Does the size of pointers vary in C? [duplicate]

Pointers are not always the same size on the same arch.

You can read more on the concept of "near", "far" and "huge" pointers, just as an example of a case where pointer sizes differ...

http://en.wikipedia.org/wiki/Intel_Memory_Model#Pointer_sizes

In days of old, using e.g. Borland C compilers on the DOS platform, there were a total of (I think) 5 memory models which could even be mixed to some extent. Essentially, you had a choice of small or large pointers to data, and small or large pointers to code, and a "tiny" model where code and data had a common address space of (If I remember correctly) 64K.

It was possible to specify "huge" pointers within a program that was otherwise built in the "tiny" model. So in the worst case it was possible to have different sized pointers to the same data type in the same program!

I think the standard doesn't even forbid this, so theoretically an obscure C compiler could do this even today. But there are doubtless experts who will be able to confirm or correct this.

Pointers to data must always be compatible with void* so generally they would be nowadays realized as types of the same width.

This statement is not true for function pointers, they may have different width. For that reason in C99 casting function pointers to void* is undefined behavior.

As I understand it there is nothing in the C standard which guarantees that pointers to different types must be the same size, so in theory an int * and a float * on the same platform could be different sizes without breaking any rules.

There is a requirement that char * and void * have the same representation and alignment requirements, and there are various other similar requirements for different subsets of pointer types but there's nothing that encompasses everything.

In practise you're unlikely to run into any implementation that uses different sized pointers unless you head into some fairly obscure places.