Document: N1798
Date: 05-March-2014

Submitter: Rajan Bhakta
Submission Date: 2014/03/05


With reference to ISO/IEC WG14 N1569, subclause The macro UINTN_C(value) shall expand to an integer constant expression corresponding to the type uint_leastN_t.

7.20.4 p1 imposes a stricter requirement on the form of the expansion; it must be an integer constant (for which paragraph 2 points to

The type described in 7.20.4 p3 for the result of the expansion has an interesting property; we observe this for uint_least16_t without reference to the UINT16_C macro by using u'\0' in a context where it will be first promoted as part of the usual arithmetic conversions:

#include <assert.h>

#if u'\0' - 1 < 0
  // Types: #if (uint_least16_t) - (signed int) < (signed int)
  // Due to 6.10.1 p4, near the reference to footnote 167,
  // after applying the integer promotions as part of p1
  // to the operands of the subtraction, the expression becomes:
  // Types: #if (unsigned int) - (signed int) < (signed int)
  // Following p1 through to the last point gives:
  // Types: #if (unsigned int) - (unsigned int) < (signed int)
  // Result: false
# error Expected large unsigned value.

int main(void) {
  // Types: assert((uint_least16_t) - (signed int) < (signed int))
  // Assuming that signed int can represent all values of uint_least16_t,
  // after applying the integer promotions as part of p1
  // to the operands of the subtraction, the expression becomes:
  // Types: assert((signed int) - (signed int) < (signed int))
  // Result: true
  assert(u'\0' - 1 < 0);
  return 0;

The code presented should neither fail to compile nor abort when executed (for example) on a system using two's complement and 8, 16 and 32 bits (respectively) for char, short and int with no padding bits.

Consider the case for N = 8 or 16 on systems with INT_MAX as +2147483647, UCHAR_MAX as 255 and USHRT_MAX as 65535: it is unclear how a macro can be formed such that it expands to an integer constant that has the promoted signed int type in phase 7 of translation and also the promoted unsigned int type in phase 4 of translation without special (non-standard) support from the compiler.

Even if the requirement for an integer constant is relaxed to only require an integer constant expression, the case for N = 8 on systems with INT_MAX as +32767 and UCHAR_MAX as 255 remains a problem without the use of casts (since uint_least16_t, for which we can form a literal, has different promotion behaviour from uint_least8_t).

Implementations seen:

  1. #define UINT8_C(c) c ## U
  2. #define UINT8_C(c) c

DR 209 seemed to try to address the issue of needing special compiler support in order to define the macros for integer constants; however, the problem seems to remain.

Suggested Technical Corrigendum

  1. Add in suffixes for char and short literals.
  2. Remove the UINT{8,16}_C macros from the standard.