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

int main()
{
    FILE *fptr;

    fptr = fopen("long_text.txt", "w");
    fprintf(fptr, "0123456789");

    fptr = freopen("long_text.txt","a",fptr);

    for(int i = 0; i < 10000-1; i++)
    {
        fprintf(fptr, "0123456789");
    }

    fclose(fptr);
}