brak konwersji ciągu cyfr

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

void changeToRoman(int a)
{    
    int s;
    int d;
    int j;
    int t;    
    int plik2;
    
   	plik2 = open("liczbyRzymskie.txt",O_WRONLY|O_APPEND);
		if(plik2 == -1)	
		{
		perror("Blad otwarcia pliku!");
		exit(1);
		}
    if(a!=0)
     {
       j = a%10;
       d = (a/10)%10;
       s = a/100;
       t = a/1000;
       
       
 	switch(t)
   { 
      case 1: write (plik2,"M",1); break;
      case 2: write (plik2,"MM",2); break;
      case 3: write (plik2,"MMM",3); break;
   }
 
	switch(s)
   { 
      case 1: write (plik2,"C",1); break;
      case 2: write (plik2,"CC",2); break;
      case 3: write (plik2,"CCC",3); break;
      case 4: write (plik2,"CD",2); break;
      case 5: write (plik2,"D",1); break;
      case 6: write (plik2,"DC",2); break;
      case 7: write (plik2,"DCC",3); break;
      case 8: write (plik2,"DCCC",4); break;
      case 9: write (plik2,"CM",2); break;
   }
   
    	switch(d)
   { 
      case 1: write (plik2,"X",1); break;
      case 2: write (plik2,"XX",2); break;
      case 3: write (plik2,"XXX",3); break;
      case 4: write (plik2,"XL",2); break;
      case 5: write (plik2,"L",1); break;
      case 6: write (plik2,"LX",2); break;
      case 7: write (plik2,"LXX",3); break;
      case 8: write (plik2,"LXXX",4); break;
      case 9: write (plik2,"XC",2); break;
   
   }
	switch(j)
   { 
      case 1: write (plik2,"I",1); break;
      case 2: write (plik2,"II",2); break;
      case 3: write (plik2,"III",3); break;
      case 4: write (plik2,"IV",2); break;
      case 5: write (plik2,"V",1); break;
      case 6: write (plik2,"VI",2); break;
      case 7: write (plik2,"VII",3); break;
      case 8: write (plik2,"VIII",4); break;
      case 9: write (plik2,"IX",2); break;
   }     
 
   write (plik2," ",1);
   close(plik2);
}
}

int main()
{
    int plik;
    int l;
	char znak[1];
	plik = open("liczbyArabskie.txt", O_RDONLY);
	if(plik == -1)
	{
	 perror("Blad otwarcia pliku!");
	 exit(1);
	}
	
   int size = 0;
   while((size=read(plik, znak, 1))>0)
	{
	l = atoi(znak);
	if(l > 0)
	{
		changeToRoman(l);
	}
	}
	int i=0;
	scanf("%c", &i);
    return 0;
}

 while((size=read(plik, znak, 1))>0)

 while((size=read(plik, znak, 4))>0)