Temporary Files

In this chapter, we describe how to create temporary files and directories, how to remove them, and how to work with programs which do not create files in ways that are safe with a shared directory for temporary files. General file system manipulation is treated in a separate chapter, File System Manipulation.

Secure creation of temporary files has four different aspects.

  • The location of the directory for temporary files must be obtained in a secure manner (that is, untrusted environment variables must be ignored, see xref:tasks/Tasks-Processes.adoc#sect-Defensive_Coding-Tasks-secure_getenv[Accessing Environment Variables).

  • A new file must be created. Reusing an existing file must be avoided (the /tmp race condition). This is tricky because traditionally, system-wide temporary directories shared by all users are used.

  • The file must be created in a way that makes it impossible for other users to open it.

  • The descriptor for the temporary file should not leak to subprocesses.

All functions mentioned below will take care of these aspects.

Traditionally, temporary files are often used to reduce memory usage of programs. More and more systems use RAM-based file systems such as tmpfs for storing temporary files, to increase performance and decrease wear on Flash storage. As a result, spooling data to temporary files does not result in any memory savings, and the related complexity can be avoided if the data is kept in process memory.

Obtaining the Location of Temporary Directory

Some functions below need the location of a directory which stores temporary files. For C/C++ programs, use the following steps to obtain that directory:

  • Use secure_getenv to obtain the value of the TMPDIR environment variable. If it is set, convert the path to a fully-resolved absolute path, using realpath(path, NULL). Check if the new path refers to a directory and is writeable. In this case, use it as the temporary directory.

  • Fall back to /tmp.

In Python, you can use the tempfile.tempdir variable.

Java does not support SUID/SGID programs, so you can use the java.lang.System.getenv(String) method to obtain the value of the TMPDIR environment variable, and follow the two steps described above. (Java’s default directory selection does not honor TMPDIR.)

Named Temporary Files

The mkostemp function creates a named temporary file. You should specify the O_CLOEXEC flag to avoid file descriptor leaks to subprocesses. (Applications which do not use multiple threads can also use mkstemp, but libraries should use mkostemp.) For determining the directory part of the file name pattern, see Obtaining the Location of Temporary Directory

The file is not removed automatically. It is not safe to rename or delete the file before processing, or transform the name in any way (for example, by adding a file extension). If you need multiple temporary files, call mkostemp multiple times. Do not create additional file names derived from the name provided by a previous mkostemp call. However, it is safe to close the descriptor returned by mkostemp and reopen the file using the generated name.

The Python class tempfile.NamedTemporaryFile provides similar functionality, except that the file is deleted automatically by default. Note that you may have to use the file attribute to obtain the actual file object because some programming interfaces cannot deal with file-like objects. The C function mkostemp is also available as tempfile.mkstemp.

In Java, you can use the java.io.File.createTempFile(String, String, File) function, using the temporary file location determined according to Obtaining the Location of Temporary Directory. Do not use java.io.File.deleteOnExit() to delete temporary files, and do not register a shutdown hook for each temporary file you create. In both cases, the deletion hint cannot be removed from the system if you delete the temporary file prior to termination of the VM, causing a memory leak.

Temporary Files without Names

The tmpfile function creates a temporary file and immediately deletes it, while keeping the file open. As a result, the file lacks a name and its space is deallocated as soon as the file descriptor is closed (including the implicit close when the process terminates). This avoids cluttering the temporary directory with orphaned files.

Alternatively, if the maximum size of the temporary file is known beforehand, the fmemopen function can be used to create a FILE * object which is backed by memory.

In Python, unnamed temporary files are provided by the tempfile.TemporaryFile class, and the tempfile.SpooledTemporaryFile class provides a way to avoid creation of small temporary files.

Java does not support unnamed temporary files.

Temporary Directories

The mkdtemp function can be used to create a temporary directory. (For determining the directory part of the file name pattern, see Obtaining the Location of Temporary Directory.) The directory is not automatically removed. In Python, this function is available as tempfile.mkdtemp. In Java 7, temporary directories can be created using the java.nio.file.Files.createTempDirectory(Path, String, FileAttribute…​) function.

When creating files in the temporary directory, use automatically generated names, e.g., derived from a sequential counter. Files with externally provided names could be picked up in unexpected contexts, and crafted names could actually point outside of the tempoary directory (due to directory traversal).

Removing a directory tree in a completely safe manner is complicated. Unless there are overriding performance concerns, the rm program should be used, with the -rf and -- options.

Compensating for Unsafe File Creation

There are two ways to make a function or program which excepts a file name safe for use with temporary files. See Creating Safe Processes for details on subprocess creation.

  • Create a temporary directory and place the file there. If possible, run the program in a subprocess which uses the temporary directory as its current directory, with a restricted environment. Use generated names for all files in that temporary directory. (See Temporary Directories.)

  • Create the temporary file and pass the generated file name to the function or program. This only works if the function or program can cope with a zero-length existing file. It is safe only under additional assumptions:

    • The function or program must not create additional files whose name is derived from the specified file name or are otherwise predictable.

    • The function or program must not delete the file before processing it.

    • It must not access any existing files in the same directory.

      It is often difficult to check whether these additional assumptions are matched, therefore this approach is not recommended.