File::Temp - return name and handle of a temporary file safely

- NAME
- PORTABILITY
- SYNOPSIS
- DESCRIPTION
- FUNCTIONS
- MKTEMP FUNCTIONS
- POSIX FUNCTIONS
- ADDITIONAL FUNCTIONS
- UTILITY FUNCTIONS
- PACKAGE VARIABLES
- WARNING

- Temporary files and NFS

- HISTORY
- SEE ALSO
- AUTHOR

# NAME

File::Temp - return name and handle of a temporary file safely

# SYNOPSIS

      use File::Temp qw/ tempfile tempdir /;

      $dir = tempdir( CLEANUP => 1 );
      ($fh, $filename) = tempfile( DIR => $dir );

      ($fh, $filename) = tempfile( $template, DIR => $dir);
      ($fh, $filename) = tempfile( $template, SUFFIX => '.dat');

      $fh = tempfile();

MkTemp family:

      use File::Temp qw/ :mktemp  /;

      ($fh, $file) = mkstemp( "tmpfileXXXXX" );
      ($fh, $file) = mkstemps( "tmpfileXXXXXX", $suffix);

      $tmpdir = mkdtemp( $template );

      $unopened_file = mktemp( $template );

POSIX functions:

      use File::Temp qw/ :POSIX /;

      $file = tmpnam();
      $fh = tmpfile();

      ($fh, $file) = tmpnam();
      ($fh, $file) = tmpfile();

Compatibility functions:

      $unopened_file = File::Temp::tempnam( $dir, $pfx );

# DESCRIPTION

`File::Temp` can be used to create and open temporary files in a safe way. The `tempfile()` function can be used to return the name and the open filehandle of a temporary file. The `tempdir()` function can be used to create a temporary directory.

The security aspect of temporary file creation is emphasized such that a filehandle and filename are returned together. This helps guarantee that a race condition can not occur where the temporary file is created by another process between checking for the existence of the file and its opening. Additional security levels are provided to check, for example, that the sticky bit is set on world writable directories. See safe_level for more information.

For compatibility with popular C library functions, Perl implementations of the `mkstemp()` family of functions are provided. These are, mkstemp(), mkstemps(), `mkdtemp()` and mktemp().

Additionally, implementations of the standard POSIX `tmpnam()` and `tmpfile()` functions are provided if required.

Implementations of mktemp(), tmpnam(), and `tempnam()` are provided, but should be used with caution since they return only a filename that was valid when function was called, so cannot guarantee that the file will not exist by the time the caller opens the filename.

# FUNCTIONS

This section describes the recommended interface for generating temporary files and directories.

tempfile
  This is the basic function to generate temporary files. The behaviour of the file can be changed using various options:

        ($fh, $filename) = tempfile();

  Create a temporary file in the directory specified for temporary files, as specified by the `tmpdir()` function in the File::Spec manpage.

        ($fh, $filename) = tempfile($template);

  Create a temporary file in the current directory using the supplied template. Trailing `X' characters are replaced with random letters to generate the filename. At least four `X' characters must be present in the template.

        ($fh, $filename) = tempfile($template, SUFFIX => $suffix)

  Same as previously, except that a suffix is added to the template after the `X' translation. Useful for ensuring that a temporary filename has a particular extension when needed by other applications. But see the WARNING at the end.

        ($fh, $filename) = tempfile($template, DIR => $dir);

  Translates the template as before except that a directory name is specified.

        ($fh, $filename) = tempfile($template, UNLINK => 1);

  Return the filename and filehandle as before except that the file is automatically removed when the program exits. Default is for the file to be removed if a file handle is requested and to be kept if the filename is requested. In a scalar context (where no filename is returned) the file is always deleted either on exit or when it is closed.

  If the template is not specified, a template is always automatically generated. This temporary file is placed in `tmpdir()` (the File::Spec manpage) unless a directory is specified explicitly with the DIR option.

