Where is the c language function declaration placed

Function declarations should be placed before the function is used, the same source file should be placed before the function definition, and different source files should be placed in the header file. Macro definitions should be used in the header file to prevent duplicate inclusion, ensuring the code style is clear and concise.

Where to put C function declaration? This problem seems simple, but it actually has a secret, which is related to how the compiler understands your code and thus affects the compilation and operation of the program. Many novices tend to fall into the pit and scratch their heads with a confused look on their faces.

To put it bluntly, the function declaration tells the compiler: "Hey, buddy, I have a function here, what is the name, what is the parameter type, what is the return value, I will give you a specific implementation in the future, just remember it for the time being." If there is no declaration, the compiler will be confused when he sees the function call, not knowing which function you are calling, how to pass the parameters, and how to handle the return value, so he will naturally report an error.

So, where should I put the function declaration? The safest way is to put the function declaration before the function is used. It’s like when you treat a meal, you have to show the menu to the guests first so that the guests can order the dishes, right? You can't just tell him what's on the menu after the customer has finished ordering.

To be more specific, this means:

• In the same source file: If the function declaration and function definition are in the same .c file, then just ensure that the function declaration appears before the function call. You can put the declaration at the beginning of the file or before calling the function, depending on how your code is organized and your personal preferences. I personally prefer to place all function declarations at the beginning of the file so that the code structure is clear and clear at a glance.
• In different source files: This is the real challenge. At this time, you need to use the header file ( .h file). Put the function declaration in the header file, and then include the header file in the .c file that needs to use the function ( #include "header.h" ). This is like establishing a "registration office" of a function, and each .c file goes to this "registration office" to query the function information. The advantage of this is that modifying the function declaration only requires modifying the header file, and all code using the function will be automatically updated.

For example, suppose we have two files: main.c and myfunc.c .

myfunc.h:

 <code class="c">#ifndef MYFUNC_H #define MYFUNC_H int add(int a, int b); // 函数声明#endif</code>

myfunc.c:

 <code class="c">#include "myfunc.h" int add(int a, int b) { return ab; }</code>

main.c:

 <code class="c">#include <stdio.h> #include "myfunc.h" int main() { int sum = add(5, 3); // 调用add函数printf("Sum: %d\n", sum); return 0; }</stdio.h></code>

Here, myfunc.h contains the declaration of the add function, myfunc.c contains the definition of the add function, main.c contains myfunc.h , so you can use the add function. If myfunc.h is not included in main.c , the compiler will report an error because main.c does not know the existence of the add function.

Go deeper and talk about your experience in trampling on pitfalls. Many newbies are prone to making mistakes: forget to add the three macro definitions of #ifndef , #define and #endif in the header file. The purpose of these three macro definitions is to prevent the header file from being repeatedly included and avoid compilation errors. If these three sentences are not added, the compiler may include the same header file multiple times, causing the function declaration to be repeated, resulting in an error. Remember, this is a very important detail, and many compilation errors originate from it.

Finally, code style and readability are also important. Clear and concise code is easier to understand and maintain, and it is easier to avoid errors. Developing good programming habits is crucial to your programming career. Remember, the code is written for people to see, and the second is for machines to see.

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