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

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