Submitter:
Rajan Bhakta
Submission Date:
2014/03/05
Summary
With reference to ISO/IEC WG14 N1569, subclause 7.20.4.1: 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 6.4.4.1).
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 6.3.1.8 p1
// to the operands of the subtraction, the expression becomes:
// Types: #if (unsigned int) - (signed int) < (signed int)
// Following 6.3.1.8 p1 through to the last point gives:
// Types: #if (unsigned int) - (unsigned int) < (signed int)
// Result: false
# error Expected large unsigned value.
#endif
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 6.3.1.8 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:
#define UINT8_C(c) c ## U#define UINT8_C(c) cDR 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
UINT{8,16}_C macros from the standard.