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File::Temp

Section: Perl Programmers Reference Guide (3pm)
Updated: perl v5.6.1
Index

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 openfilehandle 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 thata filehandle and filename are returned together. This helps guaranteethat a race condition can not occur where the temporary file iscreated by another process between checking for the existence of thefile and its opening. Additional security levels are provided tocheck, for example, that the sticky bit is set on world writabledirectories. See the section on "safe_level" for more information.

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

Additionally, implementations of the standard POSIXtmpnam() 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 filenamethat was valid when function was called, so cannot guaranteethat the file will not exist by the time the caller opens the filename.

FUNCTIONS

This section describes the recommended interface for generatingtemporary 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 temporaryfiles, 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 suppliedtemplate. Trailing `X' characters are replaced with random letters togenerate the filename. At least four `X' characters must be presentin the template.

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

Same as previously, except that a suffix is added to the templateafter the `X' translation. Useful for ensuring that a temporaryfilename 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 nameis specified.

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

Return the filename and filehandle as before except that the file isautomatically removed when the program exits. Default is for the fileto be removed if a file handle is requested and to be kept if thefilename 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 alwaysautomatically 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 returnedand 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 conditionby fumbling with the filename. On systems that can not unlinkan 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 (equivalentto 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 butwill not open this file. Cannot be used in conjunction withUNLINK set to true. Default is to always open the file to protect from possible race conditions. A warning is issuedif warnings are turned on. Consider using the tmpnam()and mktemp() functions described elsewhere in this documentif 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 issimilar to that described for tempfile(). `X' characters at the endof the template are replaced with random letters to construct thedirectory 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 templateshould not include parent directory specifications itself. Any parentdirectory specifications are removed from the template beforeprepending 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 thetemplate itself. The "TMPDIR" option is ignored if "DIR" is setexplicitly. Additionally, "TMPDIR" is implied if neither a templatenor 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 programexits. Note that an attempt will be made to remove all files fromthe directory even if they were not created by this module (otherwisewhy ask to clean it up?). The directory removal is made withthe rmtree() function from the File::Path module.Of course, if the template is not specified, the temporary directorywill 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 nameof 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 appendedto it, for example /tmp/temp.XXXX. The trailing X's are replacedwith unique alphanumeric combinations.

mkstemps

Similar to mkstemp(), except that an extra argument can be suppliedwith 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 inX'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 guaranteethat 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 usedfor the temporary file is not specified in a system includefile ("P_tmpdir") but simply depends on the choice of tmpdir()returned by File::Spec. On some implementations thislocation can be set using the "TMPDIR" environment variable, whichmay 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 doesnot already exist, but there is no guarantee that that condition willcontinue to apply.

  $file = tmpnam();

When called in list context, a filehandle to the open file anda 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 possiblerace conditions.

See the tmpdir entry in the File::Spec manpage for information on the choice of temporarydirectory 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 programexits. 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 temporarydirectory is on an NFS file system.

ADDITIONAL FUNCTIONS

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

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

tempnam

Return the name of a temporary file in the specified directoryusing a prefix. The file is guaranteed not to exist at the timethe 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 safeunlink. This is achieved by first checking that the filename andfilehandle initially point to the same file and that the number oflinks to the file is 1 (all fields returned by stat() are compared).Then the filename is unlinked and the filehandle checked once again toverify that the number of links on that file is now 0. This is theclosest you can come to making sure that the filename unlinked was thesame 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 someoccasions this is not required.

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

Additionally, on Windows NT not all the fields returned by stat() canbe compared. For example, the "dev" and "rdev" fields seem to bedifferent. 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 whenusing autoflush (this is usually overcome by waiting a while afterwriting to the tempfile before attempting to "unlink0" it).

Finally, on NFS file systems the link count of the file handle doesnot always go to zero immediately after unlinking. Currently, thiscommand 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 thetemporary file or directory before proceeding.Options are:

STANDARD: Do the basic security measures to ensure the directory exists andis writable, that the umask() is fixed before opening of the file,that temporary files are opened only if they do not already exist, andthat possible race conditions are avoided. Finally the unlink0function is used to remove files safely.
MEDIUM: In addition to the STANDARD security, the output directory is checkedto make sure that it is owned either by root or the user running theprogram. If the directory is writable by group or by other, it is thenchecked to make sure that the sticky bit is set.
Will not work on platforms that do not support the "-k" testfor sticky bit.
HIGH: In addition to the MEDIUM security checks, also check for thepossibility of ``chown() giveaway'' using the POSIXsysconf() function. If this is a possibility, each directory in thepath 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 testis 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 torun with MEDIUM or HIGH security. This is simply because the safety tests use functions from Fcntl that are notavailable in older versions of perl. The problem is that the versionnumber for Fcntl is the same in perl 5.6.0 and in 5.005_03 even thoughthey 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 willbe ignored. The decision to ignore rather than raise an exceptionallows portable programs to be written with high security in mindfor the systems that can support this without those programs failingon systems where the extra tests are irrelevant.

If you really need to see whether the change has been acceptedsimply 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 rootUID. 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 ownedby root.

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

  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 notknow that that filename is connected to the same file as the handleyou have, and attempts to check this can only trigger more raceconditions. It's far more secure to use the filehandle alone anddispense with the filename altogether.

If you need to pass the handle to something that expects a filenamethen, on a unix system, use ""/dev/fd/" . fileno($fh)" for arbitraryprograms, or more generally ""+<=&" . fileno($fh)" for Perlprograms. You will have to clear the close-on-exec bit on that filedescriptor 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 resideon NFS file systems and it is recommended that a local filesystemis used whenever possible. Some of the security tests will most probablyfail when the temp file is not local. Additionally, be aware thatthe performance of I/O operations over NFS will not be as good as fora local disk.

HISTORY

Originally began life in May 1999 as an XS interface to the systemmkstemp() function. In March 2000, the OpenBSD mkstemp() code wastranslated to Perl for total control of the code'ssecurity checking, to ensure the presence of the function regardless ofoperating system and to help with portability.

AUTHOR

Tim Jenness <t.jennessAATTjach.hawaii.edu>

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

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

This document was created byman2html,using the manual pages.