[libc] Implement perror

The perror function writes an error message directly to stderr. This
patch adds an implementation, tests, and header generation details.

Author: michaelrj-google

libc/config/linux/x86_64/entrypoints.txt

@@ -1113,6 +1113,7 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.stdio.getc_unlocked
     libc.src.stdio.getchar
     libc.src.stdio.getchar_unlocked
+    libc.src.stdio.perror
     libc.src.stdio.putc
     libc.src.stdio.putchar
     libc.src.stdio.puts

libc/include/stdio.yaml

@@ -247,6 +247,12 @@ functions:
       - POSIX
     return_type: int
     arguments: []
+  - name: perror
+    standards:
+      - stdc
+    return_type: void
+    arguments:
+      - type: const char *
   - name: printf
     standards:
       - stdc

libc/src/stdio/CMakeLists.txt

@@ -221,6 +221,7 @@ add_stdio_entrypoint_object(fopen)
 add_stdio_entrypoint_object(fclose)
 add_stdio_entrypoint_object(fread_unlocked)
 add_stdio_entrypoint_object(fread)
+add_stdio_entrypoint_object(perror)
 add_stdio_entrypoint_object(puts)
 add_stdio_entrypoint_object(fputs)
 add_stdio_entrypoint_object(fwrite_unlocked)

libc/src/stdio/generic/CMakeLists.txt

@@ -206,6 +206,21 @@ add_generic_entrypoint_object(
     libc.src.__support.File.platform_file
 )
 
+add_generic_entrypoint_object(
+  perror
+  SRCS
+    perror.cpp
+  HDRS
+    ../perror.h
+  DEPENDS
+    libc.src.errno.errno
+    libc.src.__support.StringUtil.error_to_string
+    libc.src.__support.CPP.string_view
+    libc.src.__support.File.file
+    libc.src.__support.File.platform_file
+    libc.src.__support.File.platform_stderr
+)
+
 add_generic_entrypoint_object(
   fputs
   SRCS

libc/src/stdio/generic/perror.cpp

@@ -0,0 +1,87 @@
+//===-- Implementation of perror ------------------------------------------===//
+//
+// 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/stdio/perror.h"
+#include "src/__support/CPP/string_view.h"
+#include "src/__support/File/file.h"
+#include "src/__support/StringUtil/error_to_string.h"
+#include "src/__support/macros/config.h"
+#include "src/errno/libc_errno.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+namespace {
+
+// TODO: this is copied from `puts`, it should be moved to a shared utility.
+//  Simple helper to unlock the file once destroyed.
+struct ScopedLock {
+  ScopedLock(LIBC_NAMESPACE::File *stream) : stream(stream) { stream->lock(); }
+  ~ScopedLock() { stream->unlock(); }
+
+private:
+  LIBC_NAMESPACE::File *stream;
+};
+
+int write_out(cpp::string_view str_view, File *f) {
+  if (str_view.size() > 0) {
+    auto result = f->write_unlocked(str_view.data(), str_view.size());
+    if (result.has_error())
+      return result.error;
+  }
+  return 0;
+}
+
+} // namespace
+
+// TODO: this seems like there should be some sort of queue system to
+// deduplicate this code.
+LLVM_LIBC_FUNCTION(void, perror, (const char *str)) {
+  const char empty_str[1] = {'\0'};
+  if (str == nullptr)
+    str = empty_str;
+  cpp::string_view str_view(str);
+
+  auto err_str = get_error_string(libc_errno);
+
+  // We need to lock the stream to ensure the newline is always appended.
+  ScopedLock lock(LIBC_NAMESPACE::stderr);
+  int write_err;
+
+  // FORMAT:
+  // if str != nullptr and doesn't start with a null byte:
+  //   "[str]: [strerror(errno)]\n"
+  // else
+  //   "[strerror(errno)]\n"
+  if (str_view.size() > 0) {
+    write_err = write_out(str_view, LIBC_NAMESPACE::stderr);
+    if (write_err != 0) {
+      libc_errno = write_err;
+      return;
+    }
+
+    write_err = write_out(": ", LIBC_NAMESPACE::stderr);
+    if (write_err != 0) {
+      libc_errno = write_err;
+      return;
+    }
+  }
+
+  write_err = write_out(err_str, LIBC_NAMESPACE::stderr);
+  if (write_err != 0) {
+    libc_errno = write_err;
+    return;
+  }
+
+  write_err = write_out("\n", LIBC_NAMESPACE::stderr);
+  if (write_err != 0) {
+    libc_errno = write_err;
+    return;
+  }
+}
+
+} // namespace LIBC_NAMESPACE_DECL

libc/src/stdio/perror.h

@@ -0,0 +1,20 @@
+//===-- Implementation header of perror -------------------------*- C++ -*-===//
+//
+// 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_STDIO_PERROR_H
+#define LLVM_LIBC_SRC_STDIO_PERROR_H
+
+#include "src/__support/macros/config.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+void perror(const char *s);
+
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_STDIO_PERROR_H

libc/test/src/stdio/CMakeLists.txt

@@ -357,6 +357,18 @@ add_libc_test(
     libc.src.stdio.puts
 )
 
+add_libc_test(
+  perror_test
+  HERMETIC_TEST_ONLY # writes to libc's stderr
+  SUITE
+    libc_stdio_unittests
+  SRCS
+    perror_test.cpp
+  DEPENDS
+    libc.src.stdio.perror
+    libc.src.errno.errno
+)
+
 add_libc_test(
   fputs_test
   HERMETIC_TEST_ONLY # writes to libc's stdout and stderr

libc/test/src/stdio/perror_test.cpp

@@ -0,0 +1,32 @@
+//===-- Unittests for perror ---------------------------------------------===//
+//
+// 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/stdio/perror.h"
+
+#include "src/errno/libc_errno.h"
+#include "test/UnitTest/Test.h"
+
+// The standard says perror prints directly to stderr and returns nothing. This
+// makes it rather difficult to test automatically.
+
+// TODO: figure out redirecting stderr so this test can check correctness.
+TEST(LlvmLibcPerrorTest, PrintOut) {
+  LIBC_NAMESPACE::libc_errno = 0;
+  constexpr char simple[] = "A simple string";
+  LIBC_NAMESPACE::perror(simple);
+
+  // stick to stdc errno values, specifically 0, EDOM, ERANGE, and EILSEQ.
+  LIBC_NAMESPACE::libc_errno = EDOM;
+  LIBC_NAMESPACE::perror("Print this and an error");
+
+  LIBC_NAMESPACE::libc_errno = EILSEQ;
+  LIBC_NAMESPACE::perror("\0 shouldn't print this.");
+
+  LIBC_NAMESPACE::libc_errno = ERANGE;
+  LIBC_NAMESPACE::perror(nullptr);
+}