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pointer to struct. Segmentation fault on read value of member

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I’m trying to map some area of the physical memory and then put it in struct. Mapping is OK and the assignment is OK. ( I think so, because there is no segmentation fault.) The problem is when I try to read some value from the struct.

My struct is something like this:

struct some_struct {
    unsigned long a;
    unsigned long b;
    unsigned long c;
    unsigned long d;
    unsigned long e;
    unsigned long f;
};

Then I map the memory:

void *pc = (void *) mmap(0, PageSize * 2, PROT_READ | PROT_WRITE, MAP_SHARED, fd, addr_start);

Don’t look too closely at the parameters. I’m 100% sure that this works fine. After that I do this:

struct some_struct *ptr = ((struct some_struct *)(pc + some_offset));

Then I try:

printf("DDR[%p] -> 0x%xn", &(ptr->a));

And I get the address of the member. It points at physical address in the memory. But the following gets me segmentation fault:

printf("DDR[%p] -> 0x%xn", &(ptr->a), ptr->a);

I’m not sure if I have to use physical of virtual address when I assign the struct. Or I’m missing something very obvious.

Answer

It’s not enough of your code to give a full help.

I can provide only possible problems, that you can have in your code:

From man mmap:

1) If you want to read and write, you should have open("./file.txt", O_RDWR); in your code.

The prot argument describes the desired memory protection of the mapping 
(and must not conflict with the open mode of the file).

2) You offset must be k*sysconf(_SC_PAGE_SIZE).
On my system, for example, the page size is 4096. So your addr_start must be like 4*4096, but not like 12 or an any arbitrary number.

offset must be a multiple of the page size as returned by sysconf(_SC_PAGE_SIZE)

3) Compile your code with all warnings enabled and see what compiler will write to you. If you use gcc, than add -Wall flag. You already have an undefined behavior in your printf statement (this was already mentioned in the comments) and with -Wall compiler will tell you about it.

4) If you still want compile without -Wall flag, then ensure that you have all needed headers included to your program. I’ve included:

//mmap
#include <sys/mman.h>
//printf
#include <stdio.h>
//open
#include <stdlib.h>
#include <sys/types.h>
#include <fcntl.h>
//sysconf
#include <unistd.h>

Hope this will help.

P.S. I’ve saw that you have solved your problem already just after I’ve wrote this post (but I still do not understand your problem). So I just leave this in case anyone other fill have similar behavior, but the different problem.