Submitter: UK C Panel
Submission Date: 2005-03-04
Source: Joseph Myers <joseph@codesourcery.com>
Reference Document: ISO/IEC WG14
N1102
Version: 1.0
Date: 2005-03-04
Subject: Cross-translation-unit tagged type compatibility
Summary
Compatibility of tagged types can be intransitive when there are multiple translation units:
// TU 1:
struct s;
struct t { struct s *a; };
// TU 2:
struct s { int p; };
struct t { struct s *a; };
// TU 3:
struct s { long q; };
struct t { struct s *a; };
where struct t in TU 1 is compatible with that in TU 2 and TU 3 but
they are not compatible with each other.
C++ avoids problems with such cases by giving types linkage, meaning that the incompatible definitions of named struct types yield undefined behavior. In C types do not have linkage, and such incompatibilities can give rise to several problems. The requirement of 6.2.7#2 that "All declarations that refer to the same object or function shall have compatible type; otherwise, the behavior is undefined." does not seem sufficient to avoid all such problems.
Question 1: Does 6.2.7#2 refer to the types immediately after the declarations, or the types at any point where the declarations are in scope?
Question 2: If each of the above three translation units started
extern struct t *x;, would there be undefined behavior?
Even if the requirement applies to the types anywhere in scope, this may not be enough. Each translation unit above could have prepended to it
struct t;
static void f(void) { extern struct t *x; }
and the incompatible completions are not within the scope of x. (x
might then be defined in another translation unit that just says
struct t *x;.)
The above example at least leads to incompatible "ultimate" types for
x that the object has at the end of each translation unit, albeit
outside the scope of the declaration. But now consider the following
three translation units.
// TU 1:
struct s;
struct t { struct s *a; };
int g1(struct s *);
int g2(struct s *);
int f(struct t *p, int x) { return x ? g1(p->a) : g2(p->a); }
// TU 2:
struct t { int b; };
struct s { int a; struct t *z; };
int g1(struct s *p) { return p->a + p->z->b; }
// TU 3:
struct t { long c; };
struct s { struct t *z; long a; };
int g2(struct s *p) { return p->a + p->z->c; }
Each object and function has a well-defined complete type. But a
struct t in TU 1 may contain pointers to two different versions of
struct s, and each of these contains pointers to an entirely different
struct t from that in TU 1. This requires very strange gymnastics for
an implementation inlining across translation units to inline g1 and
g2 into f. There is no single translation unit representing a natural
merger of the three above; renaming static objects with conflicting
names does not suffice (indeed, there are no such objects), and
renaming type names used in different names in different translation
units does not help either because there is no single natural
expression of a recursive completion of TU 1's struct t.
Question 3: Is an implementation required to accept compiling the three translation units above together into a program?
This issue arises directly from actual implementation problems implementing optimizations across multiple translation units in GCC. It is natural for an implementation to take multiple translation units and convert them into a language-independent intermediate representation of the whole program which is then optimized, and in so doing to unify the declarations in different translation units which refer to the same object or function. But unifying them involves unifying their types, and so recursively the types involved in the definitions of those types, and the above translation units, although apparently valid to link into a single program at present, do not admit of such a unification.
Suggested Technical Corrigendum