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<xlocnum>: Fix num_put::do_put() to avoid frexp()'s unspecified behavior for inf/nan #6168

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Merged
Stephan T. Lavavej (StephanTLavavej) merged 3 commits into from
Mar 24, 2026
Merged

<xlocnum>: Fix num_put::do_put() to avoid frexp()'s unspecified behavior for inf/nan #6168

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5cae4fb
Avoid calling frexp() for inf/nan.
StephanTLavavej Mar 19, 2026
c44af80
Avoid garbage-init for frexp()'s integer exponent.
StephanTLavavej Mar 19, 2026
c814ee3
Enhance test coverage to expect specific strings.
StephanTLavavej Mar 19, 2026
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2 changes: 1 addition & 1 deletion stl/inc/random
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Original file line number Diff line number Diff line change
Expand Up @@ -70,7 +70,7 @@ _INLINE_VAR constexpr int _Nwords = 4;
template <class _Elem, class _Traits>
basic_ostream<_Elem, _Traits>& _Write(
basic_ostream<_Elem, _Traits>& _Os, long double _Dx) { // write long double to stream
int _Ex;
int _Ex = 0;
long double _Frac = _CSTD frexpl(_Dx, &_Ex);
for (int _Nw = 0; _Nw < _Nwords; ++_Nw) { // break into 31-bit words
_Frac *= _Two31;
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16 changes: 8 additions & 8 deletions stl/inc/xlocnum
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Original file line number Diff line number Diff line change
Expand Up @@ -1385,15 +1385,15 @@ protected:
const streamsize _Precision = _Is_hex ? -1 : _Iosbase.precision(); // precision setting
const int _Desired_precision =
_Float_put_desired_precision<double>(_Precision, _Float_flags); // desired precision
size_t _Bufsize = static_cast<size_t>(_Desired_precision);
if (_Is_fixed && 1e10 < _CSTD fabs(_Val)) { // f or F format
int _Ptwo;
size_t _Bufsize = static_cast<size_t>(_Desired_precision);
const bool _Is_finite = (_STD isfinite) (_Val);
if (_Is_fixed && _Is_finite && 1e10 < _CSTD fabs(_Val)) { // f or F format
int _Ptwo = 0;
(void) _CSTD frexp(_Val, &_Ptwo);
_Bufsize += _CSTD abs(_Ptwo) * 30103L / 100000L;
}

_Buf.resize(_Bufsize + 50); // add fudge factor
const bool _Is_finite = (_STD isfinite) (_Val);
const auto _Adjusted_flags = // TRANSITION, DevCom-10519861
_Is_finite ? _Iosbase.flags() : _Iosbase.flags() & ~ios_base::showpoint;

Expand All @@ -1419,15 +1419,15 @@ protected:
const streamsize _Precision = _Is_hex ? -1 : _Iosbase.precision(); // precision setting
const int _Desired_precision =
_Float_put_desired_precision<long double>(_Precision, _Float_flags); // desired precision
size_t _Bufsize = static_cast<size_t>(_Desired_precision);
if (_Is_fixed && 1e10 < _CSTD fabsl(_Val)) { // f or F format
int _Ptwo;
size_t _Bufsize = static_cast<size_t>(_Desired_precision);
const bool _Is_finite = (_STD isfinite) (_Val);
if (_Is_fixed && _Is_finite && 1e10 < _CSTD fabsl(_Val)) { // f or F format
int _Ptwo = 0;
(void) _CSTD frexpl(_Val, &_Ptwo);
_Bufsize += _CSTD abs(_Ptwo) * 30103L / 100000L;
}

_Buf.resize(_Bufsize + 50); // add fudge factor
const bool _Is_finite = (_STD isfinite) (_Val);
const auto _Adjusted_flags = // TRANSITION, DevCom-10519861
_Is_finite ? _Iosbase.flags() : _Iosbase.flags() & ~ios_base::showpoint;

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14 changes: 9 additions & 5 deletions tests/std/tests/GH_003867_output_nan/test.cpp
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Expand Up @@ -58,7 +58,8 @@ void test_gh_3867() {
// Also test GH-4210: With setprecision(0) showpoint fixed, a bogus '.' is emitted for infinity and NaN

template <class FloatingPoint>
void test_output_nonfinite_value(const FloatingPoint x) {
void test_output_nonfinite_value(
const FloatingPoint x, const string& expected_noshowpos, const string& expected_showpos) {
const auto s1 = [x] {
ostringstream os;
os << setprecision(0) << showpoint << fixed;
Expand All @@ -72,6 +73,7 @@ void test_output_nonfinite_value(const FloatingPoint x) {
return os.str();
}();
assert(s1 == s2);
assert(s1 == expected_noshowpos);

const auto s3 = [x] {
ostringstream os;
Expand All @@ -86,17 +88,19 @@ void test_output_nonfinite_value(const FloatingPoint x) {
return os.str();
}();
assert(s3 == s4);
assert(s3 == expected_showpos);
}

template <class FloatingPoint>
void test_gh_4210() {
constexpr auto inf_val = numeric_limits<FloatingPoint>::infinity();
constexpr auto nan_val = numeric_limits<FloatingPoint>::quiet_NaN();

test_output_nonfinite_value(inf_val);
test_output_nonfinite_value(-inf_val);
test_output_nonfinite_value(nan_val);
test_output_nonfinite_value(-nan_val);
// These expected results are implementation-defined:
test_output_nonfinite_value(inf_val, "inf", "+inf");
test_output_nonfinite_value(-inf_val, "-inf", "-inf");
test_output_nonfinite_value(nan_val, "nan", "+nan");
test_output_nonfinite_value(-nan_val, "-nan(ind)", "-nan(ind)");
}

int main() {
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