        $fh = tempfile( $template, DIR => $dir );

  If called in scalar context, only the filehandle is returned and the file will automatically be deleted when closed (see the description of `tmpfile()` elsewhere in this document). This is the preferred mode of operation, as if you only have a filehandle, you can never create a race condition by fumbling with the filename. On systems that can not unlink an open file or can not mark a file as temporary when it is opened (for example, Windows NT uses the `O_TEMPORARY` flag)) the file is marked for deletion when the program ends (equivalent to setting UNLINK to 1). The `UNLINK` flag is ignored if present.

        (undef, $filename) = tempfile($template, OPEN => 0);

  This will return the filename based on the template but will not open this file. Cannot be used in conjunction with UNLINK set to true. Default is to always open the file to protect from possible race conditions. A warning is issued if warnings are turned on. Consider using the `tmpnam()` and `mktemp()` functions described elsewhere in this document if opening the file is not required.

  Options can be combined as required.

tempdir
  This is the recommended interface for creation of temporary directories. The behaviour of the function depends on the arguments:

        $tempdir = tempdir();

  Create a directory in `tmpdir()` (see File::Spec).

        $tempdir = tempdir( $template );

  Create a directory from the supplied template. This template is similar to that described for tempfile(). `X' characters at the end of the template are replaced with random letters to construct the directory name. At least four `X' characters must be in the template.

        $tempdir = tempdir ( DIR => $dir );

  Specifies the directory to use for the temporary directory. The temporary directory name is derived from an internal template.

        $tempdir = tempdir ( $template, DIR => $dir );

  Prepend the supplied directory name to the template. The template should not include parent directory specifications itself. Any parent directory specifications are removed from the template before prepending the supplied directory.

        $tempdir = tempdir ( $template, TMPDIR => 1 );

  Using the supplied template, creat the temporary directory in a standard location for temporary files. Equivalent to doing

        $tempdir = tempdir ( $template, DIR => File::Spec->tmpdir);

  but shorter. Parent directory specifications are stripped from the template itself. The `TMPDIR` option is ignored if `DIR` is set explicitly. Additionally, `TMPDIR` is implied if neither a template nor a directory are supplied.

        $tempdir = tempdir( $template, CLEANUP => 1);

  Create a temporary directory using the supplied template, but attempt to remove it (and all files inside it) when the program exits. Note that an attempt will be made to remove all files from the directory even if they were not created by this module (otherwise why ask to clean it up?). The directory removal is made with the `rmtree()` function from the File::Path module. Of course, if the template is not specified, the temporary directory will be created in `tmpdir()` and will also be removed at program exit.

# MKTEMP FUNCTIONS

The following functions are Perl implementations of the `mktemp()` family of temp file generation system calls.

mkstemp
  Given a template, returns a filehandle to the temporary file and the name of the file.

        ($fh, $name) = mkstemp( $template );

  In scalar context, just the filehandle is returned.

  The template may be any filename with some number of X's appended to it, for example /tmp/temp.XXXX. The trailing X's are replaced with unique alphanumeric combinations.

mkstemps
  Similar to mkstemp(), except that an extra argument can be supplied with a suffix to be appended to the template.

        ($fh, $name) = mkstemps( $template, $suffix );

  For example a template of `testXXXXXX` and suffix of `.dat` would generate a file similar to testhGji_w.dat.

  Returns just the filehandle alone when called in scalar context.

mkdtemp
  Create a directory from a template. The template must end in X's that are replaced by the routine.

        $tmpdir_name = mkdtemp($template);

  Returns the name of the temporary directory created. Returns undef on failure.

  Directory must be removed by the caller.

mktemp
  Returns a valid temporary filename but does not guarantee that the file will not be opened by someone else.

        $unopened_file = mktemp($template);

  Template is the same as that required by mkstemp().

# POSIX FUNCTIONS

This section describes the re-implementation of the `tmpnam()` and `tmpfile()` functions described in the POSIX manpage using the `mkstemp()` from this module.

