Point me to Pointers

Alright, the fun stuff - C pointers!

Working with variables is good but having access to the memory where these variables are stored is awesome. C allows us to work directly with the memory. Pointer is a data type that stores the address of a particular memory location. What do I need to do to use pointers?

1. Create a pointer of appropriate data type: char *pc or int *pi.
2. Initialize the pointer to appropriate data type variable, for example, if i is a variable, its address will be &i. So initializing the pointer will look like pi = &i;.
3. Use the pointers In order to access the value corresponding to a pointer address, you can use *<pointer_name> in any expression just like you would use the variable i.

Passing pointers to function arguments

Functions in C pass arguments by value. This means if a variable a is passed to a function, a copy of the variable is passed and any changes made to this variable inside the function will not be saved/reflected in the calling function. One way to manipulate the variables inside a function is to pass the address (pointer) of the variable instead. How do we do this? Let's see a typical example where we want to swap the values stored in let's say two integer variables.

// swap.c -- swap two integers (demo use of pointers)
#include <stdio.h>

void swap(int *a, int *b); // swap take two int pointers as arguments

int main(void)
{
    int a, b;
    a = 12;
    b = 29;
    printf("a = %d, b = %d\n", a, b);

    swap(&a, &b); // passing address of a and b to swap()
    printf("a = %d, b = %d\n", a, b);

    return 0;
}

// swap() swaps the two integers
void swap(int *a, int *b)
{
    int tmp;
    tmp = *a; // a is the address, *a gets the value from that address
    *a = *b;
    *b = tmp;
}

a = 12, b = 29
a = 29, b = 12