base::ReadFile() should return the number of read bytes on Windows.

base/file_util.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file contains utility functions for dealing with the local
 // filesystem.
 
 #ifndef BASE_FILE_UTIL_H_
 #define BASE_FILE_UTIL_H_
 
@@ skipping 291 matching lines @@
 // Wrapper for fopen-like calls. Returns non-NULL FILE* on success.
 BASE_EXPORT FILE* OpenFile(const FilePath& filename, const char* mode);
 
 // Closes file opened by OpenFile. Returns true on success.
 BASE_EXPORT bool CloseFile(FILE* file);
 
 // Truncates an open file to end at the location of the current file pointer.
 // This is a cross-platform analog to Windows' SetEndOfFile() function.
 BASE_EXPORT bool TruncateFile(FILE* file);
 
-// Reads the given number of bytes from the file into the buffer.  Returns
-// the number of read bytes, or -1 on error.
-BASE_EXPORT int ReadFile(const FilePath& filename, char* data, int size);
+// Reads at most the given number of bytes from the file into the buffer.
+// Returns the number of read bytes, or -1 on error.
+BASE_EXPORT int ReadFile(const FilePath& filename, char* data, int max_size);
 
 // Writes the given buffer into the file, overwriting any data that was
 // previously there.  Returns the number of bytes written, or -1 on error.
 BASE_EXPORT int WriteFile(const FilePath& filename, const char* data,
                           int size);
 
 #if defined(OS_POSIX)
 // Append the data to |fd|. Does not close |fd| when done.
 BASE_EXPORT int WriteFileDescriptor(const int fd, const char* data, int size);
 #endif
@@ skipping 118 matching lines @@
 // This function simulates Move(), but unlike Move() it works across volumes.
 // This function is not transactional.
 BASE_EXPORT bool CopyAndDeleteDirectory(const FilePath& from_path,
                                         const FilePath& to_path);
 #endif  // defined(OS_WIN)
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_FILE_UTIL_H_