Using temporary files safely

You have a lot of suggestions already, but another option that I don't think has been mentioned is using named pipes. It will depend on the library in question as to whether it works or not, but it might be worth a try. You can create a named pipe in your application using the CreateNamedPipe function, and pass the name of the pipe to the library to operate on (the filename you would pass would be \\.\pipe\PipeName). Whether the library accepts a filename like that or not is something you would have to try, but if it works the advantage is your file never has to actually be written to disk.

This can be achieved using the CreateFile and GetTempFileName functions (if you don't know if you can write to the current working directory, you may also want to use , GetTempPath).

1. Determine a directory to store your temporary file in; the current directory (".") or the result of GetTempPath would be good candidates.
2. Use GetTempFileName to create a temporary file name.
3. Finally, call CreateFile to create the temporary file.

For the last step, there are a few things to consider:

• The dwFlagsAndAttributes parameter of CreateFile should probably include FILE_ATTRIBUTE_TEMPORARY.
• The dwFlagsAndAttributes parameter should probably also include FILE_FLAG_DELETE_ON_CLOSE to make sure that the file gets deleted no matter what (this probably also works if your process crashes, in which case the system closes all handles for you).
• The dwShareMode parameter of CreateFile should probably be FILE_SHARE_READ so that other attempts to open the file will succeed, but only for reading. This means that your library code will be able to read the file, but nobody will be able to write to it.

This article should give you some good guidelines on the issue.

The gist of the matter is this:

• The POSIX mkstemp() function is the secure and preferred solution where available. Unfortunately, it is not available in Windows, so you would need to find a wrapper that properly implements this functionality using Windows API calls.
• On Windows, the tmpfile_s() function is the only one that actually opens the temporary file atomically (instead of simply generating a filename), protecting you from a race condition. Unfortunately, this function does not allow you to specify which directory the file will be created in, which is a potential security issue.