git.ozlabs.org Git - ccan/blob - ccan/cast/cast.h

   1 /* Licensed under LGPLv2.1+ - see LICENSE file for details */
   2 #ifndef CCAN_CAST_H
   3 #define CCAN_CAST_H
   4 #include "config.h"
   5 #include <stdint.h>
   6 #include <ccan/build_assert/build_assert.h>
   7
   8 /**
   9  * cast_signed - cast a (const) char * to/from (const) signed/unsigned char *.
  10  * @type: some char * variant.
  11  * @expr: expression (of some char * variant) to cast.
  12  *
  13  * Some libraries insist on an unsigned char in various places; cast_signed
  14  * makes sure (with suitable compiler) that the expression you are casting
  15  * only differs in signed/unsigned, not in type or const-ness.
  16  */
  17 #define cast_signed(type, expr)                                         \
  18         ((type)(expr)                                                   \
  19          + BUILD_ASSERT_OR_ZERO(cast_sign_compatible(type, (expr))))
  20
  21 /**
  22  * cast_const - remove a const qualifier from a pointer.
  23  * @type: some pointer type.
  24  * @expr: expression to cast.
  25  *
  26  * This ensures that you are only removing the const qualifier from an
  27  * expression.  The expression must otherwise match @type.
  28  *
  29  * If @type is a pointer to a pointer, you must use cast_const2 (etc).
  30  *
  31  * Example:
  32  *      // Dumb open-coded strstr variant.
  33  *      static char *find_needle(const char *haystack)
  34  *      {
  35  *              size_t i;
  36  *              for (i = 0; i < strlen(haystack); i++)
  37  *              if (memcmp("needle", haystack+i, strlen("needle")) == 0)
  38  *                      return cast_const(char *, haystack+i);
  39  *              return NULL;
  40  *      }
  41  */
  42 #define cast_const(type, expr)                                          \
  43         ((type)((intptr_t)(expr)                                        \
  44                 + BUILD_ASSERT_OR_ZERO(cast_const_compat1((expr), type))))
  45
  46 /**
  47  * cast_const2 - remove a const qualifier from a pointer to a pointer.
  48  * @type: some pointer to pointer type.
  49  * @expr: expression to cast.
  50  *
  51  * This ensures that you are only removing the const qualifier from an
  52  * expression.  The expression must otherwise match @type.
  53  */
  54 #define cast_const2(type, expr)                                         \
  55         ((type)((intptr_t)(expr)                                        \
  56                 + BUILD_ASSERT_OR_ZERO(cast_const_compat2((expr), type))))
  57
  58 /**
  59  * cast_const3 - remove a const from a pointer to a pointer to a pointer..
  60  * @type: some pointer to pointer to pointer type.
  61  * @expr: expression to cast.
  62  *
  63  * This ensures that you are only removing the const qualifier from an
  64  * expression.  The expression must otherwise match @type.
  65  */
  66 #define cast_const3(type, expr)                                         \
  67         ((type)((intptr_t)(expr)                                        \
  68                 + BUILD_ASSERT_OR_ZERO(cast_const_compat3((expr), type))))
  69
  70
  71 /**
  72  * cast_static - explicit mimic of implicit cast.
  73  * @type: some type.
  74  * @expr: expression to cast.
  75  *
  76  * This ensures that the cast is not to or from a pointer: it can only be
  77  * an implicit cast, such as a pointer to a similar const pointer, or between
  78  * integral types.
  79  */
  80 #if HAVE_COMPOUND_LITERALS
  81 #define cast_static(type, expr)                 \
  82         ((struct { type x; }){(expr)}.x)
  83 #else
  84 #define cast_static(type, expr)                 \
  85         ((type)(expr))
  86 #endif
  87
  88 /* Herein lies the gcc magic to evoke compile errors. */
  89 #if HAVE_BUILTIN_CHOOSE_EXPR && HAVE_BUILTIN_TYPES_COMPATIBLE_P && HAVE_TYPEOF
  90 #define cast_sign_compatible(t, e) \
  91   __builtin_choose_expr(                                                \
  92           __builtin_types_compatible_p(__typeof__(t), char *) ||        \
  93           __builtin_types_compatible_p(__typeof__(t), signed char *) || \
  94           __builtin_types_compatible_p(__typeof__(t), unsigned char *), \
  95           /* if type is not const qualified */                          \
  96           __builtin_types_compatible_p(__typeof__(e), char *) ||        \
  97           __builtin_types_compatible_p(__typeof__(e), signed char *) || \
  98           __builtin_types_compatible_p(__typeof__(e), unsigned char *), \
  99           /* and if it is... */                                         \
 100           __builtin_types_compatible_p(__typeof__(e), const char *) ||  \
 101           __builtin_types_compatible_p(__typeof__(e), const signed char *) || \
 102           __builtin_types_compatible_p(__typeof__(e), const unsigned char *) ||\
 103           __builtin_types_compatible_p(__typeof__(e), char *) ||        \
 104           __builtin_types_compatible_p(__typeof__(e), signed char *) || \
 105           __builtin_types_compatible_p(__typeof__(e), unsigned char *)  \
 106           )
 107
 108 #define cast_const_strip1(expr)                 \
 109         __typeof__(*(struct { int z; __typeof__(expr) x; }){0}.x)
 110 #define cast_const_strip2(expr) \
 111         __typeof__(**(struct { int z; __typeof__(expr) x; }){0}.x)
 112 #define cast_const_strip3(expr) \
 113         __typeof__(***(struct { int z; __typeof__(expr) x; }){0}.x)
 114 #define cast_const_compat1(expr, type)                                  \
 115         __builtin_types_compatible_p(cast_const_strip1(expr),           \
 116                                      cast_const_strip1(type))
 117 #define cast_const_compat2(expr, type)                                  \
 118         __builtin_types_compatible_p(cast_const_strip2(expr),           \
 119                                      cast_const_strip2(type))
 120 #define cast_const_compat3(expr, type)                                  \
 121         __builtin_types_compatible_p(cast_const_strip3(expr),           \
 122                                      cast_const_strip3(type))
 123 #else
 124 #define cast_sign_compatible(type, expr)                \
 125         (sizeof(*(type)0) == 1 && sizeof(*(expr)) == 1)
 126 #define cast_const_compat1(expr, type)          (1)
 127 #define cast_const_compat2(expr, type)          (1)
 128 #define cast_const_compat3(expr, type)          (1)
 129 #endif
 130 #endif /* CCAN_CAST_H */