/dev/mem access from c program

[aleale97]

Hi guys, this is my first post here so, hi!

I'm stuck in this piece of code found in a library, as far as i understood it should read a value from memory(RAM) and return it.
I fired up google and got my self up and running with almost all the functions but there're some lines that i still don't understand, here is the C code:
Code:
uint32_t readl(uint32_t addr)
{
static volatile uint32_t *gpio_vaddr ;

int fd = 0;
uint32_t val = 0;
fd = open("/dev/mem", O_RDWR | O_NDELAY);
if(-1 == fd){
printf("open /dev/mem failed.");
return -1;
}

uint32_t mmap_base = (addr & ~MAP_MASK);
uint32_t mmap_seek = ((addr - mmap_base) >> 2);

gpio_vaddr = (uint32_t *)mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, mmap_base);

if (0 == gpio_vaddr){
close(fd);
printf("mmap failed:fd(%d), addr(0x%x),size(%d)\n", fd, addr, MAP_SIZE);
} else {
val = *(gpio_vaddr + mmap_seek);
if (wiringPiDebug)
printf("readl: vaddr = 0x%x,val:0x%x\n", (gpio_vaddr + mmap_seek),val);
close(fd);
munmap((uint32_t *)gpio_vaddr, MAP_SIZE);
return val;
}
}
Where the constants are:
Code:
MAP_SIZE (4096*2)
MAP_MASK (MAP_SIZE - 1)
and the addr parameter of the function is the address that has to be red(31 bit long binary number).

• In this line there is a bitwise not(the tilde) on the MAP_MASK value and an and of the result with the address given(correct me if i'm wrong):
  Code:
  uint32_t mmap_base = (addr & ~MAP_MASK);
  (MAP_SIZE-1)=111111111111(8191dec), if i invert that i get 000000000000 that will set, with every address given, the mmap_base to 0! Is just bad code or am i missing something?
• What is hte purpose of the function munmap?

If anyone wants to help, thanks !!!
Alessandro from Italy.

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