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         (0 ? BUILD_ASSERT_OR_ZERO(cast_sign_compatible(type, (expr))) : \
  19          (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  * We cast via intptr_t to suppress gcc's -Wcast-qual (which SAMBA
  30  * uses), and via the ? : so Sun CC doesn't complain about the result
  31  * not being constant.
  32  *
  33  * If @type is a pointer to a pointer, you must use cast_const2 (etc).
  34  *
  35  * Example:
  36  *      // Dumb open-coded strstr variant.
  37  *      static char *find_needle(const char *haystack)
  38  *      {
  39  *              size_t i;
  40  *              for (i = 0; i < strlen(haystack); i++)
  41  *                      if (memcmp("needle", haystack+i, strlen("needle")) == 0)
  42  *                              return cast_const(char *, haystack+i);
  43  *              return NULL;
  44  *      }
  45  */
  46 #define cast_const(type, expr)                                          \
  47         (0 ? BUILD_ASSERT_OR_ZERO(cast_const_compat1((expr), type)) :   \
  48          (type)(intptr_t)(expr))
  49
  50 /**
  51  * cast_const2 - remove a const qualifier from a pointer to a pointer.
  52  * @type: some pointer to pointer type.
  53  * @expr: expression to cast.
  54  *
  55  * This ensures that you are only removing the const qualifier from an
  56  * expression.  The expression must otherwise match @type.
  57  */
  58 #define cast_const2(type, expr)                                         \
  59         (0 ? BUILD_ASSERT_OR_ZERO(cast_const_compat2((expr), type)) :   \
  60          (type)(intptr_t)(expr))
  61
  62 /**
  63  * cast_const3 - remove a const from a pointer to a pointer to a pointer..
  64  * @type: some pointer to pointer to pointer type.
  65  * @expr: expression to cast.
  66  *
  67  * This ensures that you are only removing the const qualifier from an
  68  * expression.  The expression must otherwise match @type.
  69  */
  70 #define cast_const3(type, expr)                                         \
  71         (0 ? BUILD_ASSERT_OR_ZERO(cast_const_compat3((expr), type)) :   \
  72          (type)(intptr_t)(expr))
  73
  74
  75 /**
  76  * cast_static - explicit mimic of implicit cast.
  77  * @type: some type.
  78  * @expr: expression to cast.
  79  *
  80  * This ensures that the cast is not to or from a pointer: it can only be
  81  * an implicit cast, such as a pointer to a similar const pointer, or between
  82  * integral types.
  83  */
  84 #if HAVE_COMPOUND_LITERALS
  85 #define cast_static(type, expr)                 \
  86         ((struct { type x; }){(expr)}.x)
  87 #else
  88 #define cast_static(type, expr)                 \
  89         ((type)(expr))
  90 #endif
  91
  92 /* Herein lies the gcc magic to evoke compile errors. */
  93 #if HAVE_BUILTIN_CHOOSE_EXPR && HAVE_BUILTIN_TYPES_COMPATIBLE_P && HAVE_TYPEOF
  94 #define cast_sign_compatible(t, e) \
  95   __builtin_choose_expr(                                                \
  96           __builtin_types_compatible_p(__typeof__(t), char *) ||        \
  97           __builtin_types_compatible_p(__typeof__(t), signed char *) || \
  98           __builtin_types_compatible_p(__typeof__(t), unsigned char *), \
  99           /* if type is not const qualified */                          \
 100           __builtin_types_compatible_p(__typeof__(e), char *) ||        \
 101           __builtin_types_compatible_p(__typeof__(e), signed char *) || \
 102           __builtin_types_compatible_p(__typeof__(e), unsigned char *), \
 103           /* and if it is... */                                         \
 104           __builtin_types_compatible_p(__typeof__(e), const char *) ||  \
 105           __builtin_types_compatible_p(__typeof__(e), const signed char *) || \
 106           __builtin_types_compatible_p(__typeof__(e), const unsigned char *) ||\
 107           __builtin_types_compatible_p(__typeof__(e), char *) ||        \
 108           __builtin_types_compatible_p(__typeof__(e), signed char *) || \
 109           __builtin_types_compatible_p(__typeof__(e), unsigned char *)  \
 110           )
 111
 112 #define cast_const_strip1(expr)                 \
 113         __typeof__(*(union { int z; __typeof__(expr) x; }){0}.x)
 114 #define cast_const_strip2(expr) \
 115         __typeof__(**(union { int z; __typeof__(expr) x; }){0}.x)
 116 #define cast_const_strip3(expr) \
 117         __typeof__(***(union { int z; __typeof__(expr) x; }){0}.x)
 118 #define cast_const_compat1(expr, type)                                  \
 119         __builtin_types_compatible_p(cast_const_strip1(expr),           \
 120                                      cast_const_strip1(type))
 121 #define cast_const_compat2(expr, type)                                  \
 122         __builtin_types_compatible_p(cast_const_strip2(expr),           \
 123                                      cast_const_strip2(type))
 124 #define cast_const_compat3(expr, type)                                  \
 125         __builtin_types_compatible_p(cast_const_strip3(expr),           \
 126                                      cast_const_strip3(type))
 127 #else
 128 #define cast_sign_compatible(type, expr)                \
 129         (sizeof(*(type)0) == 1 && sizeof(*(expr)) == 1)
 130 #define cast_const_compat1(expr, type)          (1)
 131 #define cast_const_compat2(expr, type)          (1)
 132 #define cast_const_compat3(expr, type)          (1)
 133 #endif
 134 #endif /* CCAN_CAST_H */