If the definition of the external variable occurs in the source file before its use in a particular function, then there is no need for an extern declaration in the function.

#include<stdio.h>
#include<stdlib.h>

int main()
{
    union test
    {
        int i;
        float f;
        char c;
    };
    union test *t;
    t = (union test *)malloc(sizeof(union test));
    t->f = 10.10f;
    printf("%f", t->f);
    return 0;
}

#include<stdio.h>
int main()
{
    int x=4, y, z;
    y = --x;
    z = x--;
    printf("%d, %d, %d\n", x, y, z);
    return 0;
}

#include<stdio.h>
int main()
{
    int a=100, b=200, c;
    c = (a == 100 || b > 200);
    printf("c=%d\n", c);
    return 0;
}

#include<stdio.h>
int main()
{
    int x=55;
    printf("%d, %d, %d\n", x<=55, x=40, x>=10);
    return 0;
}

#include<stdio.h>

int main()
{
    void fun(char*);
    char a[100];
    a[0] = 'A'; a[1] = 'B';
    a[2] = 'C'; a[3] = 'D';
    fun(&a[0]);
    return 0;
}
void fun(char *a)
{
    a++;
    printf("%c", *a);
    a++;
    printf("%c", *a);
}

#include<stdio.h>

void fun(void *p);
int i;

int main()
{
    void *vptr;
    vptr = &i;
    fun(vptr);
    return 0;
}
void fun(void *p)
{
    int **q;
    q = (int**)&p;
    printf("%d\n", **q);
}