Secure Programming Cookbook for C and C++

2.6. Accessing File Information Securely

Problem

Y ou need to access information about a file, such as its size or last modification date. In doing so, you want to avoid the possibility of race conditions.

Solution

Use a secure directory, as described in Recipe 2.4. Alternatively, open the file and query the needed information using the file handle. Do not use functions that operate on the name of the file, especially if multiple queries are required for the same file or if you intend to open it based on the information obtained from queries. Operating on filenames introduces the possibility of race conditions because filenames can change between calls.

On Unix, use the fstat( ) function instead of the stat( ) function. Both functions return the same information, but fstat( ) uses an open file descriptor while stat( ) uses a filename. Doing so removes the possibility of a race condition, because the file to which the file descriptor points can never change unless you reopen the file descriptor. When operating on just the filename, there is no guarantee that the underlying file pointed to by the filename remains the same after the call to stat( ).

On Windows, use the function GetFileInformationByHandle( ) instead of functions like FindFirstFile( ) or FindFirstFileEx( ). As with fstat( ) versus stat( ) on Unix (which are also available on Windows if you’re using the C runtime API), the primary difference between these functions is that one uses a file handle while the others use filenames. ...