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[libc] Support %lc in printf #169983

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7a33e23
init
Dec 3, 2025
a9c5857
fix build
Dec 3, 2025
cef5d5b
weof fix
Dec 4, 2025
a9e0814
fix test errors
Dec 4, 2025
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6 changes: 6 additions & 0 deletions libc/src/stdio/printf_core/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -67,6 +67,7 @@ add_header_library(
parser.h
DEPENDS
.core_structs
libc.hdr.types.wint_t.h
libc.src.__support.arg_list
libc.src.__support.ctype_utils
libc.src.__support.str_to_integer
Expand Down Expand Up @@ -111,6 +112,9 @@ add_header_library(
.printf_config
.writer
libc.include.inttypes
libc.hdr.types.wchar_t
libc.hdr.types.wint_t
libc.hdr.wchar_macros
libc.src.__support.big_int
libc.src.__support.common
libc.src.__support.CPP.limits
Expand All @@ -123,6 +127,8 @@ add_header_library(
libc.src.__support.integer_to_string
libc.src.__support.libc_assert
libc.src.__support.uint128
libc.src.__support.wchar.mbstate
libc.src.__support.wchar.wcrtomb
libc.src.__support.StringUtil.error_to_string
libc.src.string.memory_utils.inline_memcpy
)
Expand Down
30 changes: 27 additions & 3 deletions libc/src/stdio/printf_core/char_converter.h
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
//===-- String Converter for printf -----------------------------*- C++ -*-===//
//===-- Character Converter for printf --------------------------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
Expand All @@ -9,7 +9,12 @@
#ifndef LLVM_LIBC_SRC_STDIO_PRINTF_CORE_CHAR_CONVERTER_H
#define LLVM_LIBC_SRC_STDIO_PRINTF_CORE_CHAR_CONVERTER_H

#include "hdr/types/wchar_t.h"
#include "hdr/types/wint_t.h"
#include "hdr/wchar_macros.h"
#include "src/__support/macros/config.h"
#include "src/__support/wchar/mbstate.h"
#include "src/__support/wchar/wcrtomb.h"
#include "src/stdio/printf_core/converter_utils.h"
#include "src/stdio/printf_core/core_structs.h"
#include "src/stdio/printf_core/writer.h"
Expand All @@ -21,7 +26,6 @@ template <WriteMode write_mode>
LIBC_INLINE int convert_char(Writer<write_mode> *writer,
const FormatSection &to_conv) {
char c = static_cast<char>(to_conv.conv_val_raw);

constexpr int STRING_LEN = 1;

size_t padding_spaces =
Expand All @@ -33,7 +37,27 @@ LIBC_INLINE int convert_char(Writer<write_mode> *writer,
RET_IF_RESULT_NEGATIVE(writer->write(' ', padding_spaces));
}

RET_IF_RESULT_NEGATIVE(writer->write(c));
if (to_conv.length_modifier == LengthModifier::l) {
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@michaelrj-google michaelrj-google Dec 4, 2025

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this looks good overall, but we'll want to have a compile flag to disable wide characters to save code size. Could you add something similar to LIBC_COPT_PRINTF_DISABLE_STRERROR that just disables the wide character portion? You'll also need to guard the includes at the top of this file and the tests.

wint_t wi = static_cast<wint_t>(to_conv.conv_val_raw);

if (wi == WEOF) {
return -1;
}

char mb_str[MB_LEN_MAX];
internal::mbstate mbstate;
wchar_t wc = static_cast<wchar_t>(wi);

auto ret = internal::wcrtomb(mb_str, wc, &mbstate);
if (!ret.has_value()) {
return -1;
}

RET_IF_RESULT_NEGATIVE(writer->write({mb_str, ret.value()}));

} else {
RET_IF_RESULT_NEGATIVE(writer->write(c));
}

// If the padding is on the right side, write the spaces last.
if (padding_spaces > 0 &&
Expand Down
13 changes: 9 additions & 4 deletions libc/src/stdio/printf_core/parser.h
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@
#ifndef LLVM_LIBC_SRC_STDIO_PRINTF_CORE_PARSER_H
#define LLVM_LIBC_SRC_STDIO_PRINTF_CORE_PARSER_H

