程序代码：

#include<stdio.h>

void main()
{
    FILE *a,*b,*c;
    

    if((a=fopen("filea","r"))==NULL)
    {
        printf("sorry can't open this file\n");
        exit(0);
    }

    if((b=fopen("fileb","r"))==NULL)
    {
        printf("sorry can't open this file\n");
        exit(0);
    }

    if((c=fopen("filec","w"))==NULL)
    {
        printf("sorry can't open this file\n");
        exit(0);
    }
    fclose(c);
    if((c=fopen("filec","a"))==NULL)
    {
        printf("sorry can't open this file\n");
        exit(0);
    }


    while(!feof(a))
    {
        fputc(fgetc(a),c);
        putchar(fgetc(a));
    }
     while(!feof(b))
     {
         fputc(fgetc(b),c);
         putchar(fgetc(b));
     }
     fclose(a);
     fclose(b);
}