How to mmap() to correct address - c

Question

I am trying to write an integer (1114129) from my HPS on Cyclone V Altera FPGA from a PUTTY window to a 32bit PIO on the FPGA side via lightweight axis interface. I am using mmap() and cannot get it to map to the address i want it to which is 0xff206000, instead its mapping to the address 0xb6f31000. My code and result is shown below.
#include <sys/stat.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/mman.h>
#include <sys/time.h>
#include <math.h>
#define MAPPED_SIZE 4
#define DDR_RAM_PHYS 0xff206000
#define PAGESIZE 0x1000
//1114129
int main(void)
{
int _fdmem;
void *map;
int a;
int c = 1114129;
const char memDevice[] = "/dev/mem";
_fdmem = open( memDevice, O_RDWR | O_SYNC );
if (_fdmem < 0){
printf("Failed to open the /dev/mem !\n");
return 0;
}
else{
printf("open /dev/mem successfully !\n");
}
a = sysconf(_SC_PAGESIZE);
printf("page %d", a);
/* mmap() the opened /dev/mem */
map = mmap(0, MAPPED_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, _fdmem, DDR_RAM_PHYS & -PAGESIZE);
if (map == MAP_FAILED) { perror("npheap_alloc()"); exit(1); }
int volatile * const p = (int *) (map + (0xff206000 % PAGESIZE));
*p = c;
printf("value %p", map);
printf("\nThe memory address of variable var = ptr = %p\n", p);
printf("\nIndirect access, variable var value = *ptr = %d", *p);
/* unmap the area & error checking */
if (munmap(map,MAPPED_SIZE)==-1){
perror("Error un-mmapping the file");
}
/* close the character device */
close(_fdmem);
}
open /dev/mem successfully !
page 4096value 0xb6f31000
The memory address of variable var = ptr = 0xb6f31000
Indirect access, variable var value = *ptr = 1114129#
Thanks in Advance

Answer 1

I think you're confusing the physical address (0xff206000) with the virtual address (0xb6f31000). You don't care what the virtual address is, you just what the pages at the physical address 0xff206000 to be mapped to any virtual address you can use.

Answer 2

The offset argument to mmap is a byte offset into the file. The pointer that mmap returns is a pointer to that location in the file, which in your case is a pointer to the exact address you want to write the value to.
So when you write the value, you write it to the exact location that map is pointing to:
int volatile * const map = (int volatile * const) mmap(...);
*map = value;