How to write standard output in hexadecimal of a pointer address using write()

Question

In a task I need to print the address of a pointer in hexadecimal into the standard output. The issue is that I am only allowed to include unistd.h so no printf is allowed.

I have tried to cast it into a char* but the cast is quite impossible to achieve. So help would be so appreciated in here.

It should print something like that:

000000010a161f40

Answer 1

I've coded a print_pointer function that does the exact same thing as printf("%p",pointer)

#include <unistd.h>

void print_pointer(const void *p)
{
    static const char *table = "0123456789abcdef";
   unsigned long long value = (unsigned long long)p;
   // 1 digit = 4 bits
   char buffer[sizeof(void*)*2];
   int i,idx = 0;
   
   while(value)
   {
       int digit = value % 16;
       buffer[idx++] = table[digit];
       
       value /= 16;
   }
   // zero-padding
   for (i=idx;i<sizeof(buffer);i++)
   {
       buffer[i] = '0';
   }
   // reversed print, as division computes lowest digits first
   for (i=sizeof(buffer)-1;i>=0;i--)
   {
       write(1,buffer+i,1); // write 1 char to standard output
   }
   
    
}

int main()
{
    const char *data = "foo";
    
    print_pointer(data);
    
    return 0;
}