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

Answer 1

Pointers are always the same size on the same arch, regardless of datatype it points to. Otherwise things like casting between different pointer types be useless, and break a lot of things.

Many common coding techniques depend on casting between for instance a void pointer (or a char) to various structs, depending on size.

Take even the standard printf(), is must be able to take pointers to various kinds of data. If pointers were of different size, implementing printf could get very messy.

Answer 2

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.

Answer 3

Can I assume that pointers of different types have the same size (on one arch of course)?

For the platforms with flat memory model (== all popular/modern platforms) pointer size would be the same.

For the platforms with segmented memory model, for efficiency, often there are platform-specific pointer types of different sizes. (E.g. far pointers in the DOS, since 8086 CPU used segmented memory model.) But this is platform specific and non-standard.

You probably should keep in mind that in C++ size of normal pointer might differ from size of pointer to virtual method. Pointers to virtual methods has to preserve extra bit of information to not to work properly with polymorphism. This is probably only exception I'm aware of, which is still relevant (since I doubt that segmented memory model would ever make it back).

Answer 4

Yes, the size of a pointer is platform dependent. More specifically, the size of a pointer depends on the target processor architecture and the "bit-ness" you compile for.

As a rule of thumb, on a 64bit machine a pointer is usually 64bits, on a 32bit machine usually 32 bits. There are exceptions however.

Since a pointer is just a memory address its always the same size regardless of what the memory it points to contains. So a pointer to a float, a char or an int are all the same size.

Answer 5

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

Answer 6

I was going to write a reply saying that C99 has various pointer conversion requirements that more or less ensure that pointers to data have to be all the same size. However, on reading them carefully, I realised that C99 is specifically designed to allow pointers to be of different sizes for different types.

For instance on an architecture where the integers are 4 bytes and must be 4 byte aligned an int pointer could be two bits smaller than a char or void pointer. Provided the cast actually does the shift in both directions, you're fine with C99. It helpfully says that the result of casting a char pointer to an incorrectly aligned int pointer is undefined.

See the C99 standard. Section 6.3.2.3