Secure Programming Cookbook for C and C++

2.11. Creating Files for Temporary Use

Problem

You need to create a file to use as scratch space that may contain sensitive data.

Solution

Generate a random filename and attempt to create the file, failing if the file already exists. If the file cannot be created because it already exists, repeat the process until it succeeds. If creating the file fails for any other reason, abort the process.

Discussion

Warning

When creating temporary files, you should consider using a known-safe directory to store them, as described in Recipe 2.4.

The need for temporary files is common. More often than not, other processes have no need to access the temporary files you create, and especially if the files contain sensitive data, it is best to do everything possible to ensure that other processes cannot access them. It is also important that temporary files do not remain on the filesystem any longer than necessary. If the program creating temporary files terminates unexpectedly before it cleans up the files, temporary directories often become littered with files of no interest or value to anyone or anything. Worse, if the temporary files contain sensitive data, they are suddenly both interesting and valuable to an attacker.

Temporary files on Unix

The best solution for creating a temporary file on Unix is to use the mkstemp( ) function in the standard C runtime library. This function generates a random filename,^[2] attempts to create it, and repeats the whole process until it is successful, thus guaranteeing ...