#include "hdr/types/wint_t.h"
#include "include/llvm-libc-macros/stdfix-macros.h"
#include "src/__support/CPP/algorithm.h" // max
#include "src/__support/CPP/limits.h"
Expand Down Expand Up @@ -73,9 +74,9 @@ template <typename ArgProvider> class Parser {
ArgProvider args_cur;

#ifndef LIBC_COPT_PRINTF_DISABLE_INDEX_MODE
// args_start stores the start of the va_args, which is allows getting the
// value of arguments that have already been passed. args_index is tracked so
// that we know which argument args_cur is on.
// args_start stores the start of the va_args, which helps in getting the
// number of arguments that have already been passed. args_index is tracked
// so that we know which argument args_cur is on.
ArgProvider args_start;
size_t args_index = 1;

Expand Down Expand Up @@ -173,7 +174,11 @@ template <typename ArgProvider> class Parser {
section.has_conv = true;
break;
case ('c'):
WRITE_ARG_VAL_SIMPLEST(section.conv_val_raw, int, conv_index);
if (section.length_modifier == LengthModifier::l) {
WRITE_ARG_VAL_SIMPLEST(section.conv_val_raw, wint_t, conv_index);
} else {
WRITE_ARG_VAL_SIMPLEST(section.conv_val_raw, int, conv_index);
}
break;
case ('d'):
case ('i'):
Expand Down
3 changes: 3 additions & 0 deletions libc/test/src/stdio/printf_core/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@ add_libc_unittest(
LINK_LIBRARIES
LibcPrintfHelpers
DEPENDS
libc.hdr.types.wchar_t
libc.src.stdio.printf_core.parser
libc.src.stdio.printf_core.core_structs
libc.src.__support.CPP.string_view
Expand All @@ -32,6 +33,8 @@ add_libc_unittest(
SRCS
converter_test.cpp
DEPENDS
libc.hdr.types.wchar_t
libc.hdr.wchar_macros
libc.src.stdio.printf_core.converter
libc.src.stdio.printf_core.writer
libc.src.stdio.printf_core.core_structs
Expand Down
83 changes: 82 additions & 1 deletion libc/test/src/stdio/printf_core/converter_test.cpp
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Expand Up @@ -6,10 +6,11 @@
//
//===----------------------------------------------------------------------===//

#include "hdr/types/wchar_t.h"
#include "hdr/wchar_macros.h"
#include "src/stdio/printf_core/converter.h"
#include "src/stdio/printf_core/core_structs.h"
#include "src/stdio/printf_core/writer.h"

#include "test/UnitTest/Test.h"

class LlvmLibcPrintfConverterTest : public LIBC_NAMESPACE::testing::Test {
Expand Down Expand Up @@ -255,3 +256,83 @@ TEST_F(LlvmLibcPrintfConverterTest, OctConversion) {
ASSERT_STREQ(str, "1234");
ASSERT_EQ(writer.get_chars_written(), size_t{4});
}

TEST_F(LlvmLibcPrintfConverterTest, WideCharConversion) {
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@michaelrj-google michaelrj-google Dec 4, 2025

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For simplicity most of the functionality testing of the printf conversions is actually done in sprintf_test.cpp. Doing it that way makes the tests less verbose and more accurate to their actual use. I would recommend removing the tests from converter_test.cpp and only having the wide character tests in sprintf_test.cpp.

LIBC_NAMESPACE::printf_core::FormatSection section;
section.has_conv = true;
section.raw_string = "%lc";
section.conv_name = 'c';
section.length_modifier = LIBC_NAMESPACE::printf_core::LengthModifier::l;
section.conv_val_raw = static_cast<wchar_t>(L'€');

LIBC_NAMESPACE::printf_core::convert(&writer, section);
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@vonosmas vonosmas Dec 1, 2025

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I think you should also consider adding end-to-end test of this functionality - e.g. a test of snprintf invocation which prints converted wide characters to a multi-byte sting, and a test case in libc/test/src/stdio/printf_core/parser_test.cpp to ensure that %lc is parsed correctly.