Unlike the POSIX implementations, the directory used for the temporary file is not specified in a system include file (`P_tmpdir`) but simply depends on the choice of `tmpdir()` returned by File::Spec. On some implementations this location can be set using the `TMPDIR` environment variable, which may not be secure. If this is a problem, simply use `mkstemp()` and specify a template.

tmpnam
  When called in scalar context, returns the full name (including path) of a temporary file (uses mktemp()). The only check is that the file does not already exist, but there is no guarantee that that condition will continue to apply.

        $file = tmpnam();

  When called in list context, a filehandle to the open file and a filename are returned. This is achieved by calling `mkstemp()` after constructing a suitable template.

        ($fh, $file) = tmpnam();

  If possible, this form should be used to prevent possible race conditions.

  See tmpdir in the File::Spec manpage for information on the choice of temporary directory for a particular operating system.

tmpfile
  In scalar context, returns the filehandle of a temporary file.

        $fh = tmpfile();

  The file is removed when the filehandle is closed or when the program exits. No access to the filename is provided.

  If the temporary file can not be created undef is returned. Currently this command will probably not work when the temporary directory is on an NFS file system.

# ADDITIONAL FUNCTIONS

These functions are provided for backwards compatibility with common tempfile generation C library functions.

They are not exported and must be addressed using the full package name.

tempnam
  Return the name of a temporary file in the specified directory using a prefix. The file is guaranteed not to exist at the time the function was called, but such guarantees are good for one clock tick only. Always use the proper form of `sysopen` with `O_CREAT | O_EXCL` if you must open such a filename.

        $filename = File::Temp::tempnam( $dir, $prefix );

  Equivalent to running `mktemp()` with $dir/$prefixXXXXXXXX (using unix file convention as an example)

  Because this function uses mktemp(), it can suffer from race conditions.

# UTILITY FUNCTIONS

Useful functions for dealing with the filehandle and filename.

unlink0
  Given an open filehandle and the associated filename, make a safe unlink. This is achieved by first checking that the filename and filehandle initially point to the same file and that the number of links to the file is 1 (all fields returned by `stat()` are compared). Then the filename is unlinked and the filehandle checked once again to verify that the number of links on that file is now 0. This is the closest you can come to making sure that the filename unlinked was the same as the file whose descriptor you hold.

        unlink0($fh, $path) or die "Error unlinking file $path safely";

  Returns false on error. The filehandle is not closed since on some occasions this is not required.

  On some platforms, for example Windows NT, it is not possible to unlink an open file (the file must be closed first). On those platforms, the actual unlinking is deferred until the program ends and good status is returned. A check is still performed to make sure that the filehandle and filename are pointing to the same thing (but not at the time the end block is executed since the deferred removal may not have access to the filehandle).

  Additionally, on Windows NT not all the fields returned by `stat()` can be compared. For example, the `dev` and `rdev` fields seem to be different. Also, it seems that the size of the file returned by `stat()` does not always agree, with `stat(FH)` being more accurate than `stat(filename)`, presumably because of caching issues even when using autoflush (this is usually overcome by waiting a while after writing to the tempfile before attempting to `unlink0` it).

  Finally, on NFS file systems the link count of the file handle does not always go to zero immediately after unlinking. Currently, this command is expected to fail on NFS disks.

# PACKAGE VARIABLES

These functions control the global state of the package.

safe_level
  Controls the lengths to which the module will go to check the safety of the temporary file or directory before proceeding. Options are:
  STANDARD
    Do the basic security measures to ensure the directory exists and is writable, that the `umask()` is fixed before opening of the file, that temporary files are opened only if they do not already exist, and that possible race conditions are avoided. Finally the unlink0 function is used to remove files safely.

  MEDIUM
    In addition to the STANDARD security, the output directory is checked to make sure that it is owned either by root or the user running the program. If the directory is writable by group or by other, it is then checked to make sure that the sticky bit is set.

