How to use /dev/kmem?

Updated my post...

I got below program. It operates on /dev/kmem and /dev/mem.

I think I can learn something from the code. But when I run it on my Beagle Board, below result is given:

case 1: ( if(1) )
    root@omap:/home/ubuntu/tom# ./kmem_mem /boot/System.map-3.0.4-x3
    found jiffies at (0xc0870080) c0870080
    /dev/kmem read buf = 319317
    jiffies=319317 (read from virtual memory)

    /dev/mem: the offset is 870080
    the page size = 4096
    mmap: Invalid argument



case 2: ( if(0) )
    root@omap:/home/ubuntu/tom# ./kmem_mem /boot/System.map-3.0.4-x3 
    found jiffies at (0xc0870080) c0870080
    /dev/kmem read buf = 333631
    jiffies=333631 (read from virtual memory)

    /dev/mem: the offset is 870080
    /dev/mem read failed: Bad address
    jiffies=0 (read from physical memory)

And I used below command so that mmap can use NULL as its first parameter.

root@omap:/home/ubuntu/tom# echo 0 > /proc/sys/vm/mmap_min_addr
root@omap:/home/ubuntu/tom# cat /proc/sys/vm/mmap_min_addr
0

As you can see, read_kmem() works fine but read_mem() doesn't work, and it seems that the 'offset' transferred to it is wrong. But kernel address - PAGE_OFFSET(0xC0000000) = physical address, is it wrong?

My questions are: (1) Why "mmap: Invalid argument" in case 1? (2) Why the mmap only maps PAGE_SIZE length space? (3) What's wrong with read_mem?

Can anyone help? Thanks!

/*
* getjiff.c
*
* this toolkit shows how to get jiffies value from user space:
* 1. find jiffies's address from kernel image.
* 2. access virtual address space to get jiffies value.
* 3. access physical address sapce to get jiffies value.
*
* demostrate following techniques:
* o get ELF object symbol address by calling nlist()
* o access virtual memory space from /dev/kmem
* o access virtual memory