[C Advanced] 8, goto and void analysis

Summary

1) In general engineering development, is required to disable the goto statement. Different compilers may have different error handling for the goto statement.

2) In C language, if the function does not write a return value, the default return value type of is int; if the function does not write a parameter list, the function accepts any number of parameters.
Therefore, if the function does not return a value, you must explicitly declare the return value type as void; if the function has no parameters, you must declare the parameter list as void.

3) Void is a basic type, but is not a basic data type, so cannot be used to define variables (the C language does not specify how much void is an alias for memory space).

4) For standard C language specification, sizeof(void) is not allowed, such as bcc compiler; for extended C language, sizeof(void) may be allowed, such as gcc compiler, the output is 1.

5) The main function of the void* used to receive any type of pointer. In C language, void* and type* can be converted to each other; in C++, void* can be used as an lvalue to receive any type of pointer, when used as an rvalue, must be forced to convert.

goto and void

1、goto

The goto statement has strong assembly language features and can jump to the specified statement, just like the jump instruction of assembly language.
In engineering practice, generally disables "goto". Goto will destroy the structure of the program and bring unexpected errors in .

// 代码示例：以下代码输出什么？

int main()
{
goto End;
    int i = 0;
End:
    printf("i = %d\n", i);
    return 0;
}

• gcc compiler: Compile error error , suggesting that the initialization of the i variable was skipped.
• VS compiler: Compiler warning warning , indicating that i is not initialized, but an executable program has been compiled. The result of the operation is a random value, an unexpected error!

2、void

2.1 Void modifies the return value and parameters

In C language:

• If the function does not return a value, it should be declared as void
• If the function has no parameters, its parameters should be declared as void

Note that in C language:

• If the function does not write a return value, the default return value type of

• If the function does not declare a parameter list, accepts any number of parameters by default

  f()
  {
  }
  
  int main()
  {
    int i = f(1, 2, 3);        // ok
  
    return 0;
  }

2.2 Is there a variable of type void?

Conclusion: does not exist a variable of type void

Void is a basic type, but is not a basic data type; the C language does not define how much memory void is an alias, so the corresponding variable cannot be defined.

int main()
{
    void var;        // error
    void arr[5];     // error

    return 0;
}

Tips:

• ANSI C : Standard C language specification, such as bcc compiler

• extended C: extended on the basis of ANSI C, such as gcc compiler

  // void类型有大小么？
  
  int main()
  {
    prinft("%d\n", sizeof(void));
    return 0;
  }

• gcc compiler, Demo can be edited, the output is 1
• Bcc compiler, compile error, prompt void is not allowed type.

2.3 Pointer of void type

• In the C language, the pointer type check is not so strict, so the pointer of the void type can be converted with the pointer of any data type .

  // test.c
  int main()
  {
    int* p = nullptr;
  
    void* pv = p;         // ok
    
    char* pc = pv;        // ok
    return 0;
  }

• In the C++ language, the type has been enhanced, and colleagues are also compatible with the writing in the C language. Void can be used as an lvalue to accept any type of pointer, However, when void used as an rvalue, must be forced type conversion.

  // test.cpp
  
  int main()
  {
    void* pv = nullptr;        // 初始化是个好习惯
    
    int* pi = pv;          // error, invalid conversion from void* to int*
    int* pi = (int*)pv;    // ok, due to cast
  
    return 0;
  }

Summary: void* pointer is to receive pointers of any type.
Example Demo: Set all the memory occupied by the array to 0

void MemSet(void* src, int length, unsigned char n)
{
    unsigned char* p = (unsigned char*)src;
    int i = 0;
    for(i=0; i<length; i++)
    {
        p[i] = n;
    }
}

int main()
{
    int a[5];
    int i = 0;

    MemSet(a, sizeof(a), 0);

    for(i=0; i<5; i++)
    {
        printf("%d\n", a[i]);
    }

    return 0;
}

This article is summarized from "C Language Advanced Course" by Tang Zuolin from "Ditai Software Academy".
If there are any errors or omissions, please correct me.