    Will not work on platforms that do not support the `-k` test for sticky bit.

  HIGH
    In addition to the MEDIUM security checks, also check for the possibility of ``chown() giveaway'' using the POSIX `sysconf()` function. If this is a possibility, each directory in the path is checked in turn for safeness, recursively walking back to the root directory.

    For platforms that do not support the POSIX `_PC_CHOWN_RESTRICTED` symbol (for example, Windows NT) it is assumed that ``chown() giveaway'' is possible and the recursive test is performed.

  The level can be changed as follows:

        File::Temp->safe_level( File::Temp::HIGH );

  The level constants are not exported by the module.

  Currently, you must be running at least perl v5.6.0 in order to run with MEDIUM or HIGH security. This is simply because the safety tests use functions from Fcntl that are not available in older versions of perl. The problem is that the version number for Fcntl is the same in perl 5.6.0 and in 5.005_03 even though they are different versions.

  On systems that do not support the HIGH or MEDIUM safety levels (for example Win NT or OS/2) any attempt to change the level will be ignored. The decision to ignore rather than raise an exception allows portable programs to be written with high security in mind for the systems that can support this without those programs failing on systems where the extra tests are irrelevant.

  If you really need to see whether the change has been accepted simply examine the return value of `safe_level`.

        $newlevel = File::Temp->safe_level( File::Temp::HIGH );
        die "Could not change to high security"
            if $newlevel != File::Temp::HIGH;

TopSystemUID
  This is the highest UID on the current system that refers to a root UID. This is used to make sure that the temporary directory is owned by a system UID (`root`, `bin`, `sys` etc) rather than simply by root.

  This is required since on many unix systems `/tmp` is not owned by root.

  Default is to assume that any UID less than or equal to 10 is a root UID.

        File::Temp->top_system_uid(10);
        my $topid = File::Temp->top_system_uid;

  This value can be adjusted to reduce security checking if required. The value is only relevant when `safe_level` is set to MEDIUM or higher.

# WARNING

For maximum security, endeavour always to avoid ever looking at, touching, or even imputing the existence of the filename. You do not know that that filename is connected to the same file as the handle you have, and attempts to check this can only trigger more race conditions. It's far more secure to use the filehandle alone and dispense with the filename altogether.

If you need to pass the handle to something that expects a filename then, on a unix system, use `"/dev/fd/" . fileno($fh)` for arbitrary programs, or more generally `"+<=&" . fileno($fh)` for Perl programs. You will have to clear the close-on-exec bit on that file descriptor before passing it to another process.

        use Fcntl qw/F_SETFD F_GETFD/;
        fcntl($tmpfh, F_SETFD, 0)
            or die "Can't clear close-on-exec flag on temp fh: $!\n";

## Temporary files and NFS

Some problems are associated with using temporary files that reside on NFS file systems and it is recommended that a local filesystem is used whenever possible. Some of the security tests will most probably fail when the temp file is not local. Additionally, be aware that the performance of I/O operations over NFS will not be as good as for a local disk.

# HISTORY

Originally began life in May 1999 as an XS interface to the system `mkstemp()` function. In March 2000, the OpenBSD `mkstemp()` code was translated to Perl for total control of the code's security checking, to ensure the presence of the function regardless of operating system and to help with portability.

# SEE ALSO

tmpnam in the POSIX manpage, tmpfile in the POSIX manpage, the File::Spec manpage, the File::Path manpage

See the IO::File manpage and the File::MkTemp manpage for different implementations of temporary file handling.

# AUTHOR

Tim Jenness <t.jenness@jach.hawaii.edu>

Copyright (C) 1999-2001 Tim Jenness and the UK Particle Physics and Astronomy Research Council. All Rights Reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

Original Perl implementation loosely based on the OpenBSD C code for mkstemp(). Thanks to Tom Christiansen for suggesting that this module should be written and providing ideas for code improvements and security enhancements.

File::Temp - return name and handle of a temporary file safely