"C Language Learning" 04__goto,void,extern,sizeof Analysis __c language

1:goto disabled

The meaning of 2:void

void modifier function return value and parameters
If the function does not return a value, it should be declared as a void type
If the function has no arguments, it should declare that its argument is void
The void modifier returns values and parameters only to indicate that none

void variable not present

The C language does not define void as the alias of how much memory

The meaning of void pointers
C language stipulates that only pointers of the same type can assign values to each other
void* pointer as a left value for "receive" any type of pointer
The void* pointer is assigned to the other pointer as a right value to force type conversion

int* pi = (int*) malloc (sizeof (int));
char* pc = (char*) malloc (sizeof (char));
void* p = NULL;
int*  PNI = NULL;
char* PNC = NULL;

p = Pi;//ok
PNI = p;//oops

p = pc;//ok
PNC = p;//oops

The Hidden meaning in 3:extern

extern for declaring externally defined variables and functions
extern is used to "tell" compilers to compile with the C method
C + + compilers and a few variants the C. compiler defaults to compile in its own way
Functions and variables, through extern keys, you can command the compiler to "compile in standard C mode."

4:sizeof

sizeof is a built-in indicator of the compiler, not a function
sizeof used to "calculate" the amount of memory that the corresponding entity occupies
The value of the sizeof is determined at compile time

#include <stdio.h>

int main ()
{  
    int A;
    
    printf ("%d\n", sizeof (a));//4
    printf ("%d\n", sizeof a);//4
    printf ("%d\n", sizeof (int));//4
    
    GetChar ()
    ; return 0;
}