File::Temp
Temp(3) User Contributed Perl Documentation Temp(3)
NAME
File::Temp - return name and handle of a temporary file safely
SYNOPSIS
use File::Temp qw/ tempfile tempdir /;
$fh = tempfile();
($fh, $filename) = tempfile();
($fh, $filename) = tempfile( $template, DIR => $dir);
($fh, $filename) = tempfile( $template, SUFFIX => '.dat');
$dir = tempdir( CLEANUP => 1 );
($fh, $filename) = tempfile( DIR => $dir );
Object interface:
require File::Temp;
use File::Temp ();
use File::Temp qw/ :seekable /;
$fh = new File::Temp();
$fname = $fh->filename;
$fh = new File::Temp(TEMPLATE => $template);
$fname = $fh->filename;
$tmp = new File::Temp( UNLINK => 0, SUFFIX => '.dat' );
print $tmp "Some data\n";
print "Filename is $tmp\n";
$tmp->seek( 0, SEEK_END );
The following interfaces are provided for compatibility with existing
APIs. They should not be used in new code.
MkTemp family:
use File::Temp qw/ :mktemp /;
($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();
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 constructor 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 directories. See
"safe_level" for more information.
For compatibility with popular C library functions, Perl implementa-
tions of the mkstemp() family of functions are provided. These are,
mkstemp(), mkstemps(), mkdtemp() and mktemp().
Additionally, implementations of the standard POSIX tmpnam() and tmp-
file() 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.
OBJECT-ORIENTED 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 tempo-
rary file, and can be compared to a filename directly. The object isa
"IO::Handle" and isa "IO::Seekable" so all those methods are available.
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 tempo-
rary 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.
Can call croak() if an error occurs.
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.
unlink_on_destroy
Control whether the file is unlinked when the object goes out of
scope. The file is removed if this value is true and $KEEP_ALL is
not.
$fh->unlink_on_destroy( 1 );
Default is for the file to be removed.
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.
If the global variable $KEEP_ALL is true, the file will not be
removed.
FUNCTIONS
This section describes the recommended interface for generating tempo-
rary files and directories.
tempfile
This is the basic function to generate temporary files. The
behaviour of the file can be changed using various options:
$fh = tempfile();
($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 filename. At least four `X' characters must be
present at the end of the template.
($fh, $filename) = tempfile($template, SUFFIX => $suffix)
Same as previously, except that a suffix is added to the template
after the `X' translation. Useful for ensuring that a temporary
filename has a particular extension when needed by other applica-
tions. But see the WARNING at the end.
($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 (dependent on
$KEEP_ALL). 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 (depending on the operating system) on exit or when
it is closed (unless $KEEP_ALL is true when the temp file is cre-
ated).
Use the object-oriented interface if fine-grained control of when a
file is removed is required.
If the template is not specified, a template is always automati-
cally 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 on operating
systems that support this (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 con-
dition 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 else-
where in this document if opening the file is not required.
Options can be combined as required.
Will croak() if there is an error.
tempdir
This is the recommended interface for creation of temporary direc-
tories. 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 tem-
plate.
$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 par-
ent directory specifications 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 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 tem-
plate nor a directory are supplied.
$tempdir = tempdir( $template, CLEANUP => 1);
Create a temporary directory using the supplied template, but
attempt to remove it (and all files inside it) when the program
exits. Note that an attempt will be made to remove all files from
the directory even if they were not created by this module (other-
wise why ask to clean it up?). The directory removal is made with
the rmtree() function from the File::Path module. Of course, if
the template is not specified, the temporary directory will be cre-
ated in tmpdir() and will also be removed at program exit.
Will croak() if there is an error.
MKTEMP FUNCTIONS
The following functions are Perl implementations of the mktemp() family
of temp file generation system calls.
mkstemp
Given a template, returns a filehandle to the temporary file and
the name of the file.
($fh, $name) = mkstemp( $template );
In scalar context, just the filehandle is returned.
The template may be any filename with some number of X's appended
to it, for example /tmp/temp.XXXX. The trailing X's are replaced
with unique alphanumeric combinations.
