Question

I’ve a theoretical question about these two statements:

Assuming p a pointer to an Integer and a an Integer:

a) if(p==a){.....} or if(p>a)..

b) p=a;

All of them are illegal and b is especially dangerous but how does standard C consider them?

Reading the standard, I haven’t found if they are errors, undefined behaviors, unspecified behavior, constraint violation, if one of them is legal or other.

Looking in the countless similar question, I haven’t found a solution.

Answer 1

C11 (n1570) §6.5.8 [Relational operators] / 2:

Constraints

One of the following shall hold:

• both operands have real type; or
• both operands are pointers to qualified or unqualified versions of compatible object
  types.

(real type means the integer types, enums, real floating point types and char (§6.2.5/17))

§6.5.9 [Equality operators]

Constraints

One of the following shall hold:

• both operands have arithmetic type;
• both operands are pointers to qualified or unqualified versions of compatible types;
• one operand is a pointer to an object type and the other is a pointer to a qualified or unqualified version of void; or
• one operand is a pointer and the other is a null pointer constant.

§6.5.16.1 [Simple assignment] / 1:

Constraints

One of the following shall hold:

• the left operand has atomic, qualified, or unqualified arithmetic type, and the righ has arithmetic type;
• the left operand has an atomic, qualified, or unqualified version of a structure or union type compatible with the type of the right;
• the left operand has atomic, qualified, or unqualified pointer type, and (considering
  the type the left operand would have after lvalue conversion) 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;
• the left operand has atomic, qualified, or unqualified pointer type, and (considering
  the type the left operand would have after lvalue conversion) one operand is a pointer
  to an object 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 an atomic, qualified, or unqualified pointer, and the right is a null
  pointer constant; or
  the left operand has type atomic, qualified, or unqualified _Bool, and the right is a
  pointer.

Basically, all 3 expressions violated the constraints set in the expression.

---

What does violating constraints mean? Errors? But nowhere says that violation of constraints shall cause the translation to fail. The C standard doesn’t seem to be very clear, but the closest interpretation would be:

§4 [Conformance] / 2:

If a “shall” or “shall not” requirement that appears outside of a constraint or runtime-constraint is violated, the behavior is undefined.

but this talks about outside of constraints; and §5.1.1.3 [Diagnostics] / 1:

A conforming implementation shall produce at least one diagnostic message (identified in
an implementation-defined manner) if a preprocessing translation unit or translation unit
contains a violation of any syntax rule or constraint, even if the behavior is also explicitly specified as undefined or implementation-defined. Diagnostic messages need not be produced in other circumstances.

so the compiler needs to at least produce a message (as we can see in gcc), but did not say what they should do after that.

The example after this paragraph mentions that

EXAMPLE

An implementation shall issue a diagnostic for the translation unit:

   char i;
   int i;

because in those cases where wording in this International Standard describes the behavior for a construct as being both a constraint error and resulting in undefined behavior, the constraint error shall be diagnosed.

and §6.7 [Declarations] / 3 does not explicitly mention that violating the Constraint will cause undefined behavior.

Therefore, I deduce that your 3 expressions are all undefined behaviors, accompanied with diagnostic messages.