Libraries

Libraries are very important in C because the C language supports only the most basic features that it needs. C does not even contain I/O functions to read from the keyboard and write to the screen. Anything that extends beyond the basic language must be written by a programmer. The resulting chunks of code are often placed in libraries to make them easily reusable. We have seen the standard I/O, or stdio, library already: Standard libraries exist for standard I/O, math functions, string handling, time manipulation, and so on. You can use libraries in your own programs to split up your programs into modules. This makes them easier to understand, test, and debug, and also makes it possible to reuse code from other programs that you write.

You can create your own libraries easily. As an example, we will take some code from a previous article in this series and make a library out of two of its functions. Here's the code we will start with:

#include <stdio.h>

#define MAX 10

int a[MAX];
int rand_seed=10;

int rand()
/* from K&R
   - produces a random number between 0 and 32767.*/
{
    rand_seed = rand_seed * 1103515245 +12345;
    return (unsigned int)(rand_seed / 65536) % 32768;
}

void main()
{
    int i,t,x,y;

    /* fill array */
    for (i=0; i < MAX; i++)
    {
        a[i]=rand();
        printf("%d\n",a[i]);
    }

    /* bubble sort the array */
    for (x=0; x < MAX-1; x++)
        for (y=0; y < MAX-x-1; y++)
            if (a[y] > a[y+1])
            {
                t=a[y];
                a[y]=a[y+1];
                a[y+1]=t;
            }

    /* print sorted array */
    printf("--------------------\n");
    for (i=0; i < MAX; i++)
        printf("%d\n",a[i]);
}

This code fills an array with random numbers, sorts them using a bubble sort, and then displays the sorted list.

Take the bubble sort code, and use what you learned in the previous article to make a function from it. Since both the array a and the constant MAX are known globally, the function you create needs no parameters, nor does it need to return a result. However, you should use local variables for x, y, and t.

Once you have tested the function to make sure it is working, pass in the number of elements as a parameter rather than using MAX:

#include <stdio.h>

#define MAX 10

int a[MAX];
int rand_seed=10;

/* from K&R
   - returns random number between 0 and 32767.*/
int rand()
{
    rand_seed = rand_seed * 1103515245 +12345;
    return (unsigned int)(rand_seed / 65536) % 32768;
}

void bubble_sort(int m)
{
    int x,y,t;
     for (x=0; x < m-1; x++)
        for (y=0; y < m-x-1; y++)
            if (a[y] > a[y+1])
            {
                t=a[y];
                a[y]=a[y+1];
                a[y+1]=t;
            }
}

void main()
{
    int i,t,x,y;
    /* fill array */
    for (i=0; i < MAX; i++)
    {
        a[i]=rand();
        printf("%d\n",a[i]);
    }
    bubble_sort(MAX);
    /* print sorted array */
    printf("--------------------\n");
    for (i=0; i < MAX; i++)
        printf("%d\n",a[i]);
}

You can also generalize the bubble_sort function even more by passing in a as a parameter:

bubble_sort(int m, int a[])

This line says, "Accept the integer array a of any size as a parameter." Nothing in the body of the bubble_sort function needs to change. To call bubble_sort, change the call to:

bubble_sort(MAX, a);

Note that &a has not been used in the function call even though the sort will change a. The reason for this will become clear once you understand pointers.