common/include/compiler-macros.h

   1 /* nbdkit
   2  * Copyright Red Hat
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions are
   6  * met:
   7  *
   8  * * Redistributions of source code must retain the above copyright
   9  * notice, this list of conditions and the following disclaimer.
  10  *
  11  * * Redistributions in binary form must reproduce the above copyright
  12  * notice, this list of conditions and the following disclaimer in the
  13  * documentation and/or other materials provided with the distribution.
  14  *
  15  * * Neither the name of Red Hat nor the names of its contributors may be
  16  * used to endorse or promote products derived from this software without
  17  * specific prior written permission.
  18  *
  19  * THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND
  20  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
  21  * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
  22  * PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR
  23  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  24  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  25  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
  26  * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
  27  * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  28  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
  29  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  30  * SUCH DAMAGE.
  31  */
  32
  33 #ifndef NBDKIT_COMPILER_MACROS_H
  34 #define NBDKIT_COMPILER_MACROS_H
  35
  36 #ifndef __cplusplus
  37
  38 /* BUILD_BUG_UNLESS_TRUE(1) => 0
  39  * BUILD_BUG_UNLESS_TRUE(0) => compile-time failure
  40  *
  41  * It works by constructing a struct which has an impossible
  42  * (negative-sized) bitfield in the false case.  Anonymous bitfields
  43  * are permitted in C99 and above.
  44  *
  45  * The Linux kernel calls this BUILD_BUG_ON_ZERO(!cond) which is a
  46  * confusing name.  It has the same semantics except cond is negated.
  47  */
  48 #define BUILD_BUG_STRUCT_SIZE(cond) \
  49   (sizeof (struct { int: (cond) ? 1 : -1; }))
  50 #define BUILD_BUG_UNLESS_TRUE(cond) \
  51   (BUILD_BUG_STRUCT_SIZE (cond) - BUILD_BUG_STRUCT_SIZE (cond))
  52
  53 /* Each of TYPE_IS_POINTER() and TYPE_IS_ARRAY() produces a build failure if it
  54  * is invoked with an object that has neither pointer-to-object type nor array
  55  * type.
  56  *
  57  * C99 6.5.2.1 constrains one of the operands of the subscript operator to have
  58  * pointer-to-object type, and the other operand to have integer type. In the
  59  * replacement text of TYPE_IS_POINTER(), we use [0] as subscript (providing the
  60  * integer operand), therefore the macro argument (p) is constrained to have
  61  * pointer-to-object type.
  62  *
  63  * If TYPE_IS_POINTER() is invoked with a pointer that has pointer-to-object
  64  * type, the constraint is directly satisfied, and TYPE_IS_POINTER() evaluates,
  65  * at compile time, to 1.
  66  *
  67  * If TYPE_IS_POINTER() is invoked with an array, the constraint of the
  68  * subscript operator is satisfied again -- because the array argument "decays"
  69  * to a pointer to the array's initial element (C99 6.3.2p3) --, and
  70  * TYPE_IS_POINTER() evaluates, at compile time, to 0.
  71  *
  72  * If TYPE_IS_POINTER() is invoked with an argument having any other type, then
  73  * the subscript operator constraint is not satisfied, and C99 5.1.1.3p1
  74  * requires the emission of a diagnostic message -- the build breaks. Therefore,
  75  * TYPE_IS_ARRAY() can be defined simply as the logical negation of
  76  * TYPE_IS_POINTER().
  77  */
  78 #define TYPE_IS_POINTER(p) \
  79   (__builtin_types_compatible_p (typeof (p), typeof (&(p)[0])))
  80 #define TYPE_IS_ARRAY(a) (!TYPE_IS_POINTER (a))
  81
  82 #else /* __cplusplus */
  83
  84 #define BUILD_BUG_UNLESS_TRUE(cond) 0
  85 #define TYPE_IS_POINTER(p) 1
  86 #define TYPE_IS_ARRAY(a) 1
  87
  88 #endif /* __cplusplus */
  89
  90 #endif /* NBDKIT_COMPILER_MACROS_H */