Delete open file in Windows (creating an anonymous file)? - windows

Question

Under Linux, my program would do something like this:
Process 1 opens a file (e.g. by mapping it into memory). Let's call this file#1
Process 2 unlinks the file, and creates a new file with the same name. Let's call this file#2.
Process 1 continues to work with file#1. When it is closed, it is deleted (since it has no links). Process 1 continues to work with the content in file#1, and does not see content from file#2.
When both processes have exited, file#2 remains on disk.
I want to achieve the same semantics in Windows. After reading this question, I think FILE_SHARE_DELETE does basically this. Is opening the file with FILE_SHARE_DELETE enough, or do I need to consider something more?
The above execution flow is just an example, I know there are other ways of solving that exact problem in Windows, but I want to understand how to make such code portable between Windows and Linux.
Edit: to clarify: The use cases would be to reuse a filename for different unrelated files, but let existing processes keep their data (think transactional update of a config file for example), and to make a file anonymous (unnamed), but continue to use it like an anonymous memory map. Again I know both are possible on Windows through other means, but I am trying to find a way that is portable across platforms.

Answer 1

You can achieve this by using a combination of CreateFile, CreateFileMapping and MapViewOfFile calls. MapViewOfFile will give you a memory-mapped buffer of the file backed by the file on disk.
Following code when executed from different processes, will write the process id of last closing process in the file at c:\temp\temp.txt
int main()
{
TCHAR szMsg[256];
HANDLE hMapFile;
LPCTSTR pBuf;
HANDLE hFile = CreateFileW(
L"C:\\Temp\\temp.txt",
GENERIC_WRITE|GENERIC_READ,
FILE_SHARE_DELETE|FILE_SHARE_READ|FILE_SHARE_WRITE,
NULL,
CREATE_ALWAYS,
FILE_ATTRIBUTE_NORMAL,NULL);
hMapFile = CreateFileMapping(
hFile, // Handle of file opened with rw access
NULL, // default security
PAGE_READWRITE, // read/write access
0, // maximum object size (high-order DWORD)
BUF_SIZE, // maximum object size (low-order DWORD)
szName); // name of mapping object
if (hMapFile == NULL)
{
printf( "Could not create file mapping object (%d).\n", GetLastError());
return 1;
}
pBuf = (LPTSTR) MapViewOfFile(hMapFile, // handle to map object
FILE_MAP_ALL_ACCESS, // read/write permission
0,
0,
BUF_SIZE);
if (pBuf == NULL)
{
printf("Could not map view of file (%d).\n", GetLastError());
CloseHandle(hMapFile);
return 1;
}
wsprintfW(szMsg, L"This is process id %d", GetCurrentProcessId());
CopyMemory((PVOID)pBuf, szMsg, (wcslen(szMsg) * sizeof(TCHAR)));
MessageBoxW(NULL, szMsg, L"Check", MB_OK);
UnmapViewOfFile(pBuf);
CloseHandle(hMapFile);
CloseHandle(hFile);
return 0;
}
Make sure you open the file with GENERIC_READ|GENERIC_WRITE access and allow FILE_SHARE_READ|FILE_SHARE_WRITE|FILE_SHARE_DELETE access to subsequent opens.
Also note the use of CREATE_ALWAYS in CreateFile which will delete the old file and open a new one every-time CreateFile is called. This is the 'unlink' effect you talk about.
Code inspired from Creating Named Shared Memory at msdn.