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Stop the strtod retry loop when the decimal point is unchanged

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convert_float_locale_aware() repeated the conversion until strtod
consumed the whole token, assuming an early stop can only mean a locale
change. Under a locale whose decimal point is not a single character
(e.g. the two-byte U+066B of ar_EG.UTF-8, ar_SA.UTF-8, or fa_IR.UTF-8,
all available on macOS), the in-place substitution can never succeed,
so parsing any float that reaches the strtod fallback (for example
3.14159265358979323846 at C++11) hung forever. Before this branch, the
same input was truncated.

Retry only if the decimal point changed since the previous attempt;
otherwise keep the value strtod parsed so far, as before. Add a test
that parses such numbers under a multi-byte decimal point locale; it
hangs without this change.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-27 18:22:43 +02:00
parent b69794bd80
commit f37492a6d6
3 changed files with 72 additions and 8 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/nlohmann/detail/input/lexer.hpp
+16 -4
View File
@@ -1587,9 +1587,13 @@ scan_number_done:
looked up right before the conversion instead of once when the lexer is
constructed: a locale change in between (by a parser callback, a SAX
handler, or another thread) must not truncate the value (#5198). The
token has been validated before, so if the conversion still stops early,
the locale changed between the lookup and the call, and the conversion is
repeated with the new decimal point.
token has been validated before, so if the conversion stops early and the
decimal point changed in the meantime, the locale changed between the
lookup and the call, and the conversion is repeated with the new decimal
point. If the decimal point did not change, a retry cannot succeed: the
locale's decimal point is not a single character (e.g., the two-byte
U+066B of ar_EG.UTF-8 or fa_IR.UTF-8) and cannot be substituted in place.
The value strtod parsed up to that point is kept, as before this change.
Note that changing the locale in another thread *while* strtod runs is
undefined behavior of the C library, which this function cannot prevent.
@@ -1597,9 +1601,9 @@ scan_number_done:
void convert_float_locale_aware()
{
const bool has_dot = decimal_point_position != std::string::npos;
char decimal_point = get_decimal_point();
for (;;)
{
const char decimal_point = get_decimal_point();
const bool substitute = has_dot && decimal_point != '.';
if (substitute)
{
@@ -1619,6 +1623,14 @@ scan_number_done:
{
return;
}
// retry only if the locale changed; otherwise, this would loop forever
const char current_decimal_point = get_decimal_point();
if (current_decimal_point == decimal_point)
{
return;
}
decimal_point = current_decimal_point;
}
}
single_include/nlohmann/json.hpp
+16 -4
View File
@@ -10679,9 +10679,13 @@ scan_number_done:
looked up right before the conversion instead of once when the lexer is
constructed: a locale change in between (by a parser callback, a SAX
handler, or another thread) must not truncate the value (#5198). The
token has been validated before, so if the conversion still stops early,
the locale changed between the lookup and the call, and the conversion is
repeated with the new decimal point.
token has been validated before, so if the conversion stops early and the
decimal point changed in the meantime, the locale changed between the
lookup and the call, and the conversion is repeated with the new decimal
point. If the decimal point did not change, a retry cannot succeed: the
locale's decimal point is not a single character (e.g., the two-byte
U+066B of ar_EG.UTF-8 or fa_IR.UTF-8) and cannot be substituted in place.
The value strtod parsed up to that point is kept, as before this change.
Note that changing the locale in another thread *while* strtod runs is
undefined behavior of the C library, which this function cannot prevent.
@@ -10689,9 +10693,9 @@ scan_number_done:
void convert_float_locale_aware()
{
const bool has_dot = decimal_point_position != std::string::npos;
char decimal_point = get_decimal_point();
for (;;)
{
const char decimal_point = get_decimal_point();
const bool substitute = has_dot && decimal_point != '.';
if (substitute)
{
@@ -10711,6 +10715,14 @@ scan_number_done:
{
return;
}
// retry only if the locale changed; otherwise, this would loop forever
const char current_decimal_point = get_decimal_point();
if (current_decimal_point == decimal_point)
{
return;
}
decimal_point = current_decimal_point;
}
}
tests/src/unit-locale-cpp.cpp
+40
View File
@@ -345,3 +345,43 @@ TEST_CASE("locale changes between lexer construction and number conversion (#519
std::setlocale(LC_NUMERIC, "C");
}
TEST_CASE("locale with a multi-byte decimal point")
{
// Some locales use a decimal point that is not a single character, e.g.
// U+066B ARABIC DECIMAL SEPARATOR (two bytes in UTF-8). It cannot be
// substituted in place for '.', so the strtod fallback stops early. The
// conversion must still terminate rather than retry forever.
const std::array<const char*, 6> names = {{"ar_EG.UTF-8", "ar_SA.UTF-8", "fa_IR.UTF-8", "ps_AF.UTF-8", "ar_EG", "fa_IR"}};
bool tested = false;
for (const char* name : names)
{
if (std::setlocale(LC_NUMERIC, name) == nullptr)
{
continue;
}
const std::string decimal_point = std::localeconv()->decimal_point;
if (decimal_point.size() < 2)
{
continue;
}
CAPTURE(name);
tested = true;
// too many significant digits for Clinger's fast path, and an underflow
// that std::from_chars rejects: both reach the strtod fallback
json j;
CHECK_NOTHROW(j = json::parse("[3.14159265358979323846, 1.5e-400, -0.000123456789012345678]"));
CHECK(j.is_array());
CHECK(json::accept("3.14159265358979323846"));
// a value the locale-independent paths convert is not affected
CHECK(json::parse("12.5") == 12.5);
}
if (!tested)
{
MESSAGE("no locale with a multi-byte decimal point is usable");
}
std::setlocale(LC_NUMERIC, "C");
}