the existence of the function, but does not contain the implementation details of the function; the definition contains the complete implementation of the function. You can think of a declaration as a "trailer" of a function, and the definition is the "true film". If there is no definition, only declaration, the compiler will look for the definition of the function in the linking stage. Not found, the link failed, the program cannot run.

In-depth details: Parameter passing and return value

The parameter passing method determines how the function handles the incoming data. C language mainly uses value transfer, that is, the function receives a copy of the parameters rather than the original data. This means that modifying the value of the parameter inside the function does not affect the value of the external variable. The pointer passes is different. It passes the memory address of the variable, and the function can directly modify the original data.

The return value determines the result of the function execution. Functions can return various types of values, including integers, floating point, character, pointers, etc., and may even not return values ??(void). The type of the return value must be consistent with the type specified in the function declaration.

Potential pitfalls and experience sharing

• Forgot to declare: This is the most common mistake made by novices. The compiler will report an error, prompting that the function cannot be found. Develop good programming habits and declare it before using a function.
• Parameter type mismatch: This will cause the program to run incorrectly and even crash. Double-check parameter types to make sure they are consistent with the type specified in the function declaration.
• Memory Leak: If a function dynamically allocates memory but forgets to release it, it will cause a memory leak. Develop good habits to free all dynamically allocated memory before the function ends.
• Stack Overflow of function recursive: The number of layers of recursive calls is too deep, which may cause stack overflow. Recursive functions need to be designed carefully to avoid infinite recursion.

Performance optimization: Inline functions

For some simple, small-code functions, inline functions can be used to improve performance. Inline functions will insert the function code directly into the call position during the compilation stage, avoiding the overhead of function calls. However, inline functions can also increase the size of the code, so use it with caution.

 <code class="c">inline int inline_add(int a, int b) { return ab; }</code>

Best Practice: Modular Design

Breaking the code into multiple independent functions can improve the readability, maintainability and reusability of the code. Each function should only be responsible for a specific task, which can make the code clearer and easier to understand.

In short, the understanding of C language functions cannot be on the surface. Only by deeply understanding the details of its declaration, definition, parameter passing, return values, etc., and mastering some performance optimization techniques and best practices can you write high-quality C code. Remember, details determine success or failure!

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