[libc] wcpncpy implementation (#145430)

Implemented wcpncpy and tests.

---------

Co-authored-by: Sriya Pratipati <sriyap@google.com>

libc/test/src/wchar/CMakeLists.txt

@@ -255,3 +255,13 @@ add_libc_test(
   DEPENDS
     libc.src.wchar.wcpcpy
 )
+
+add_libc_test(
+  wcpncpy_test
+  SUITE
+    libc_wchar_unittests
+  SRCS
+    wcpncpy_test.cpp
+  DEPENDS
+    libc.src.wchar.wcpncpy
+)

libc/test/src/wchar/wcpncpy_test.cpp

@@ -0,0 +1,84 @@
+//===-- Unittests for wcpncpy --------------------------------------------===//
+//
+// 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/wcpncpy.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcWCPNCpyTest, EmptySrc) {
+  // Empty src should lead to empty destination.
+  wchar_t dest[4] = {L'a', L'b', L'c', L'\0'};
+  const wchar_t *src = L"";
+  LIBC_NAMESPACE::wcpncpy(dest, src, 3);
+  ASSERT_TRUE(dest[0] == src[0]);
+  ASSERT_TRUE(dest[0] == L'\0');
+  // Rest should also be padded with L'\0'
+  ASSERT_TRUE(dest[1] == L'\0');
+  ASSERT_TRUE(dest[2] == L'\0');
+}
+
+TEST(LlvmLibcWCPNCpyTest, Untouched) {
+  wchar_t dest[] = {L'a', L'b'};
+  const wchar_t src[] = {L'x', L'\0'};
+  LIBC_NAMESPACE::wcpncpy(dest, src, 0);
+  ASSERT_TRUE(dest[0] == L'a');
+  ASSERT_TRUE(dest[1] == L'b');
+}
+
+TEST(LlvmLibcWCPNCpyTest, CopyOne) {
+  wchar_t dest[] = {L'a', L'b'};
+  const wchar_t src[] = {L'x', L'y'};
+  wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, src, 1);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_EQ(dest + 1, res);
+}
+
+TEST(LlvmLibcWCPNCpyTest, CopyNull) {
+  wchar_t dest[] = {L'a', L'b'};
+  const wchar_t src[] = {L'\0', L'y'};
+  wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, src, 1);
+  ASSERT_TRUE(dest[0] == L'\0');
+  ASSERT_TRUE(dest[1] == L'b');
+  ASSERT_EQ(dest + 1, res);
+}
+
+TEST(LlvmLibcWCPNCpyTest, CopyPastSrc) {
+  wchar_t dest[] = {L'a', L'b'};
+  const wchar_t src[] = {L'\0', L'y'};
+  wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, src, 2);
+  ASSERT_TRUE(dest[0] == L'\0');
+  ASSERT_TRUE(dest[1] == L'\0');
+  ASSERT_EQ(dest + 2, res);
+}
+
+TEST(LlvmLibcWCPNCpyTest, CopyTwoNoNull) {
+  wchar_t dest[] = {L'a', L'b'};
+  const wchar_t src[] = {L'x', L'y'};
+  wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, src, 2);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'y');
+  ASSERT_EQ(dest + 2, res);
+}
+
+TEST(LlvmLibcWCPNCpyTest, CopyTwoWithNull) {
+  wchar_t dest[] = {L'a', L'b'};
+  const wchar_t src[] = {L'x', L'\0'};
+  wchar_t *res = LIBC_NAMESPACE::wcpncpy(dest, src, 2);
+  ASSERT_TRUE(dest[0] == L'x');
+  ASSERT_TRUE(dest[1] == L'\0');
+  ASSERT_EQ(dest + 2, res);
+}
+
+#if defined(LIBC_ADD_NULL_CHECKS) && !defined(LIBC_HAS_SANITIZER)
+TEST(LlvmLibcWCPNCpyTest, NullptrCrash) {
+  // Passing in a nullptr should crash the program.
+  EXPECT_DEATH([] { LIBC_NAMESPACE::wcpncpy(nullptr, nullptr, 1); },
+               WITH_SIGNAL(-1));
+}
+#endif // LIBC_HAS_ADDRESS_SANITIZER