pointer-arithmetic

int main() { int a; void *p; p = &a; printf("%ld\n",(long)p); p = p+1; printf("%ld\n",(long)p); } In this program, p+1 is just incrementing the value of p by 1. I know void pointer arithmetic is not possible in C , so GCC is doing it implicitly. And if yes, then is it taking it as char pointer . Also, why dereferencing is not possible for void pointer, if it is implicitly doing pointer arithmetic. C does not allow pointer arithmetic with void * pointer type. GNU C allows it by considering the size of void is 1 . From 6.23 Arithmetic on void- and Function-Pointers : In GNU C, addition and

Given this code: int *p, *q; p = (int *) 1000; q = (int *) 2000; What is q - p and how? It's actually undefined, according to the standard. Pointer arithmetic is not guaranteed to work unless the pointers are both pointing to either an element in, or just beyond, the same array. The relevant section of the standard is 6.5.6:9 (n1362 draft of c1x but this hasn't changed since c99) which states: When two pointers are subtracted, both shall point to elements of the same array object, or one past the last element of the array object; the result is the difference of the subscripts of the two array

In C/C++, addition or subtraction on pointer is defined only if the resulting pointer lies within the original pointed complete object . Moreover, comparison of two pointers can only be performed if the two pointed objects are subobjects of a unique complete object . What are the reasons of such limitations? I supposed that segmented memory model (see here §1.2.1) could be one of the reasons but since compilers can actually define a total order on all pointers as demonstrated by this answer , I am doubting this. There are architectures where program and data spaces are separated, and it's

In Visual Studio C++ version 9 (and probably other versions too), the following code: int a = sizeof(void); void const *b = static_cast<void const *>("hello world"); b += 6; Generates these errors : error C2070: 'void': illegal sizeof operand error C2036: 'const void *' : unknown size This code works under GCC, which treats sizeof(void) as 1 . Is there some way around this limitation, as casting explicitly to char * for purposes of pointer arithmetic adds to the confusion ( void * is well recognised and used as a typeless pointer to raw memory). Update0 Please note, I'm well aware of the

On AMD64 compliant architectures, addresses need to be in canonical form before being dereferenced. From the Intel manual, section 3.3.7.1 : In 64-bit mode, an address is considered to be in canonical form if address bits 63 through to the most-significant implemented bit by the microarchitecture are set to either all ones or all zeros. Now, the most significat implemented bit on current operating systems and architectures is the 47th bit. This leaves us with a 48-bit address space. Especially when ASLR is enabled, user programs can expect to receive an address with the 47th bit set. If

If I do *ptr[x] , is that equivalent to *(ptr[x]) , or (*ptr)[x] ? *(ptr[x]) See the Wikipedia operator precedence table , or, for a more detailed table, this C/C++ specific table . In C, all postfix operators have higher precedence than prefix operators, and prefix operators have higher precedence than infix operators. So its *(ptr[x]) t0mm13b Using the counter-clockwise movement of analyzing and parsing that simple example 1. starting with ptr, work in counter-clockwise until you hit asterisk operator 2. asterisk, in counter-clockwise until you hit subscript operator 3. we arrive here, at