[libc] Add implementation of fgetc.

[michaelrj]

LGTM with nits

libc/src/stdio/fgetc.cpp

19

this should be size_t not ssize_t

20

nit: you don't need the braces here.

[sivachandra]

Address comments.

libc/config/linux/x86_64/entrypoints.txt

list(APPEND TARGET_LIBC_ENTRYPOINTS
     libc.src.stdio.clearerr
     libc.src.stdio.clearerr_unlocked
     libc.src.stdio.fclose
     libc.src.stdio.flockfile
     libc.src.stdio.feof
     libc.src.stdio.feof_unlocked
     libc.src.stdio.ferror
     libc.src.stdio.ferror_unlocked
+    libc.src.stdio.fgetc
     libc.src.stdio.fflush
     libc.src.stdio.fopen
     libc.src.stdio.fputc
     libc.src.stdio.fputs
     libc.src.stdio.fopencookie
     libc.src.stdio.fread
     libc.src.stdio.fread_unlocked
     libc.src.stdio.fseek

libc/spec/stdc.td

HeaderSpec StdIO = HeaderSpec<
               [ArgSpec<FILEPtr>]
           >,
           FunctionSpec<
               "ferror",
               RetValSpec<IntType>,
               [ArgSpec<FILEPtr>]
           >,
           FunctionSpec<
+              "fgetc",
+              RetValSpec<IntType>,
+              [ArgSpec<FILEPtr>]
+          >,
+          FunctionSpec<
               "fflush",
               RetValSpec<IntType>,
               [ArgSpec<FILEPtr>]
           >,
           FunctionSpec<
               "fopen",
               RetValSpec<FILEPtr>,
               [ArgSpec<ConstCharPtr>,

libc/src/stdio/CMakeLists.txt

HDRS
     ferror_unlocked.h
   DEPENDS
     libc.include.stdio
     libc.src.__support.File.file
     libc.src.__support.File.platform_file
 )
 
 add_entrypoint_object(
+  fgetc
+  SRCS
+    fgetc.cpp
+  HDRS
+    fgetc.h
+  DEPENDS
+    libc.include.stdio
+    libc.src.__support.File.file
+    libc.src.__support.File.platform_file
+)
+
+add_entrypoint_object(
   fflush
   SRCS
     fflush.cpp
   HDRS
     fflush.h
   DEPENDS
     libc.include.stdio
     libc.src.__support.File.file

libc/src/stdio/fgetc.h

+//===-- Implementation header of fgetc --------------------------*- 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_FGETC_H
+#define LLVM_LIBC_SRC_STDIO_FGETC_H
+
+#include <stdio.h>
+
+namespace __llvm_libc {
+
+int fgetc(::FILE *f);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STDIO_FGETC_H

libc/src/stdio/fgetc.cpp

+//===-- Implementation of fgetc -------------------------------------------===//
+//
+// 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/fgetc.h"
+#include "src/__support/File/file.h"
+
+#include <stdio.h>
+
+namespace __llvm_libc {
+
+LLVM_LIBC_FUNCTION(int, fgetc, (::FILE * stream)) {
+  unsigned char c;
+  size_t r = reinterpret_cast<__llvm_libc::File *>(stream)->read(&c, 1);
+  if (r != 1)
+    return EOF;
+  return c;
+}
+
+} // namespace __llvm_libc

libc/test/src/stdio/CMakeLists.txt

DEPENDS
       libc.src.fcntl.open
       libc.src.stdio.remove
       libc.src.sys.stat.mkdirat
       libc.src.unistd.access
       libc.src.unistd.close
   )
 endif()
 
+add_libc_unittest(
+  fgetc_test
+  SUITE
+    libc_stdio_unittests
+  SRCS
+    fgetc_test.cpp
+  DEPENDS
+    libc.include.errno
+    libc.include.stdio
+    libc.src.stdio.fclose
+    libc.src.stdio.feof
+    libc.src.stdio.ferror
+    libc.src.stdio.fgetc
+    libc.src.stdio.fopen
+    libc.src.stdio.fwrite
+)
+
 add_subdirectory(printf_core)
 add_subdirectory(testdata)

libc/test/src/stdio/fgetc_test.cpp

+//===-- Unittests for fgetc -----------------------------------------------===//
+//
+// 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/clearerr.h"
+#include "src/stdio/fclose.h"
+#include "src/stdio/feof.h"
+#include "src/stdio/ferror.h"
+#include "src/stdio/fgetc.h"
+#include "src/stdio/fopen.h"
+#include "src/stdio/fwrite.h"
+#include "utils/UnitTest/Test.h"
+
+#include <errno.h>
+#include <stdio.h>
+
+TEST(LlvmLibcFGetCTest, WriteAndReadCharacters) {
+  constexpr char FILENAME[] = "testdata/fgetc.test";
+  ::FILE *file = __llvm_libc::fopen(FILENAME, "w");
+  ASSERT_FALSE(file == nullptr);
+  constexpr char CONTENT[] = "123456789";
+  constexpr size_t WRITE_SIZE = sizeof(CONTENT) - 1;
+  ASSERT_EQ(WRITE_SIZE, __llvm_libc::fwrite(CONTENT, 1, WRITE_SIZE, file));
+  // This is a write-only file so reads should fail.
+  ASSERT_EQ(__llvm_libc::fgetc(file), EOF);
+  // This is an error and not a real EOF.
+  ASSERT_EQ(__llvm_libc::feof(file), 0);
+  ASSERT_NE(__llvm_libc::ferror(file), 0);
+  errno = 0;
+
+  ASSERT_EQ(0, __llvm_libc::fclose(file));
+
+  file = __llvm_libc::fopen(FILENAME, "r");
+  ASSERT_FALSE(file == nullptr);
+
+  for (size_t i = 0; i < WRITE_SIZE; ++i) {
+    int c = __llvm_libc::fgetc(file);
+    ASSERT_EQ(c, int('1' + i));
+  }
+  // Reading more should return EOF but not set error.
+  ASSERT_EQ(__llvm_libc::fgetc(file), EOF);
+  ASSERT_NE(__llvm_libc::feof(file), 0);
+  ASSERT_EQ(__llvm_libc::ferror(file), 0);
+
+  ASSERT_EQ(0, __llvm_libc::fclose(file));
+}