[libc-commits] [libc] [libc] Support %lc in printf (PR #169301)

[Shubh Pachchigar via libc-commits]

https://github.com/shubhe25p updated https://github.com/llvm/llvm-project/pull/169301

>From 5bed042ffb9439115c0a67756e2d6fefd727dbb0 Mon Sep 17 00:00:00 2001
From: "shubh at DOE" <shubhp at mbm3a24.local>
Date: Fri, 28 Nov 2025 01:44:44 -0800
Subject: [PATCH] [libc] Support %lc in printf

Add %lc support to libc printf by utilizing
wcrtomb internal function, also added relevant
unit tests.
---
 libc/src/stdio/printf_core/CMakeLists.txt     |  3 +
 libc/src/stdio/printf_core/char_converter.h   | 24 +++++++-
 .../test/src/stdio/printf_core/CMakeLists.txt |  1 +
 .../src/stdio/printf_core/converter_test.cpp  | 55 ++++++++++++++++++-
 4 files changed, 80 insertions(+), 3 deletions(-)

diff --git a/libc/src/stdio/printf_core/CMakeLists.txt b/libc/src/stdio/printf_core/CMakeLists.txt
index 624129b2b36e7..f35a65de1f110 100644
--- a/libc/src/stdio/printf_core/CMakeLists.txt
+++ b/libc/src/stdio/printf_core/CMakeLists.txt
@@ -111,6 +111,7 @@ add_header_library(
     .printf_config
     .writer
     libc.include.inttypes
+    libc.hdr.types.wchar_t
     libc.src.__support.big_int
     libc.src.__support.common
     libc.src.__support.CPP.limits
@@ -123,6 +124,8 @@ 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
 )
diff --git a/libc/src/stdio/printf_core/char_converter.h b/libc/src/stdio/printf_core/char_converter.h
index fd2eb2553887a..3c6dc854b9fda 100644
--- a/libc/src/stdio/printf_core/char_converter.h
+++ b/libc/src/stdio/printf_core/char_converter.h
@@ -9,7 +9,10 @@
 #ifndef LLVM_LIBC_SRC_STDIO_PRINTF_CORE_CHAR_CONVERTER_H
 #define LLVM_LIBC_SRC_STDIO_PRINTF_CORE_CHAR_CONVERTER_H
 
+#include "libc/hdr/types/wchar_t.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"
@@ -20,8 +23,12 @@ namespace printf_core {
 template <WriteMode write_mode>
 LIBC_INLINE int convert_char(Writer<write_mode> *writer,
                              const FormatSection &to_conv) {
+  wchar_t wc;
+  char mb_str[MB_LEN_MAX];
+  static internal::mbstate internal_mbstate;
+  int ret = 0;
+  
   char c = static_cast<char>(to_conv.conv_val_raw);
-
   constexpr int STRING_LEN = 1;
 
   size_t padding_spaces =
@@ -33,7 +40,20 @@ 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) {
+    wc = static_cast<wchar_t>(c);
+    ret = internal::wcrtomb(mb_str, wc, &internal_mbstate);
+    if (ret <= 0) {
+      return -1;
+    }
+
+    for (int i = 0; i < ret; i++) {
+      RET_IF_RESULT_NEGATIVE(writer->write(mb_str[i]));
+    }
+
+  } else {
+    RET_IF_RESULT_NEGATIVE(writer->write(c));
+  }
 
   // If the padding is on the right side, write the spaces last.
   if (padding_spaces > 0 &&
diff --git a/libc/test/src/stdio/printf_core/CMakeLists.txt b/libc/test/src/stdio/printf_core/CMakeLists.txt
index ff7ebbc4f5fd0..a4c919420777d 100644
--- a/libc/test/src/stdio/printf_core/CMakeLists.txt
+++ b/libc/test/src/stdio/printf_core/CMakeLists.txt
@@ -35,4 +35,5 @@ add_libc_unittest(
     libc.src.stdio.printf_core.converter
     libc.src.stdio.printf_core.writer
     libc.src.stdio.printf_core.core_structs
+    libc.hdr.types.wchar_t
 )
diff --git a/libc/test/src/stdio/printf_core/converter_test.cpp b/libc/test/src/stdio/printf_core/converter_test.cpp
index 2dae2a22c864c..c181175e6314f 100644
--- a/libc/test/src/stdio/printf_core/converter_test.cpp
+++ b/libc/test/src/stdio/printf_core/converter_test.cpp
@@ -9,7 +9,7 @@
 #include "src/stdio/printf_core/converter.h"
 #include "src/stdio/printf_core/core_structs.h"
 #include "src/stdio/printf_core/writer.h"
-
+#include "libc/hdr/types/wchar_t.h"
 #include "test/UnitTest/Test.h"
 
 class LlvmLibcPrintfConverterTest : public LIBC_NAMESPACE::testing::Test {
@@ -255,3 +255,56 @@ TEST_F(LlvmLibcPrintfConverterTest, OctConversion) {
   ASSERT_STREQ(str, "1234");
   ASSERT_EQ(writer.get_chars_written(), size_t{4});
 }
+
+TEST_F(LlvmLibcPrintfConverterTest, WideCharConversion) {
+
+  LIBC_NAMESPACE::printf_core::FormatSection section;
+  section.has_conv = true;
+  section.raw_string = "%c";
+  section.conv_name = 'c';
+  section.length_modifier = LIBC_NAMESPACE::printf_core::LengthModifier::l;
+  section.conv_val_raw = static_cast<wchar_t>(L'S');
+
+  LIBC_NAMESPACE::printf_core::convert(&writer, section);
+
+  wb.buff[wb.buff_cur] = '\0';
+
+  ASSERT_STREQ(str, "S");
+  ASSERT_EQ(writer.get_chars_written(), size_t{1});
+}
+
+TEST_F(LlvmLibcPrintfConverterTest, WideCharConversionLeftJustified) {
+  LIBC_NAMESPACE::printf_core::FormatSection left_justified_conv;
+  left_justified_conv.has_conv = true;
+  left_justified_conv.raw_string = "%-4c";
+  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'S');
+
+  LIBC_NAMESPACE::printf_core::convert(&writer, left_justified_conv);
+  wb.buff[wb.buff_cur] = '\0';
+
+  ASSERT_STREQ(str, "S   ");
+  ASSERT_EQ(writer.get_chars_written(), size_t{4});
+}
+
+TEST_F(LlvmLibcPrintfConverterTest, WideCharConversionRightJustified) {
+  LIBC_NAMESPACE::printf_core::FormatSection right_justified_conv;
+  right_justified_conv.has_conv = true;
+  right_justified_conv.raw_string = "%4c";
+  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'S');
+
+  LIBC_NAMESPACE::printf_core::convert(&writer, right_justified_conv);
+  wb.buff[wb.buff_cur] = '\0';
+
+  ASSERT_STREQ(str, "   S");
+  ASSERT_EQ(writer.get_chars_written(), size_t{4});
+}