/dev/mem access from c program

[Guest 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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