[Linux C] Map reserved memory block

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdint.h>
#include <sys/mman.h>
// Mapping to reserved DDR3 mmeory: 196MB / 0xC400000
#define HW_REGS_BASE ( 196*1024*1024 )

// mapping address range 1MB
#define HW_REGS_SPAN ( 0x100000 )
#define HW_REGS_MASK ( HW_REGS_SPAN - 1 )

void HexDump( unsigned char * buffer, int bytes)
{
  int i;
  for(i=0; i<bytes; i++)
  {
    if(i%16 == 0)
    printf("\n [%04X] " , i);
    printf("%02X " , buffer[i]);
  }
  printf( "\n");
}

int main() {

  void *virtual_base;
  int fd;

  // Broj bajta koje zelim da mapiram na fizicki adresni prostor i broj
  // njih koje cu posle ispisati
  int bytes_to_map  = 4;
  int bytes_to_dump = 4;

  // map the address space for the LED registers into user space so we can interact with them.
  // we'll actually map in the entire CSR span of the HPS since we want to access various registers within that span
  if( ( fd = open( "/dev/mem", ( O_RDWR | O_SYNC ) ) ) == -1 ) {
    printf( "ERROR: could not open \"/ dev/mem \"...\n" );
    return(1);
  }

  printf( "mapping DDR3 offset from 0x%X, range = 0x%X. \r\n" , HW_REGS_BASE, HW_REGS_SPAN);
  //virtual_base = mmap( NULL, HW_REGS_SPAN, ( PROT_READ | PROT_WRITE ), MAP_SHARED, fd, HW_REGS_BASE );
  virtual_base = mmap( NULL, bytes_to_map, ( PROT_READ | PROT_WRITE ), MAP_SHARED, fd, HW_REGS_BASE );

  if( virtual_base == MAP_FAILED ) {
    printf( "ERROR: mmap() failed...\n" );
    close( fd );
    return(1);
  }

  //HexDump(virtual_base, 0x40);
  // Okej ovde mu kazem ispisi mi 8 bajta od ovih gore 16 koje si mapirao
  // pocev od HW_REGS_BASE
  HexDump(virtual_base, bytes_to_dump);

  // clean up our memory mapping and exit
  // if( munmap( virtual_base, HW_REGS_SPAN ) != 0 ) {

  // Ovde unmap odradim kako bi virtuelne adrese vratio kernelu na raspolaganje.
  if( munmap( virtual_base, bytes_to_map ) != 0 ) {
    printf( "ERROR: munmap() failed...\n" );
    close( fd );
    return(1);
  }
  close( fd );
  return(0);
}