Listing 2. writeport.c: a simple program that writes a
32-bit integer to a physical memory location.

#include <stdio.h>
#include <fcntl.h>      //needed for O_RDWR and O_SYNC
#include <sys/mman.h>   //needed for PROT_READ etc.
#define GRAB_SIZE 1024UL
#define GRAB_MASK (GRAB_SIZE - 1)

int main(int argc, char **argv)
{
   void *grab_base, *virt_addr;
   unsigned int md, read_result, writeval;
   off_t phys_addr = strtoul(argv[1], 0, 0);
   /*open memory interface*/
   if((md = open("/dev/mem", O_RDWR | O_SYNC)) == -1)
   {
      printf("ERR - /dev/mem open failed\n");
      exit(1);
   }
   /* Map one page to the physical address given*/
   if(grab_base = mmap(0, GRAB_SIZE, PROT_READ |
     PROT_WRITE, MAP_SHARED, md, phys_addr &
     ~GRAB_MASK), grab_base == (void *) -1)
   {
      printf("ERROR: failed to map\n");
      exit(1);
   }
   /*write to virtual memory that is now mapped to
   the requested physical address*/
   *((unsigned long *)grab_base + (phys_addr &
     GRAB_MASK)) = strtoul(argv[2], 0, 0);
   /*close memory interface*/
   if(munmap(grab_base, GRAB_SIZE) == -1)
   {
      printf("ERR - unmap failed\n");
      exit(1);
   }
   close(md);
}