[Libguestfs] [libnbd PATCH v5 1/5] common/include: add TYPE_IS_POINTER() macro

Because the current definition of TYPE_IS_ARRAY() already contains a negation, defining TYPE_IS_POINTER() in terms of TYPE_IS_ARRAY() would lead to double negation, which I consider less than ideal style. Therefore, introduce TYPE_IS_POINTER() from scratch, and rebase TYPE_IS_ARRAY() on top of TYPE_IS_POINTER(). Suggested-by: Richard W.M. Jones <rjones(a)redhat.com&gt; Signed-off-by: Laszlo Ersek <lersek(a)redhat.com&gt; Reviewed-by: Eric Blake <eblake(a)redhat.com&gt; --- Notes: v5: - pick up Eric's R-b - no change v4: - new patch in v4 common/include/compiler-macros.h | 31 ++++++++++++++++++-- 1 file changed, 29 insertions(+), 2 deletions(-) diff --git a/common/include/compiler-macros.h b/common/include/compiler-macros.h index 7933bb87a5bf..beb3f4fe4e5c 100644 --- a/common/include/compiler-macros.h +++ b/common/include/compiler-macros.h @@ -50,12 +50,39 @@ #define BUILD_BUG_UNLESS_TRUE(cond) \ (BUILD_BUG_STRUCT_SIZE (cond) - BUILD_BUG_STRUCT_SIZE (cond)) -#define TYPE_IS_ARRAY(a) \ - (!__builtin_types_compatible_p (typeof (a), typeof (&(a)[0]))) +/* Each of TYPE_IS_POINTER() and TYPE_IS_ARRAY() produces a build failure if it + * is invoked with an object that has neither pointer-to-object type nor array + * type. + * + * C99 6.5.2.1 constrains one of the operands of the subscript operator to have + * pointer-to-object type, and the other operand to have integer type. In the + * replacement text of TYPE_IS_POINTER(), we use [0] as subscript (providing the + * integer operand), therefore the macro argument (p) is constrained to have + * pointer-to-object type. + * + * If TYPE_IS_POINTER() is invoked with a pointer that has pointer-to-object + * type, the constraint is directly satisfied, and TYPE_IS_POINTER() evaluates, + * at compile time, to 1. + * + * If TYPE_IS_POINTER() is invoked with an array, the constraint of the + * subscript operator is satisfied again -- because the array argument "decays" + * to a pointer to the array's initial element (C99 6.3.2p3) --, and + * TYPE_IS_POINTER() evaluates, at compile time, to 0. + * + * If TYPE_IS_POINTER() is invoked with an argument having any other type, then + * the subscript operator constraint is not satisfied, and C99 5.1.1.3p1 + * requires the emission of a diagnostic message -- the build breaks. Therefore, + * TYPE_IS_ARRAY() can be defined simply as the logical negation of + * TYPE_IS_POINTER(). + */ +#define TYPE_IS_POINTER(p) \ + (__builtin_types_compatible_p (typeof (p), typeof (&(p)[0]))) +#define TYPE_IS_ARRAY(a) (!TYPE_IS_POINTER (a)) #else /* __cplusplus */ #define BUILD_BUG_UNLESS_TRUE(cond) 0 +#define TYPE_IS_POINTER(p) 1 #define TYPE_IS_ARRAY(a) 1 #endif /* __cplusplus */