id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
2996	[iostreams] Can't open mapped_file data file for read/write access on Windows	Allen Cronce <acronce@…>	Jonathan Turkanis	"I see access errors when trying to use mapped_file open a data file for reading and writing under Windows (built with VS2005). The same code works fine under Mac OS X, which is posix, of course.

Note that I originally found this problem under boost 1.35, but it is still present in the just released 1.39 version. Unfortunately it looks like trac has not been updated, so I cannot set the ""version"" to 1.38, or the ""milestone"" to 1.40.

Here's a code sample that works on Mac OS X and fails under Windows:

--snip--

/**
Test boost iostream mapped_file.
*/
static	int	BoostMappedFileTest(void)
{
	int result	= 0;
	
	std::cout << ""\nRunning BoostMappedFileTest..."" << std::endl;
	
	// Initialize our test directory
	fs::path testDirPath( ""BoostMappedFileTest"" );
	create_directory(testDirPath);
	
	// Initialize a test file buffer with some data
	const size_t kTestFileSize = 4096;
	boost::scoped_array<char> fileBuffer( new char[kTestFileSize] );
	char* pCur = fileBuffer.get();
	char curChar = 0;
	for( size_t i = 0; i < kTestFileSize; ++i )
	{
		*pCur++	= curChar++;
	}
	
	// Write the data to a test file. Be sure to explicitly close it.
	fs::path testFilePath( testDirPath / ""test_mapped_file"" );
	boost::filesystem::ofstream testFile( testFilePath, ios::trunc | ios::binary );
	testFile.write(fileBuffer.get(), kTestFileSize);
	testFile.close();
	
	// Try and open the test file as a memory mapped file.
	string testFilePathStr = testFilePath.file_string();
	mapped_file	mappedFile(testFilePathStr);
	
	// We never get here under Windows. The file fails to
	// open with an access error.
	
	// Compare the mapped file data to our original buffer
	const char* mmConstData = mappedFile.const_data();
	bool compareSucceeded = (0 == memcmp(fileBuffer.get(), mmConstData, kTestFileSize));
	if (!compareSucceeded)
	{
		throw runtime_error(""The boost memory mapped file data doesn't match our buffer!"");
	}
	
	return result;
}

--snip--

The problem seems to come from passing GENERIC_ALL as the ""dwDesiredAccess"" parameter to the CreateFileA call. If I change the implementation to pass GENERIC_READ | GENERIC_WRITE, then it works in my test scenario. Please note that I haven't tried pointing the current code at an actual executable to see if it works with GENERIC_ALL.

If you assume a strict interpretation of the mapped_file API, the mode of BOOST_IOS::in would map to GENERIC_READ in the Windows world, and BOOST_IOS::out would map to GENERIC_WRITE. There is no corresponding ""execute"" flag that I can see in this API.

I've attached a diff created from the boost 1.39 release that fixes the problem.

My email address (in case my trac preferences doesn't work) is acronce@gmail.com."	Bugs	closed	Boost 1.39.0	iostreams	Boost 1.38.0	Problem	fixed