What are the default definition types for the return value of C language function?

The default return value type of C language function is int, but if it is not explicitly declared, it may cause errors such as overflow, precision loss, etc. Therefore, it is crucial to develop the habit of explicitly declaring return value types, including: Returning floating point numbers should be declared as float or double Return pointers should be explicitly declared that the pointer type should be dynamically allocated memory using malloc, freeing memory where the function is called to avoid memory leaks

The secret world of C function returns values: default types and hidden traps

Many beginners will ask: C function returns a value. If the type is not explicitly declared, what exactly is it? The answer is: it defaults to int . But this is just a superficial phenomenon, and there are many details that are worth pondering behind it, and it may even become a breeding ground for difficult bugs in your code.

Let's first review the basics of C language. Functions are the basic component of C programs, which receive input, process, and return results. This "result" is the return value, which is passed to the caller through return statement. If the return value type is not explicitly specified, the compiler will interpret it as int type by default. This seems simple and clear, but this default mechanism has hidden mystery.

Imagine that you wrote a function that calculates the sum of two numbers, but forgot to declare the return value type:

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

The compiler will silently treat it as int add(int a, int b) . This seems to be fine, but if your values ​​of a and b are large and exceed the representation range of int , overflow will occur, resulting in an error in the result. Worse, this error may not appear immediately, but lurk in your program until a specific input condition triggers, revealing its ugly face.

In order to avoid this hidden danger, it is crucial to develop good programming habits. Always explicitly declare the return value type of a function , which not only improves the readability of the code, but also avoids potential errors. A clear type declaration allows the compiler to perform stricter type checks to detect and report potential problems early.

Let's take a look at some more advanced usages. If you need to return a float, you must declare it as float or double :

 <code class="c">double calculateAverage(double a, double b) { return (ab) / 2.0; }</code>

Here, we explicitly declare the return value type as double , ensuring the accuracy of the calculation result. If int is still used by default here, the fractional part will be lost, resulting in a loss of accuracy.

Let’s look at a more complicated situation: return pointer. The pointer type also needs to be explicitly declared:

 <code class="c">char* getString() { char* str = (char*)malloc(100); // 动态分配内存strcpy(str, "Hello, world!"); return str; }</code>

Here is a pointer to a character array. If the return value type is declared incorrectly, it will cause the program to crash or unpredictable behavior. More importantly, after using malloc to dynamically allocate memory, remember to free up memory where the function is called to avoid memory leaks. This part of memory management is a very important aspect of C language programming, and a little carelessness will cause huge problems.

In short, although the default function returns a value type int in the C language, this does not mean that you can ignore the declaration of the return value type. Clearly and explicitly declare the return value type of a function is the key to writing high-quality, robust C code . This is not only a matter of coding specifications, but also an important factor related to the correctness and stability of the program. Remember to develop good coding habits and start from every detail to avoid those "mines" lurking in the code.

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