Using intptr_t instead of void*?

No, you can't be guaranteed that any particular type is a reasonable way of storing both pointers and integers, and besides, it makes your code confusing. There is a better way.

If you want to store an integer and a pointer in the same object, the clean and portable method is to use a union:

union foo {
   int integer_foo;
   void *pointer_foo;
};

This is portable and allows you to store both sorts of things in the size of storage needed for the larger of the two. It is guaranteed to always work.

Is it a good idea to use intptr_t as a general-purpose storage (to hold pointers and integer values) instead of void*?

No.

intptr_t is not guaranteed to exist. First, as you note, it was introduced in C99. Second, implementations are not required to have an integer type big enough to hold converted pointer values without loss of information.

Converting an int to intptr_t and back is unlikely to lose information but there's no actual guarantee that intptr_t is wider than int.

If you want to store pointer values, store them in pointer objects. That's what pointer objects are for.

Any pointer to an object or incomplete type can be converted to void* and back again without loss of information. There is no such guarantee for pointers to functions -- but any pointer-to-function type can be converted to any other pointer-to-function-type and back without loss of information. (I'm referring to the C standard; I think POSIX provides some additional guarantees.)

If you want to store either an integer or a pointer value in the same object, the first thing you should do is re-think your design. If you've already done so, and concluded that you really do want to do this, consider using a union (and keeping careful track of what kind of value you've stored most recently).

There are APIs that use a void* argument to allow arbitrary data to be passed; see, for example, the POSIX pthread_create() function. This can be abused by casting an integer value to void* but it's safer to pass the address of an integer object.