程序代码：

#include<stdio.h>
#include<stdlib.h>
#include<conio.h>
#define L 1
main()
{
    FILE *fp;
    char ch[200],c;
    int i=0,j,n;
    if((fp=fopen("file1.txt","w+"))==NULL)
    {
        printf("Can't open this file");
        exit(0);
    }
    printf("gets in file1:\t");
    while((c=getchar())!='\n')
        fputc(c,fp);
    rewind(fp);
    while((c=fgetc(fp))!=EOF)
        ch[i++]=c;
    #ifdef L
    printf("i=%d",i);
    #endif
    fclose(fp);
    if((fp=fopen("file2.txt","w+"))==NULL)
    {
        printf("can't open this file");
        exit(0);
    }
    printf("gets in file2:\t");
    while((c=getchar())!='\n');
        fputc(c,fp);
    rewind(fp);
    while((c=fgetc(fp))!=EOF)
    {ch[i++]=c;

    #ifdef L
    printf("i=%d",i);
    #endif
    }
    fclose(fp);
    n=i;
    for(i=1;i<n;i++)
    for(j=0;j<n;j++)
        if(ch[j]<ch[i])
        {
            c=ch[j];
            ch[j]=ch[i];
            ch[i]=c;
        }
    if((fp=fopen("file3.txt","w+"))==NULL)
    {
        printf("can't open this file");
        exit(0);
    }
    for(i=0;i<n;i++)
    fputc(ch[i],fp);
    rewind(fp);
    while((c=fgetc(fp))!=EOF)
        putchar(c);
    printf("\n");
    fclose(fp);
    getch();
}