Where is `%p` useful with printf?

Question

After all, both these statements do the same thing...

int a = 10; int *b = &a; printf("%p\n",b); printf("%08X\n",b); 

For example (with different addresses):

0012FEE0 0012FEE0 

It is trivial to format the pointer as desired with %x, so is there some good use of the %p option?

Answer 1

They do not do the same thing. The latter printf statement interprets b as an unsigned int, which is wrong, as b is a pointer.

Pointers and unsigned ints are not always the same size, so these are not interchangeable. When they aren't the same size (an increasingly common case, as 64-bit CPUs and operating systems become more common), %x will only print half of the address. On a Mac (and probably some other systems), that will ruin the address; the output will be wrong.

Always use %p for pointers.

Answer 2

At least on one system that is not very uncommon, they do not print the same:

~/src> uname -m i686 ~/src> gcc -v Using built-in specs. Target: i686-pc-linux-gnu [some output snipped] gcc version 4.1.2 (Gentoo 4.1.2) ~/src> gcc -o printfptr printfptr.c ~/src> ./printfptr 0xbf8ce99c bf8ce99c 

Notice how the pointer version adds a 0x prefix, for instance. Always use %p since it knows about the size of pointers, and how to best represent them as text.