[libc] wcsncpy implementation

libc/config/linux/x86_64/entrypoints.txt

@@ -374,6 +374,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.wchar.wmemcmp
     libc.src.wchar.wmempcpy
     libc.src.wchar.wmemcpy
+    libc.src.wchar.wcsncpy
     libc.src.wchar.wcscat
     libc.src.wchar.wcsstr
     libc.src.wchar.wcsncat

libc/include/wchar.yaml

@@ -95,6 +95,14 @@ functions:
       - type: const wchar_t *
       - type: size_t
   - name: wmemcpy
+    standards:
+      - stdc
+    return_type: wchar_t *
+    arguments: 
+      - type: __restrict wchar_t *
+      - type: const __restrict wchar_t *
+      - type: size_t
+  - name: wcsncpy
     standards:
       - stdc
     return_type: wchar_t *

libc/src/wchar/CMakeLists.txt

@@ -137,6 +137,18 @@ add_entrypoint_object(
     libc.src.__support.wctype_utils
 )
 
+add_entrypoint_object(
+  wcsncpy
+  SRCS
+    wcsncpy.cpp
+  HDRS
+    wcsncpy.h
+  DEPENDS
+    libc.hdr.types.size_t
+    libc.hdr.wchar_macros
+    libc.src.string.string_utils
+)
+
 add_entrypoint_object(
   wcscat
   SRCS

libc/src/wchar/wcsncpy.cpp

@@ -0,0 +1,33 @@
+//===-- Implementation of wcsncpy -----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/wchar/wcsncpy.h"
+
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/string/memory_utils/inline_memcpy.h"
+#include "src/string/string_utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(wchar_t *, wcsncpy,
+                   (wchar_t *__restrict s1, const wchar_t *__restrict s2,
+                    size_t n)) {
+  size_t i = 0;
+  // Copy up until \0 is found.
+  for (; i < n && s2[i] != L'\0'; ++i)
+    s1[i] = s2[i];
+  // When s2 is shorter than n, append \0.
+  for (; i < n; ++i)
+    s1[i] = L'\0';
+  return s1;
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/wchar/wcsncpy.h

@@ -0,0 +1,23 @@
+//===-- Implementation header for wcsncpy ---------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_WCHAR_WCSNCPY_H
+#define LLVM_LIBC_SRC_WCHAR_WCSNCPY_H
+
+#include "hdr/types/size_t.h"
+#include "hdr/types/wchar_t.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+wchar_t *wcsncpy(wchar_t *__restrict s1, const wchar_t *__restrict s2,
+                 size_t n);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_WCHAR_WCSNCPY_H

libc/test/src/wchar/CMakeLists.txt

@@ -135,6 +135,16 @@ add_libc_test(
     libc.src.wchar.wmemcpy
 )
 
+add_libc_test(
+  wcsncpy_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcsncpy_test.cpp
+  DEPENDS
+    libc.src.wchar.wcsncpy
+)
+
 add_libc_test(
   wcscat_test
   SUITE

libc/test/src/wchar/wcsncpy_test.cpp

@@ -0,0 +1,66 @@
+//===-- Unittests for wcsncpy ---------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "hdr/types/wchar_t.h"
+#include "src/wchar/wcsncpy.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcWCSNCpyTest, CopyZero) {
+  // Dest should remain unchanged.
+  wchar_t dest[3] = {L'a', L'b', L'\0'};
+  const wchar_t *src = L"x";
+  LIBC_NAMESPACE::wcsncpy(dest, src, 0);
+  ASSERT_TRUE(dest[0] == L'a');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_TRUE(dest[2] == L'\0');
+}
+
+TEST(LlvmLibcWCSNCpyTest, CopyFullIntoEmpty) {
+  // Dest should be the exact same as src.
+  wchar_t dest[15];
+  const wchar_t *src = L"aaaaabbbbccccc";
+  LIBC_NAMESPACE::wcsncpy(dest, src, 15);
+  for (int i = 0; i < 15; i++)
+    ASSERT_TRUE(dest[i] == src[i]);
+}
+
+TEST(LlvmLibcWCSNCpyTest, CopyPartial) {
+  // First two characters of dest should be the first two characters of src.
+  wchar_t dest[] = {L'a', L'b', L'c', L'd', L'\0'};
+  const wchar_t *src = L"1234";
+  LIBC_NAMESPACE::wcsncpy(dest, src, 2);
+  ASSERT_TRUE(dest[0] == L'1');
+  ASSERT_TRUE(dest[1] == L'2');
+  ASSERT_TRUE(dest[2] == L'c');
+  ASSERT_TRUE(dest[3] == L'd');
+  ASSERT_TRUE(dest[4] == L'\0');
+}
+
+TEST(LlvmLibcWCSNCpyTest, CopyNullTerminator) {
+  // Null terminator should copy into dest.
+  wchar_t dest[] = {L'a', L'b', L'c', L'd', L'\0'};
+  const wchar_t src[] = {L'\0', L'y'};
+  LIBC_NAMESPACE::wcsncpy(dest, src, 1);
+  ASSERT_TRUE(dest[0] == L'\0');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_TRUE(dest[2] == L'c');
+  ASSERT_TRUE(dest[3] == L'd');
+  ASSERT_TRUE(dest[4] == L'\0');
+}
+
+TEST(LlvmLibcWCSNCpyTest, CopyPastSrc) {
+  // Copying past src should fill with null terminator.
+  wchar_t dest[] = {L'a', L'b', L'c', L'd', L'\0'};
+  const wchar_t src[] = {L'x', L'\0'};
+  LIBC_NAMESPACE::wcsncpy(dest, src, 4);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'\0');
+  ASSERT_TRUE(dest[2] == L'\0');
+  ASSERT_TRUE(dest[3] == L'\0');
+  ASSERT_TRUE(dest[4] == L'\0');
+}