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

[Shubh Pachchigar via libc-commits]

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

Add %lc support to libc printf by utilizing wctomb internal function, also added relevant unit tests. Resolves #166598 

>From 731814a7e40f75023751c677e21cc84ec506f473 Mon Sep 17 00:00:00 2001
From: "shubh at DOE" <shubhp at mbm3a24.local>
Date: Mon, 24 Nov 2025 01:21:03 -0800
Subject: [PATCH] [libc] Support %lc in printf

Add %lc support to libc printf by utilizing
wctomb internal function, also added unit
tests.
---
 libc/src/stdio/printf_core/char_converter.h   | 22 ++++++--
 .../test/src/stdio/printf_core/CMakeLists.txt |  2 +
 .../src/stdio/printf_core/converter_test.cpp  | 51 +++++++++++++++++++
 3 files changed, 72 insertions(+), 3 deletions(-)

diff --git a/libc/src/stdio/printf_core/char_converter.h b/libc/src/stdio/printf_core/char_converter.h
index fd2eb2553887a..58ade4fbf9a53 100644
--- a/libc/src/stdio/printf_core/char_converter.h
+++ b/libc/src/stdio/printf_core/char_converter.h
@@ -13,6 +13,7 @@
 #include "src/stdio/printf_core/converter_utils.h"
 #include "src/stdio/printf_core/core_structs.h"
 #include "src/stdio/printf_core/writer.h"
+#include "src/wchar/wctomb.h"
 
 namespace LIBC_NAMESPACE_DECL {
 namespace printf_core {
@@ -20,8 +21,10 @@ namespace printf_core {
 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);
-
+  char c;
+  wchar_t wc;
+  char mb_str[MB_LEN_MAX];
+  int ret = 0;
   constexpr int STRING_LEN = 1;
 
   size_t padding_spaces =
@@ -33,7 +36,20 @@ 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>(to_conv.conv_val_raw);
+    ret = wctomb(mb_str, wc);
+    if (ret <= 0) {
+      return INT_CONVERSION_ERROR;
+    }
+    for (int i = 0; i < ret; ++i) {
+      RET_IF_RESULT_NEGATIVE(writer->write(mb_str[i]));
+    }
+  }
+  else {
+    c = static_cast<char>(to_conv.conv_val_raw);
+    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..f075b8bd5ec52 100644
--- a/libc/test/src/stdio/printf_core/CMakeLists.txt
+++ b/libc/test/src/stdio/printf_core/CMakeLists.txt
@@ -35,4 +35,6 @@ add_libc_unittest(
     libc.src.stdio.printf_core.converter
     libc.src.stdio.printf_core.writer
     libc.src.stdio.printf_core.core_structs
+    libc.src.wchar.wctomb
+    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..2f7842fa97168 100644
--- a/libc/test/src/stdio/printf_core/converter_test.cpp
+++ b/libc/test/src/stdio/printf_core/converter_test.cpp
@@ -255,3 +255,54 @@ 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});
+}