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File::Temp(3pm) Perl Programmers Reference Guide File::Temp(3pm)
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();
Object interface:
require File::Temp;
use File::Temp ();
$fh = new File::Temp($template);
$fname = $fh->filename;
$tmp = new File::Temp( UNLINK => 0, SUFFIX => ’.dat’ );
print $tmp "Some data\n";
print "Filename is $tmp\n";
MkTemp family:
use File::Temp qw/ :mktemp /;
($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 = 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. There
is both a function interface and an object-oriented interface. The File::Temp con-
structor or 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 direc-
tories. 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() func-
tions 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.
OO INTERFACE
This is the primary interface for interacting with "File::Temp". Using the OO
interface a temporary file can be created when the object is constructed and the
file can be removed when the object is no longer required.
Note that there is no method to obtain the filehandle from the "File::Temp" object.
The object itself acts as a filehandle. Also, the object is configured such that it
stringifies to the name of the temporary file.
new Create a temporary file object.
my $tmp = new File::Temp();
by default the object is constructed as if "tempfile" was called without
options, but with the additional behaviour that the temporary file is removed
by the object destructor if UNLINK is set to true (the default).
Supported arguments are the same as for "tempfile": UNLINK (defaulting to
true), DIR and SUFFIX. Additionally, the filename template is specified using
the TEMPLATE option. The OPEN option is not supported (the file is always
opened).
$tmp = new File::Temp( TEMPLATE => ’tempXXXXX’,
DIR => ’mydir’,
SUFFIX => ’.dat’);
Arguments are case insensitive.
filename
Return the name of the temporary file associated with this object.
$filename = $tmp->filename;
This method is called automatically when the object is used as a string.
DESTROY
When the object goes out of scope, the destructor is called. This destructor
will attempt to unlink the file (using "unlink1") if the constructor was called
with UNLINK set to 1 (the default state if UNLINK is not specified).
No error is given if the unlink fails.
FUNCTIONS
This section describes the recommended interface for generating temporary files and
directories.
tempfile
This is the basic function to generate temporary files. The behaviour of the
file can be changed using various options:
($fh, $filename) = tempfile();
Create a temporary file in the directory specified for temporary files, as
specified by the tmpdir() function in File::Spec.
($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 file-
name. At least four ‘X’ characters must be present at the end of 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 automati-
cally 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() (File::Spec) 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() else-
where 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_TEMPO-
RARY" 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 specifica-
tions are removed from the template before prepending the supplied directory.
$tempdir = tempdir ( $template, TMPDIR => 1 );
Using the supplied template, create the temporary directory in a standard loca-
tion 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. Addition-
ally, "TMPDIR" is implied if neither a template nor a directory are supplied.
$tempdir = tempdir( $template, CLEANUP => 1);
Create a temporary directory using the supplied template, but attempt to remove
it (and all files inside it) when the program exits. Note that an attempt will
be made to remove all files from the directory even if they were not created by
this module (otherwise why ask to clean it up?). The directory removal is made
with the rmtree() function from the File::Path module. Of course, if the tem-
plate is not specified, the temporary directory will be created in tmpdir() and
will also be removed at program exit.
MKTEMP FUNCTIONS
The following functions are Perl implementations of the mktemp() family of temp
file generation system calls.
mkstemp
Given a template, returns a filehandle to the temporary file and the name of
the file.
($fh, $name) = mkstemp( $template );
In scalar context, just the filehandle is returned.
The template may be any filename with some number of X’s appended to it, for
example /tmp/temp.XXXX. The trailing X’s are replaced with unique alphanumeric
combinations.
mkstemps
Similar to mkstemp(), except that an extra argument can be supplied with a suf-
fix to be appended to the template.
($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() func-
tions described in POSIX 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 tem-
porary 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 File::Spec for information on the choice of temporary directory
for a particular operating system.
tmpfile
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 gen-
eration 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 con-
vention 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 file-
handle 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 $path safely";
Returns false on error. The filehandle is not closed since on some occasions
this is not required.
On some platforms, for example Windows NT, it is not possible to unlink an open
file (the file must be closed first). On those platforms, the actual unlinking
is deferred until the program ends and good status is returned. A check is
still performed to make sure that the filehandle and filename are pointing to
the same thing (but not at the time the end block is executed since the
deferred removal may not have access to the filehandle).
Additionally, on Windows NT not all the fields returned by stat() can be com-
pared. For example, the "dev" and "rdev" fields seem to be different. Also, it
seems that the size of the file returned by stat() does not always agree, with
"stat(FH)" being more accurate than "stat(filename)", presumably because of
caching issues even when using autoflush (this is usually overcome by waiting a
while after writing to the tempfile before attempting to "unlink0" it).
Finally, on NFS file systems the link count of the file handle does not always
go to zero immediately after unlinking. Currently, this command is expected to
fail on NFS disks.
cmpstat
Compare "stat" of filehandle with "stat" of provided filename. This can be
used to check that the filename and filehandle initially point to the same file
and that the number of links to the file is 1 (all fields returned by stat()
are compared).
cmpstat($fh, $path) or die "Error comparing handle with file";
Returns false if the stat information differs or if the link count is greater
than 1.
On certain platofms, eg Windows, not all the fields returned by stat() can be
compared. For example, the "dev" and "rdev" fields seem to be different in Win-
dows. Also, it seems that the size of the file returned by stat() does not
always agree, with "stat(FH)" being more accurate than "stat(filename)", pre-
sumably because of caching issues even when using autoflush (this is usually
overcome by waiting a while after writing to the tempfile before attempting to
"unlink0" it).
Not exported by default.
unlink1
Similar to "unlink0" except after file comparison using cmpstat, the filehandle
is closed prior to attempting to unlink the file. This allows the file to be
removed without using an END block, but does mean that the post-unlink compari-
son of the filehandle state provided by "unlink0" is not available.
unlink1($fh, $path) or die "Error closing and unlinking file";
Usually called from the object destructor when using the OO interface.
Not exported by default.
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 possibil-
ity 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" sym-
bol (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 $topid = File::Temp->top_system_uid;
This value can be adjusted to reduce security checking if required. The value
is only relevant when "safe_level" is set to MEDIUM or higher.
WARNING
For maximum security, endeavour always to avoid ever looking at, touching, or even
imputing the existence of the filename. You do not know that that filename is con-
nected to the same file as the handle you have, and attempts to check this can only
trigger more race conditions. It’s far more secure to use the filehandle alone and
dispense with the filename altogether.
If you need to pass the handle to something that expects a filename then, on a unix
system, use ""/dev/fd/" . fileno($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() func-
tion. 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 POSIX, "tmpfile" in POSIX, File::Spec, File::Path
See IO::File and File::MkTemp for different implementations of temporary file han-
dling.
AUTHOR
Tim Jenness <tjenness AT cpan.org>
Copyright (C) 1999-2003 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.
perl v5.8.6 2001-09-21 File::Temp(3pm)
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