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Fix printf conversion of a zero value with a precision of zero #4942

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Fix printf conversion of a zero value with a precision of zero #4942

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735ca7b
Fix printf conversion of a zero value with a precision of zero
sachhg Sep 10, 2026
1fcee22
Keep the zero precision handling in printf.h
sachhg Sep 13, 2026
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30 changes: 29 additions & 1 deletion include/fmt/printf.h
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Original file line number Diff line number Diff line change
Expand Up @@ -246,6 +246,19 @@ class printf_arg_formatter : public arg_formatter<Char> {
detail::write<Char>(this->out, value, this->specs, this->locale);
}

// C requires no characters when converting a zero value with a precision of
// zero, so only the sign and the padding remain.
void write_zero_with_zero_precision() {
auto s = this->specs;
const char* sign_str = s.sign() == sign::plus ? "+"
: s.sign() == sign::space ? " "
: "";
s.set_type(presentation_type::none);
if (s.align() == align::none || s.align() == align::numeric)
s.set_align(align::right);
write_bytes<Char>(this->out, sign_str, s);
}

public:
printf_arg_formatter(basic_appender<Char> iter, format_specs& s,
context_type& ctx)
Expand All @@ -258,6 +271,13 @@ class printf_arg_formatter : public arg_formatter<Char> {
// MSVC2013 fails to compile separate overloads for bool and Char so use
// std::is_same instead.
if (!std::is_same<T, Char>::value) {
auto t = this->specs.type();
if (value == 0 && this->specs.precision == 0 &&
(t == presentation_type::dec || t == presentation_type::oct ||
t == presentation_type::hex)) {
write_zero_with_zero_precision();
return;
}
write(value);
return;
}
Expand Down Expand Up @@ -490,7 +510,10 @@ void vprintf(buffer<Char>& buf, basic_string_view<Char> format,
str, to_unsigned(nul != str_end ? nul - str : specs.precision));
arg = sv;
}
if (specs.alt() && arg.visit(is_zero_int())) specs.clear_alt();
// '#' has no effect on a zero value except for octal, where it forces a
// single '0'. The conversion specifier is not parsed yet, so remember it.
bool alt_zero = specs.alt() && arg.visit(is_zero_int());
if (alt_zero) specs.clear_alt();
if (specs.fill_unit<Char>() == '0') {
if (is_arithmetic_type(arg.type()) && specs.align() != align::left) {
specs.set_align(align::numeric);
Expand Down Expand Up @@ -550,6 +573,11 @@ void vprintf(buffer<Char>& buf, basic_string_view<Char> format,
if (specs.type() == presentation_type::none)
report_error("invalid format specifier");
if (upper) specs.set_upper();
// For '#o', C requires a single '0' when the value and the precision are
// both zero.
if (alt_zero && specs.type() == presentation_type::oct &&
specs.precision == 0)
specs.precision = 1;

start = it;

Expand Down
33 changes: 33 additions & 0 deletions test/printf-test.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -278,6 +278,39 @@ TEST(printf_test, int_precision) {
EXPECT_PRINTF("00042 ", "%-#10.5o", 042);
}

// C99 7.21.6.1: the result of converting a zero value with a precision of zero
// is no characters.
TEST(printf_test, zero_int_with_zero_precision) {
EXPECT_PRINTF("", "%.0d", 0);
EXPECT_PRINTF("", "%.d", 0);
EXPECT_PRINTF("", "%.0i", 0);
EXPECT_PRINTF("", "%.0o", 0);
EXPECT_PRINTF("", "%.0u", 0);
EXPECT_PRINTF("", "%.0x", 0);
EXPECT_PRINTF("", "%.0X", 0);

// '#' forces a single '0' for octal, but has no effect on other conversions.
EXPECT_PRINTF("0", "%#.0o", 0);
EXPECT_PRINTF("", "%#.0x", 0);
EXPECT_PRINTF("", "%#.0X", 0);

// The sign, space and width still apply.
EXPECT_PRINTF("+", "%+.0d", 0);
EXPECT_PRINTF(" ", "% .0d", 0);
EXPECT_PRINTF(" ", "%5.0d", 0);
EXPECT_PRINTF(" ", "%-5.0d", 0);
EXPECT_PRINTF(" ", "%05.0d", 0);
EXPECT_PRINTF(" 0", "%#5.0o", 0);
EXPECT_PRINTF("0 ", "%#-5.0o", 0);

// A nonzero value or a nonzero precision is unaffected.
EXPECT_PRINTF("42", "%.0d", 42);
EXPECT_PRINTF("-42", "%.0d", -42);
EXPECT_PRINTF("ff", "%.0x", 255);
EXPECT_PRINTF("0", "%.1d", 0);
EXPECT_PRINTF("00", "%.2d", 0);
}

TEST(printf_test, float_precision) {
char buffer[256];
safe_sprintf(buffer, "%.3e", 1234.5678);
Expand Down
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