For example, the Parser should accept the arguments of type wint_t in this case, according to specification - so probably you'd need to update the parsing code.


wb.buff[wb.buff_cur] = '\0';

ASSERT_STREQ(str, "€");
ASSERT_EQ(writer.get_chars_written(), size_t{3});
}

TEST_F(LlvmLibcPrintfConverterTest, WideCharConversionLeftJustified) {
LIBC_NAMESPACE::printf_core::FormatSection left_justified_conv;
left_justified_conv.has_conv = true;
left_justified_conv.raw_string = "%-4lc";
left_justified_conv.conv_name = 'c';
left_justified_conv.length_modifier =
LIBC_NAMESPACE::printf_core::LengthModifier::l;
left_justified_conv.flags =
LIBC_NAMESPACE::printf_core::FormatFlags::LEFT_JUSTIFIED;
left_justified_conv.min_width = 4;
left_justified_conv.conv_val_raw = static_cast<wchar_t>(L'€');

LIBC_NAMESPACE::printf_core::convert(&writer, left_justified_conv);
wb.buff[wb.buff_cur] = '\0';

ASSERT_STREQ(str, "€ ");
ASSERT_EQ(writer.get_chars_written(), size_t{6});
}

TEST_F(LlvmLibcPrintfConverterTest, WideCharConversionRightJustified) {
LIBC_NAMESPACE::printf_core::FormatSection right_justified_conv;
right_justified_conv.has_conv = true;
right_justified_conv.raw_string = "%4lc";
right_justified_conv.conv_name = 'c';
right_justified_conv.length_modifier =
LIBC_NAMESPACE::printf_core::LengthModifier::l;
right_justified_conv.min_width = 4;
right_justified_conv.conv_val_raw = static_cast<wchar_t>(L'€');

LIBC_NAMESPACE::printf_core::convert(&writer, right_justified_conv);
wb.buff[wb.buff_cur] = '\0';

ASSERT_STREQ(str, " €");
ASSERT_EQ(writer.get_chars_written(), size_t{6});
}

TEST_F(LlvmLibcPrintfConverterTest, WideCharConversionInvalid) {
LIBC_NAMESPACE::printf_core::FormatSection section;
section.has_conv = true;
section.raw_string = "%lc";
section.conv_name = 'c';
section.length_modifier = LIBC_NAMESPACE::printf_core::LengthModifier::l;
// An invalid wide character.
section.conv_val_raw = static_cast<wchar_t>(0xFFFFFFFF);

int ret = LIBC_NAMESPACE::printf_core::convert(&writer, section);

ASSERT_EQ(ret, -1);
}

TEST_F(LlvmLibcPrintfConverterTest, WideCharWEOFConversion) {
LIBC_NAMESPACE::printf_core::FormatSection section;
section.has_conv = true;
section.raw_string = "%lc";
section.conv_name = 'c';
section.length_modifier = LIBC_NAMESPACE::printf_core::LengthModifier::l;
// WEOF value.
section.conv_val_raw = static_cast<wchar_t>(WEOF);

int ret = LIBC_NAMESPACE::printf_core::convert(&writer, section);

ASSERT_EQ(ret, -1);
}
19 changes: 19 additions & 0 deletions libc/test/src/stdio/printf_core/parser_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@
//
//===----------------------------------------------------------------------===//

#include "hdr/types/wchar_t.h"
#include "src/__support/CPP/bit.h"
#include "src/__support/CPP/string_view.h"
#include "src/__support/arg_list.h"
Expand Down Expand Up @@ -370,6 +371,24 @@ TEST(LlvmLibcPrintfParserTest,
ASSERT_PFORMAT_EQ(expected, format_arr[0]);
}

TEST(LlvmLibcPrintfParserTest, EvalOneArgWithWideCharacter) {
LIBC_NAMESPACE::printf_core::FormatSection format_arr[2];
const char *str = "%lc";
wchar_t arg1 = L'€';
evaluate(format_arr, str, arg1);

LIBC_NAMESPACE::printf_core::FormatSection expected;
expected.has_conv = true;

expected.raw_string = {str, 3};
expected.length_modifier = LIBC_NAMESPACE::printf_core::LengthModifier::l;
expected.conv_val_raw =
static_cast<LIBC_NAMESPACE::fputil::FPBits<double>::StorageType>(arg1);
expected.conv_name = 'c';

ASSERT_PFORMAT_EQ(expected, format_arr[0]);
}

#ifndef LIBC_COPT_PRINTF_DISABLE_INDEX_MODE

TEST(LlvmLibcPrintfParserTest, IndexModeOneArg) {
Expand Down
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