CODEWITHSUNDEEP
cmacrosc99
user6357139
user6357139

Reputation:

Get number of bits in size_t at compile time

I am trying to get the number of value bits in size_t to be used in the preprocessor directives. Perhaps there is a macro for this? In essence, I would like to achieve something akin to this code where SIZE_T_BITS is a hypothetical macro for the sake of demonstration.

#if SIZE_T_BITS == 32
    // code for 32 bit size_t
#elif SIZE_T_BITS == 64
    // code for 64 bit size_t
#else
    // code for other bit sizes of size_t
#endif

Upvotes: 2

Views: 1325

Answers (2)

chux
chux

Reputation: 153468

size_t is some unsigned type. Compare the max value to common candidates. The max value is certainly some 2SIZE_T_BITS - 1. The smallest SIZE_MAX may be is 0xFFFF.

#include <stdint.h>
#if (SIZE_MAX == 0xFFFF)
  #define SIZE_T_BITS 16
#elif (SIZE_MAX == 0xFFFFFFFF)
  #define SIZE_T_BITS 32
#elif (SIZE_MAX == 0xFFFFFFFFFFFFFFFF)
  #define SIZE_T_BITS 64
#else
  #error TBD code SIZE_T_BITS
#endif

Although size_t may have paddings bits (this is rare), the about method reflects the number of value bits in size_t. This could differ from the total bits.

Note: SIZE_MAX is defined such that

Each instance of these macros shall be replaced by a constant expression suitable for use in #if preprocessing directives, and this expression shall have the same type as would an expression that is an object of the corresponding type converted according to the integer promotions. C11 §7.20.3 2

Upvotes: 6

c-smile
c-smile

Reputation: 27460

This will work for GCC, CLang and MSVC:

#if defined(__x86_64__) || defined(_IA64) || defined(__IA64__) || defined(_M_X64)
  #define SIZE_T_BITS 64
#else 
  #define SIZE_T_BITS 32

Upvotes: -1

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