Question

I’m wondering why C99 allows conversions between incompatible pointer types:

void f(int* p)
{
}

void c(char* p)
{
   f(p);
}

Does C11 also allow them? Are conforming implementations required to diagnose such conversions?

Answer 1

C99 doesn’t permit implicit conversion between pointers of different types (except to/from void*). here’s what the C99 Rationale says:

It is invalid to convert a pointer to an object of any type to a pointer to an object of a different type without an explicit cast.

This is a consequence of the rules for assignment, which has a constraint that one of the following shall hold (when pointers are involved) (C99 6.5.16.1 “Simple assignment”):

• both operands are pointers to qualified or unqualified versions of compatible types, and the type pointed to by the left has all the
  qualifiers of the type pointed to by the right;
• one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void, and
  the type pointed to by the left has all the qualifiers of the type
  pointed to by the right;
• the left operand is a pointer and the right is a null pointer constant

Passing a pointer as an argument to a prototyped function follows the same rules because (C99 6.5.2.2/7 “Function calls”):

If the expression that denotes the called function has a type that
does include a prototype, the arguments are implicitly converted, as
if by assignment, to the types of the corresponding parameters

Both C90 and C11 have similar wording.

I believe that many compilers (including GCC) relax this constraint to issue only a warning because there’s too much legacy code that depends on it. Keep in mind that void* was an invention of the ANSI C standard, so pre-standard, and probably a lot of post-standard, code generally used char* or int* as a ‘generic’ pointer type.