C

#include <stdio.h>

void doubleNum(int n)
{
	printf("In fuction: %d, %p\n", n, &n);
	n *= 2;
	printf("In fuction: %d, %p\n", n, &n);
}

int main()
{
	int a = 10;

	printf("Value: %d, Address: %p\n", a, &a);

	doubleNum(a);

	printf("After calling function: %d, %p\n", a, &a);
}

#include <stdio.h>

void doubleNum(int *n)
{
	printf("In fuction: %d, %p\n", *n, n);
	*n *= 2;
	printf("In fuction: %d, %p\n", *n, n);
}

int main()
{
	int a = 10;

	printf("Value: %d, Address: %p\n", a, &a);

	doubleNum(&a);

	printf("After calling function: %d, %p\n", a, &a);
}

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

int doubleNum(int n)
{
	int num;
	num = 2*n;

	return num;
}

int main()
{
	srand(time(NULL)); // take a random seed
	int r = rand()%100; // generate a random number within [0, 100)

	printf("Value: %d\n", doubleNum(r));
}

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

int doubleNum(int);

int main()
{
	srand(time(NULL)); // take a random seed
	int r = rand()%100; // generate a random number within [0, 100)

	printf("Value: %d\n", doubleNum(r));
}

int doubleNum(int n)
{
	int num;
	num = 2*n;

	return num;
}

// f3.c
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include "util.h"

int main()
{
	srand(time(NULL)); // take a random seed
	int r = rand()%100; // generate a random number within [0, 100)

	printf("Value: %d\n", doubleNum(r));
}

// util.c
#include "util.h"

int doubleNum(int n)
{
	int num;
	num = 2*n;

	return num;
}

// util.h
#ifndef UTIL_H
#define UTIL_H
int doubleNum(int);
#endif

#include <stdio.h>

int factorial(int n)
{
	if(n <= 1) // basis step
		return 1;
	else // recursive step
		return n*factorial(n-1);
}

int main()
{
	printf("%d\n", factorial(10));

	return 0;
}

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

void factorial(int *p, int n)
{
	if(n <= 1) // basis step
	{
		*p *= 1;
	}
	else // recursive step
	{
		*p *= n;
		factorial(p, n-1);
	}
}

int main()
{
	int *ptr;
	ptr = (int *) malloc(sizeof(int));

	*ptr = 1;

	factorial(ptr, 10);

	printf("%d\n", *ptr);

	return 0;
}

#include <stdio.h>

void count()
{
	static int num = 0;
	num++;
	printf("%d\n", num);
}

int main()
{
	count(); // 1
	count(); // 2

	return 0;
}

// main.c
#include <stdio.h>
#include "util.h"

extern int count;

int main()
{
	increase();
	increase();

	printf("%d\n", count);

	return 0;
}

// util.h
#ifndef UTIL_H
#define UTIL_H

void increase();

#endif

// util.c
#include "util.h"

int count = 0; // global variable

void increase()
{
	count++;
}

A static global variable is a global variable that can only be accessed by functions in the same C program file as the variable

// main.c
#include <stdio.h>
#include "util.h"

int main()
{
	increase();

	printf("%d\n", increase()); // 2

	return 0;
}

// util.h
#ifndef UTIL_H
#define UTIL_H

int increase();

#endif

// util.c
#include "util.h"

static int count = 0; // global static variable

int increase()
{
	count++;

	return count;
}

A static function is a function that can only be accessed by functions in the same C program file

// main.c
#include <stdio.h>
#include "util.h"

int main()
{
	display();

	return 0;
}

// util.h
#ifndef UTIL_H
#define UTIL_H

static int getNum();
void display();

#endif

// util.c
#include <stdio.h>
#include <stdlib.h>
#include "util.h"

static int getNum()
{
	return rand()%10;
}

void display()
{
	printf("%d\n", getNum());
}