External Sorting

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

void shownums ( char * ) ;
void split ( char * ) ;
void sort ( char * ) ;

void main( )
{
    char str[67] ;

    clrscr( ) ;

    printf ( \"Enter file name: \" ) ;
    scanf ( \"%s\", str ) ;

    printf ( \"Numbers before sorting:\\n\" ) ;
    shownums ( str ) ;

    split ( str ) ;
    sort ( str ) ;

    printf ( \"\\nNumbers after sorting:\\n\" ) ;
    shownums ( str ) ;

    getch( ) ;
}

/* Displays the contents of file */
void shownums ( char *p )
{
    FILE *fp ;
    int i ;

    fp = fopen ( p, \"rb\" ) ;
    if ( fp == NULL )
    {
        printf ( \"Unable to open file.\" ) ;
        getch( ) ;
        exit ( 0 ) ;
    }

    while ( fread ( &i, sizeof ( int ), 1, fp ) != 0 )
        printf ( \"%d \\t\", i ) ;

    fclose ( fp ) ;
}

/* Splits the original file into two files */
void split ( char *p )
{
    FILE *fs, *ft ;
    long int l, count ;
    int i ;

    fs = fopen ( p, \"rb\" ) ;
    if ( fs == NULL )
    {
        printf ( \"Unable to open file.\" ) ;
        getch( ) ;
        exit ( 0 ) ;
    }

    ft = fopen ( \"temp1.dat\", \"wb\" ) ;
    if ( ft == NULL )
    {
        fclose ( fs ) ;
        printf ( \"Unable to open file.\" ) ;
        getch( ) ;
        exit ( 1 ) ;
    }

    fseek ( fs, 0L, SEEK_END ) ;
    l = ftell ( fs ) ;
    fseek ( fs, 0L, SEEK_SET ) ;

    l = l / ( sizeof ( int ) * 2 ) ;

    for ( count = 1 ; count <=  l ; count++ )
    {
        fread ( &i, sizeof ( int ), 1, fs ) ;
        fwrite ( &i, sizeof ( int ), 1, ft ) ;
    }

    fclose ( ft ) ;

    ft = fopen ( \"temp2.dat\", \"wb\" ) ;
    if ( ft == NULL )
    {
        fclose ( fs ) ;
        printf ( \"Unable to open file.\" ) ;
        getch( ) ;
        exit ( 2 ) ;
    }

    while (    fread ( &i, sizeof ( int ), 1, fs ) != 0 )
        fwrite ( &i, sizeof ( int ), 1, ft ) ;

    fcloseall ( ) ;
}

/* Sorts the file */
void sort ( char *p )
{
    FILE *fp[4] ;
    char *fnames[ ] =
                    {
                        \"temp1.dat\",
                        \"temp2.dat\",
                        \"final1.dat\",
                        \"final2.dat\"
                    } ;

    int i, j = 1, i1, i2, flag1, flag2, p1, p2 ;
    long int l ;

    while ( 1 )
    {
        for ( i = 0 ; i <= 1 ; i++ )
        {
            fp[i] = fopen ( fnames[i], \"rb+\" ) ;
            if ( fp[i] == NULL )
            {
                fcloseall( ) ;
                printf ( \"Unable to open file.\" ) ;
                getch( ) ;
                exit ( i ) ;
            }

            fseek ( fp[i], 0L, SEEK_END ) ;
            l = ftell ( fp[i] ) ;
            if ( l == 0 )
                goto out ;
            fseek ( fp[i], 0L, SEEK_SET ) ;
        }

        for ( i = 2 ; i <= 3 ; i++ )
        {
            fp[i] = fopen ( fnames[i], \"wb\" ) ;
            if ( fp[i] == NULL )
            {
                fcloseall( ) ;
                printf ( \"Unable to open file.\" ) ;
                getch( ) ;
                exit ( i ) ;
            }
        }

        i = 2 ;
        i1 = i2 = 0 ;
        flag1 = flag2 = 1 ;

        while ( 1 )
        {
            if ( flag1 )
            {
                if ( fread ( &p1, sizeof ( int ), 1, fp[0] ) == 0 )
                {
                    /* If first file ends then the whole content of second
                       file is written in the respective target file */
                    while ( fread ( &p2, sizeof ( int ), 1, fp[1] ) != 0 )
                        fwrite ( &p2, sizeof ( int ), 1, fp[i] ) ;
                    break ;
                }
            }

            if ( flag2 )
            {
                if ( fread ( &p2, sizeof ( int ), 1, fp[1] ) == 0 )
                {
                    /* If second file ends then the whole content of first
                       file is written in the respective target file */
                    fwrite ( &p1, sizeof ( int ), 1, fp[i] ) ;
                    while ( fread ( &p1, sizeof ( int ), 1, fp[0] ) != 0 )
                        fwrite ( &p1, sizeof ( int ), 1, fp[i] ) ;
                    break ;
                }
            }

            if ( p1 < p2 )
            {
                flag2 = 0 ;
                flag1 = 1 ;
                fwrite ( &p1, sizeof ( int ), 1, fp[i] ) ;
                i1++ ;
            }
            else
            {
                flag1 = 0 ;
                flag2 = 1 ;
                fwrite ( &p2, sizeof ( int ), 1, fp[i] ) ;
                i2++ ;
            }

            if ( i1 == j )
            {
                flag1 = flag2 = 1 ;
                fwrite ( &p2, sizeof ( int ), 1, fp[i] ) ;
                for ( i2++ ; i2 < j ; i2++ )
                {
                    if ( fread ( &p2, sizeof ( int ), 1, fp[1] ) != 0 )
                        fwrite ( &p2, sizeof ( int ), 1, fp[i] ) ;
                }
                i1 = i2 = 0 ;
                i == 2 ? ( i = 3 ) : ( i = 2 ) ;
            }

            if ( i2 == j )
            {
                flag1 = flag2 = 1 ;
                fwrite ( &p1, sizeof ( int ), 1, fp[i] ) ;
                for ( i1++ ; i1 < j ; i1++ )
                {
                    if ( fread ( &p1, sizeof ( int ), 1, fp[0] ) != 0 )
                        fwrite ( &p1, sizeof ( int ), 1, fp[i] ) ;
                }
                i1 = i2 = 0 ;
                i == 2 ? ( i = 3 ) : ( i = 2 ) ;
            }
        }

        fcloseall( ) ;
        remove ( fnames[0] ) ;
        remove ( fnames[1] ) ;
        rename ( fnames[2], fnames[0] ) ;
        rename ( fnames[3], fnames[1] ) ;
        j *= 2 ;
    }

    out :

    fcloseall( ) ;
    remove ( p ) ;
    rename ( fnames[0], p ) ;
    remove ( fnames[1] ) ;
}