Question

I’m implementing a puts (print a string on screen) system call in a custom OS project I’m working on. The absolute memory address of the string is given to me by an unsigned int variable representing the esi register, and I need to initialize a pointer to an unsigned char array to read the string from.

The registers contents are represented by:

struct regs
{
    unsigned int gs, fs, es, ds;
    unsigned int edi, esi, ebp, esp, ebx, edx, ecx, eax;
    unsigned int int_no, err_code;
    unsigned int eip, cs, eflags, useresp, ss;
};

I have tried the following to initialize a pointer to the address in esi:

void fault_handler(struct regs *r) {
   void *p = (void*)r->esi;
   unsigned char* s = (unsigned char*)p;
   // take s and print it to the screen
}

But I don’t get the “Hello\n” I’m supposed to get, instead I get garbage. I verified that the address of esi indeed points to the correct string. The problem I have is to initialize a pointer to this address.

Thanks!

Update: I will close this question and move the discussion to a new question as the original answer is answered. Thank you everyone!

Answer 1

Your code correctly assigns esi to p and then s. Thus I can only assume your problem is not in fact related to this step.

As an aside I don’t see why you need p, just assign esi directly to s.