Unknown problem in C programm.

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programming

Look guys...i must create a programm that reads 2 numbers from the first line of text file named: function.in and then create a new file named: function.out and write inside this all the prime numbers inside the two that i been given...here is an example: if the programm takes 1 and 4 it will accually check 2 and 3.
Here is my code:

#include <stdio.h>
#include <stdlib.h>
#include <conio.h>
int checkprime (int num)


{
 int counter = 2;
  while (1)
   {
    if (!(num %  counter)) return 0;
    if ((num/counter) < counter) return counter;
    counter ++;
   }
}

int main(){
    FILE *fp;
    int n, m, number, small, big;
    
    fp=fopen("function.in","r");
    fscanf(fp, "%d %d", &n, &m);
    fclose(fp);
    if(n>m)
    {
    small=m;
    big=n;
    }
    else
    {
    small=n;
    big=m;
    }
    
    fp=fopen("function.out","w+");
    for(number=small+1; number<big;>                           if (checkprime (number)){
                                          fprintf(fp,"%d ", number);
                                          }
    }
    fclose(fp);
}

It accually runs very well..but if the largest is number is 1500 or more the output is like glyphs and i dont know why..but if it is 1000 or less it is ok.
Please help me as soon as possible.
Thanks!!!!