mmap for I/O access

My call to mmap is causing a Segmentation fault.  Is this the right way to do
it?

I have some general-purpose I/O on this Embest ARM-based board.  According to
the manual for the board, the control register and the data register are at
absolute physical addresses

  0x56000010   and
  0x56000014   respectively.

So here is how I tried to test my access to these registers:

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/ioctl.h>

main() {
  int fm = open("/dev/mem",O_RDWR);
  printf("handle to /dev/mem = %08x\r\n",fm);

  char *mptr = (char*)mmap(
                 (void*)0,
                16,    //..16 bytes
                PROT_READ | PROT_WRITE,
                MAP_SHARED,
                fm,
    0x56000010);        //..starting with Port B CON reg

  printf("mptr = %08x", mptr);

  printf("GPBCON = %08x\r\n", *((int*)(mptr+0)));
  printf("GPBDAT = %08x\r\n", *((int*)(mptr+4)));
 }

When I run this program, the open() returns 4, and then the Segmentation Fault
occurs during the execution of mmap, because there was no printf telling what
mptr is.  Here is the output:


handle to /dec/mem = 00000004


 : [<000086b4>]    lr : [<00000000>]    Tainted: P

sp : bffffdd8  ip : bffff7d8  fp : bffffea4

r10: 40139280  r9 : 000084a0  r8 : 00000001

r7 : 4000ee5c  r6 : 0000830c  r5 : bffffec4  r4 : 40020c34

r3 : ffffffff  r2 : 0000000f  r1 : 00008823  r0 : 0000883c

Flags: NzCv  IRQs on  FIQs on  Mode USER_32  Segment user

Control: C000317F  Table: 33E90000  DAC: 00000015

Segmentation fault

 - - - - - - - - - - - - - - - - - - - - - - - - - - -
I am running as root, so I should be able to do this, right?


Robert Scott
Ypsilanti, Michigan