C Converting 4 digit numbers to binary - c

Question

I want to print the binary equivalent of the numbers from a file in a 20 bit field with spaces after every 4 bits but when the number exceeds 1024 (I've tested this), it no longer outputs in binary but in a base I don't know (i.e. 1234 = 0000 0014 1006 5408)
Here is my code
#include <stdio.h>
long convertToBinary(long);
int main()
{
char binNum[20]; //binary number char array
int i;
long b, decNum; //b is output binary number, decNum is each input number from file
FILE *filePtr;
filePtr = fopen("numbers.txt", "r");
while (!feof(filePtr))
{
fscanf(filePtr, "%li", &decNum);
if (decNum < 0 || decNum > 65535)
{
printf("Error, %5li is out of range.\n", decNum);
continue;
}
b = convertToBinary(decNum); //function call for binary conversion
i = 18;
while (i >= 0)
{
if (i == 4 || i == 9 || i == 14)
{
binNum[i] = ' ';
}
else if (b != 0)
{
binNum[i] = b % 10 + '0';
b /= 10;
}
else
{
binNum[i] = '0';
}
i--;
}
binNum[19] = '.';
printf("The binary representation of %5li is: ", decNum);
for (i = 0; i<20; i++)
{
printf("%c", binNum[i]);
}
printf("\n");
}
return 0;
}
//Recursion function
long convertToBinary(long number)
{
if (number == 0)
{
return 0;
}
else
{
return (number % 2 + 10 * convertToBinary(number / 2));
}
}
The file numbers.txt has numbers like : 0 1 2 3 16 17 1234 5678 65535

Answer 1

Your convertToBinary function is producing a decimal-coded-binary (as opposed to a binary-coded-decimal) which uses a decimal digit per bit.
It looks like long has the same range as int on your system, i.e. 231, which covers ten decimal digits. Once you get eleventh digit, you get an overflow.
Switching to unsigned long long, which has 64 bits, will fix the problem. However, this is sub-optimal: you would be better off writing a to-binary conversion that writes zeros and ones into an array of chars that you provide. You can do it recursively if you'd like, but an iterative solution will do just fine.
Note that the to-binary conversion does not need to insert spaces into char array. This can be done by your main function.