[libc] Add strncpy implementation.

Add libc strncpy implementation.

Reviewed By: sivachandra, gchatelet

Differential Revision: https://reviews.llvm.org/D91399

2 files changed, 67 insertions, 0 deletions

diff --git a/libc/test/src/string/CMakeLists.txt b/libc/test/src/string/CMakeLists.txt
index df824cd18ecc..8b78fcfdd020 100644
--- a/libc/test/src/string/CMakeLists.txt
+++ b/libc/test/src/string/CMakeLists.txt
@@ -73,6 +73,16 @@ add_libc_unittest(
 )
 
 add_libc_unittest(
+  strncpy_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strncpy_test.cpp
+  DEPENDS
+    libc.src.string.strncpy
+)
+
+add_libc_unittest(
   strnlen_test
   SUITE
     libc_string_unittests
diff --git a/libc/test/src/string/strncpy_test.cpp b/libc/test/src/string/strncpy_test.cpp
new file mode 100644
index 000000000000..814870613251
--- /dev/null
+++ b/libc/test/src/string/strncpy_test.cpp
@@ -0,0 +1,57 @@
+//===-- Unittests for strncpy ---------------------------------------------===//
+//
+// 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/string/strncpy.h"
+#include "utils/CPP/ArrayRef.h"
+#include "utils/UnitTest/Test.h"
+#include <stddef.h> // For size_t.
+
+class StrncpyTest : public __llvm_libc::testing::Test {
+public:
+  void check_strncpy(__llvm_libc::cpp::MutableArrayRef<char> dst,
+                     const __llvm_libc::cpp::ArrayRef<char> src, size_t n,
+                     const __llvm_libc::cpp::ArrayRef<char> expected) {
+    // Making sure we don't overflow buffer.
+    ASSERT_GE(dst.size(), n);
+    // Making sure strncpy returns dst.
+    ASSERT_EQ(__llvm_libc::strncpy(dst.data(), src.data(), n), dst.data());
+    // Expected must be of the same size as dst.
+    ASSERT_EQ(dst.size(), expected.size());
+    // Expected and dst are the same.
+    for (size_t i = 0; i < expected.size(); ++i)
+      ASSERT_EQ(expected[i], dst[i]);
+  }
+};
+
+TEST_F(StrncpyTest, Untouched) {
+  char dst[] = {'a', 'b'};
+  const char src[] = {'x', '\0'};
+  const char expected[] = {'a', 'b'};
+  check_strncpy(dst, src, 0, expected);
+}
+
+TEST_F(StrncpyTest, CopyOne) {
+  char dst[] = {'a', 'b'};
+  const char src[] = {'x', 'y'};
+  const char expected[] = {'x', 'b'}; // no \0 is appended
+  check_strncpy(dst, src, 1, expected);
+}
+
+TEST_F(StrncpyTest, CopyNull) {
+  char dst[] = {'a', 'b'};
+  const char src[] = {'\0', 'y'};
+  const char expected[] = {'\0', 'b'};
+  check_strncpy(dst, src, 1, expected);
+}
+
+TEST_F(StrncpyTest, CopyPastSrc) {
+  char dst[] = {'a', 'b'};
+  const char src[] = {'\0', 'y'};
+  const char expected[] = {'\0', '\0'};
+  check_strncpy(dst, src, 2, expected);
+}