4 replies to this topic

[veda87]

I did a small program

#include <stdio.h>


typedef unsigned short u32;


int main()

{


        u32 *ptr = NULL;

        u32 *ptr1 = NULL;

        u32 temp;

        int i;


        ptr = malloc(512);

        if( ptr  == NULL) {

                printf("memory allocation failed\n");

                exit(1);

        }


        for(i=0 ; i< 512; i++){

                ptr[i] = 0;

        }


        for(i=0 ; i< 512; i++){

                printf("%d", ptr[i]);

        }


        temp = (u32)ptr;

        ptr1 = (u32 *)temp;


        for(i=0 ; i< 512; i++){

              printf("%d", ptr1[i]);

        }


        return 0;

}

when I compiled it with gcc, I got some warning

eg.c:29: warning: cast from pointer to integer of different size

eg.c:30: warning: cast to pointer from integer of different size

when I executed the code, it gave me segmentation fault...

can anyone tell me what is wrong with this code...

[ZekeDragon]

Unsigned short is too small to hold a pointer. You need to use unsigned long (or unsigned int in 32 bit machines). I'd try to avoid manual input of pointers at all costs though, or even conversion to integers/longs. Unsigned short is 2 bytes in memory, whereas a pointer is 4 bytes in size on 32 bit machines.

[TkTech]

@h4x: ....even with the paging memory model, segments are still used. You just generally don't have to deal with them once you've setup your GDT.

@op: if your using GCC or a standards compliant compiler, check to see if your stdint.h has a uintptr_t defined. If so, that is the proffered type for storing a pointer. (However, note it is an optional type, although its almost always present)

[dargueta]

Always use unsigned long for pointers. And don't listen to h4x, he has a history of spewing his opinion without knowing how the things he's criticizing even work.

H4x, really. It would be most enlightening if you would back up your "segmentation sucks" claims with concrete evidence, examples, etc. Just stating your opinion alone (in a very unprofessional manner, I might add) is not going to help anyone. To be honest, it's getting annoying.