[libc-commits] [libc] [libc] implement ungetwc (PR #196162)
Michael Jones via libc-commits
libc-commits at lists.llvm.org
Wed May 6 12:55:21 PDT 2026
https://github.com/michaelrj-google created https://github.com/llvm/llvm-project/pull/196162
Add ungetwc function and tests. Part 6/11. All build file changes are in
part 11.
>From f127ff1cce825c097159753f03d3783c5408298d Mon Sep 17 00:00:00 2001
From: Michael Jones <michaelrj at google.com>
Date: Wed, 6 May 2026 18:27:55 +0000
Subject: [PATCH] [libc] implement ungetwc
Add ungetwc function and tests. Part 6/11. All build file changes are in
part 11.
---
libc/src/wchar/ungetwc.cpp | 26 ++++
libc/src/wchar/ungetwc.h | 22 +++
libc/test/src/wchar/ungetwc_test.cpp | 208 +++++++++++++++++++++++++++
3 files changed, 256 insertions(+)
create mode 100644 libc/src/wchar/ungetwc.cpp
create mode 100644 libc/src/wchar/ungetwc.h
create mode 100644 libc/test/src/wchar/ungetwc_test.cpp
diff --git a/libc/src/wchar/ungetwc.cpp b/libc/src/wchar/ungetwc.cpp
new file mode 100644
index 0000000000000..ddefbfaa79c2d
--- /dev/null
+++ b/libc/src/wchar/ungetwc.cpp
@@ -0,0 +1,26 @@
+//===-- Implementation of ungetwc -----------------------------------------===//
+//
+// 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/ungetwc.h"
+#include "hdr/types/FILE.h"
+#include "hdr/types/wint_t.h"
+#include "src/__support/File/file.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/__support/macros/null_check.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(wint_t, ungetwc, (wint_t wc, ::FILE *stream)) {
+ LIBC_CRASH_ON_NULLPTR(stream);
+
+ auto *f = reinterpret_cast<File *>(stream);
+ return f->ungetwc(wc);
+}
+
+} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/wchar/ungetwc.h b/libc/src/wchar/ungetwc.h
new file mode 100644
index 0000000000000..9cb34b154da02
--- /dev/null
+++ b/libc/src/wchar/ungetwc.h
@@ -0,0 +1,22 @@
+//===-- Implementation header for ungetwc ---------------------------------===//
+//
+// 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_UNGETWC_H
+#define LLVM_LIBC_SRC_WCHAR_UNGETWC_H
+
+#include "hdr/types/FILE.h"
+#include "hdr/types/wint_t.h"
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+wint_t ungetwc(wint_t wc, ::FILE *stream);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_WCHAR_UNGETWC_H
diff --git a/libc/test/src/wchar/ungetwc_test.cpp b/libc/test/src/wchar/ungetwc_test.cpp
new file mode 100644
index 0000000000000..01c8bd00852b1
--- /dev/null
+++ b/libc/test/src/wchar/ungetwc_test.cpp
@@ -0,0 +1,208 @@
+//===-- Unittests for ungetwc ---------------------------------------------===//
+//
+// 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/stdio_macros.h" // For SEEK_CUR
+#include "hdr/wchar_macros.h" // For WEOF
+#include "src/stdio/fclose.h"
+#include "src/stdio/feof.h"
+#include "src/stdio/ferror.h"
+#include "src/stdio/fopen.h"
+#include "src/stdio/fseek.h"
+#include "src/stdio/fwrite.h"
+#include "src/wchar/fgetwc.h"
+#include "src/wchar/fwide.h"
+#include "src/wchar/ungetwc.h"
+#include "test/UnitTest/Test.h"
+
+TEST(LlvmLibcUngetwcTest, PushBackAndRead) {
+ auto FILENAME =
+ libc_make_test_file_path(APPEND_LIBC_TEST("ungetwc_push.test"));
+ ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w");
+ ASSERT_FALSE(file == nullptr);
+
+ // Write "12"
+ constexpr char CONTENT[] = "12";
+ ASSERT_EQ(LIBC_NAMESPACE::fwrite(CONTENT, 1, sizeof(CONTENT) - 1, file),
+ sizeof(CONTENT) - 1);
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+
+ // Open for reading
+ file = LIBC_NAMESPACE::fopen(FILENAME, "r");
+ ASSERT_FALSE(file == nullptr);
+
+ // Read first char -> '1'
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'1'));
+
+ // Push back 'X'
+ EXPECT_EQ(LIBC_NAMESPACE::ungetwc(L'X', file), static_cast<wint_t>(L'X'));
+
+ // Read again -> should get 'X'
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'X'));
+
+ // Read again -> should get '2'
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'2'));
+
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+}
+
+TEST(LlvmLibcUngetwcTest, PushBackWEOF) {
+ auto FILENAME =
+ libc_make_test_file_path(APPEND_LIBC_TEST("ungetwc_weof.test"));
+ ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w");
+ ASSERT_FALSE(file == nullptr);
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+
+ file = LIBC_NAMESPACE::fopen(FILENAME, "r");
+ ASSERT_FALSE(file == nullptr);
+
+ // Push back WEOF should do nothing and return WEOF
+ EXPECT_EQ(LIBC_NAMESPACE::ungetwc(WEOF, file), static_cast<wint_t>(WEOF));
+
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+}
+
+TEST(LlvmLibcUngetwcTest, ByteModeFailure) {
+ auto FILENAME =
+ libc_make_test_file_path(APPEND_LIBC_TEST("ungetwc_bytemode.test"));
+ ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w+");
+ ASSERT_FALSE(file == nullptr);
+
+ // Orient to byte mode
+ EXPECT_LT(LIBC_NAMESPACE::fwide(file, -1), 0);
+
+ // Push back should fail
+ EXPECT_EQ(LIBC_NAMESPACE::ungetwc(L'a', file), static_cast<wint_t>(WEOF));
+
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+}
+
+TEST(LlvmLibcUngetwcTest, OrientUnorientedStream) {
+ auto FILENAME =
+ libc_make_test_file_path(APPEND_LIBC_TEST("ungetwc_orient.test"));
+ ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w+");
+ ASSERT_FALSE(file == nullptr);
+
+ // Verify initial unoriented state
+ EXPECT_EQ(LIBC_NAMESPACE::fwide(file, 0), 0);
+
+ // Push back a char
+ EXPECT_EQ(LIBC_NAMESPACE::ungetwc(L'A', file), static_cast<wint_t>(L'A'));
+
+ // Verify stream is now wide-oriented
+ EXPECT_GT(LIBC_NAMESPACE::fwide(file, 0), 0);
+
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+}
+
+TEST(LlvmLibcUngetwcTest, ClearEofIndicator) {
+ auto FILENAME =
+ libc_make_test_file_path(APPEND_LIBC_TEST("ungetwc_cleareof.test"));
+ ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w");
+ ASSERT_FALSE(file == nullptr);
+
+ // Write "1"
+ constexpr char CONTENT[] = "1";
+ ASSERT_EQ(LIBC_NAMESPACE::fwrite(CONTENT, 1, sizeof(CONTENT) - 1, file),
+ sizeof(CONTENT) - 1);
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+
+ file = LIBC_NAMESPACE::fopen(FILENAME, "r");
+ ASSERT_FALSE(file == nullptr);
+
+ // Read '1'
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'1'));
+
+ // Read past EOF
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(WEOF));
+ EXPECT_NE(LIBC_NAMESPACE::feof(file), 0);
+
+ // ungetwc should clear EOF indicator
+ EXPECT_EQ(LIBC_NAMESPACE::ungetwc(L'1', file), static_cast<wint_t>(L'1'));
+ EXPECT_EQ(LIBC_NAMESPACE::feof(file), 0);
+ EXPECT_EQ(LIBC_NAMESPACE::ferror(file), 0); // error remains unmodified
+
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+}
+
+TEST(LlvmLibcUngetwcTest, DiscardOnFilePositioning) {
+ auto FILENAME =
+ libc_make_test_file_path(APPEND_LIBC_TEST("ungetwc_discard.test"));
+ ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w");
+ ASSERT_FALSE(file == nullptr);
+
+ // Write "12"
+ constexpr char CONTENT[] = "12";
+ ASSERT_EQ(LIBC_NAMESPACE::fwrite(CONTENT, 1, sizeof(CONTENT) - 1, file),
+ sizeof(CONTENT) - 1);
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+
+ file = LIBC_NAMESPACE::fopen(FILENAME, "r");
+ ASSERT_FALSE(file == nullptr);
+
+ // Read first char -> '1'
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'1'));
+
+ // Push back 'X'
+ EXPECT_EQ(LIBC_NAMESPACE::ungetwc(L'X', file), static_cast<wint_t>(L'X'));
+
+ // Seek to absolute position 1 (after '1') to discard pushed-back char
+ EXPECT_EQ(LIBC_NAMESPACE::fseek(file, 1, SEEK_SET), 0);
+
+ // Read again -> should get '2' instead of 'X'
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'2'));
+
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+}
+
+TEST(LlvmLibcUngetwcTest, LifoMultiplePushbacks) {
+ auto FILENAME =
+ libc_make_test_file_path(APPEND_LIBC_TEST("ungetwc_lifo.test"));
+ ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w+");
+ ASSERT_FALSE(file == nullptr);
+
+ // Push back 'X' then 'Y'
+ EXPECT_EQ(LIBC_NAMESPACE::ungetwc(L'X', file), static_cast<wint_t>(L'X'));
+ // If multiple push-backs are supported, verify LIFO ordering
+ wint_t second_push = LIBC_NAMESPACE::ungetwc(L'Y', file);
+ if (second_push == static_cast<wint_t>(L'Y')) {
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'Y'));
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'X'));
+ } else {
+ // Only one push-back supported on this platform/implementation
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'X'));
+ }
+
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+}
+
+TEST(LlvmLibcUngetwcTest, PushbackAtStartOfFile) {
+ auto FILENAME =
+ libc_make_test_file_path(APPEND_LIBC_TEST("ungetwc_start.test"));
+ ::FILE *file = LIBC_NAMESPACE::fopen(FILENAME, "w");
+ ASSERT_FALSE(file == nullptr);
+
+ // Write "1"
+ constexpr char CONTENT[] = "1";
+ ASSERT_EQ(LIBC_NAMESPACE::fwrite(CONTENT, 1, sizeof(CONTENT) - 1, file),
+ sizeof(CONTENT) - 1);
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+
+ file = LIBC_NAMESPACE::fopen(FILENAME, "r");
+ ASSERT_FALSE(file == nullptr);
+
+ // Push back 'Z' at the very beginning without prior reads
+ EXPECT_EQ(LIBC_NAMESPACE::ungetwc(L'Z', file), static_cast<wint_t>(L'Z'));
+
+ // Read first char -> should be 'Z'
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'Z'));
+
+ // Read next char -> should be '1'
+ EXPECT_EQ(LIBC_NAMESPACE::fgetwc(file), static_cast<wint_t>(L'1'));
+
+ ASSERT_EQ(LIBC_NAMESPACE::fclose(file), 0);
+}
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