Will croak() if there is an error.
mkstemps
Similar to mkstemp(), except that an extra argument can be supplied
with a suffix to be appended to the template.
($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.
Will croak() if there is an error.
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.
Directory must be removed by the caller.
Will croak() if there is an error.
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().
Will croak() if there is an error.
POSIX FUNCTIONS
This section describes the re-implementation of the tmpnam() and tmp-
file() functions described in POSIX using the mkstemp() from this mod-
ule.
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 con-
ditions.
See "tmpdir" in File::Spec for information on the choice of tempo-
rary directory for a particular operating system.
Will croak() if there is an error.
tmpfile
Returns the filehandle of a temporary file.
$fh = tmpfile();
The file is removed when the filehandle is closed or when the pro-
gram exits. No access to the filename is provided.
If the temporary file can not be created undef is returned. Cur-
rently this command will probably not work when the temporary
directory is on an NFS file system.
Will croak() if there is an error.
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 condi-
tions.
Will croak() if there is an error.
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 com-
pared). 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 file-
name 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 but croaks() if there is a security anomaly.
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 plat-
forms, 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.
This function is disabled if the global variable $KEEP_ALL is true
and an unlink on open file is supported. If the unlink is to be
deferred to the END block, the file is still registered for
removal.
This function should not be called if you are using the object ori-
ented interface since the it will interfere with the object
destructor deleting the file.
cmpstat
Compare "stat" of filehandle with "stat" of provided filename.
This can be used to check that the filename and filehandle ini-
tially point to the same file and that the number of links to the
file is 1 (all fields returned by stat() are compared).
cmpstat($fh, $path)
or die "Error comparing handle with file";
Returns false if the stat information differs or if the link count
is greater than 1. Calls croak if there is a security anomaly.
On certain platforms, for example Windows, not all the fields
returned by stat() can be compared. For example, the "dev" and
"rdev" fields seem to be different in Windows. Also, it seems that
the size of the file returned by stat() does not always agree, with
"stat(FH)" being more accurate than "stat(filename)", presumably
because of caching issues even when using autoflush (this is usu-
ally overcome by waiting a while after writing to the tempfile
before attempting to "unlink0" it).
Not exported by default.
unlink1
Similar to "unlink0" except after file comparison using cmpstat,
the filehandle is closed prior to attempting to unlink the file.
This allows the file to be removed without using an END block, but
does mean that the post-unlink comparison of the filehandle state
provided by "unlink0" is not available.
unlink1($fh, $path)
or die "Error closing and unlinking file";
Usually called from the object destructor when using the OO inter-
face.
Not exported by default.
This function is disabled if the global variable $KEEP_ALL is true.
Can call croak() if there is a security anomaly during the stat()
comparison.
cleanup
Calling this function will cause any temp files or temp directories
that are registered for removal to be removed. This happens auto-
matically when the process exits but can be triggered manually if
the caller is sure that none of the temp files are required. This
method can be registered as an Apache callback.
On OSes where temp files are automatically removed when the temp
file is closed, calling this function will have no effect other
than to remove temporary directories (which may include temporary
files).
File::Temp::cleanup();
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 condi-
tions 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 direc-
tory in the path is checked in turn for safeness, recur-
sively 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 ver-
sions 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 differ-
ent 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 fail-
ing on systems where the extra tests are irrelevant.
If you really need to see whether the change has been accepted sim-
ply 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.
$KEEP_ALL
Controls whether temporary files and directories should be retained
regardless of any instructions in the program to remove them auto-
matically. This is useful for debugging but should not be used in
production code.
$File::Temp::KEEP_ALL = 1;
Default is for files to be removed as requested by the caller.
In some cases, files will only be retained if this variable is true
when the file is created. This means that you can not create a tem-
porary file, set this variable and expect the temp file to still be
around when the program exits.
$DEBUG
Controls whether debugging messages should be enabled.
$File::Temp::DEBUG = 1;
Default is for debugging mode to be disabled.
WARNING
For maximum security, endeavour always to avoid ever looking at, touch-
ing, or even imputing the existence of the filename. You do not know
that that filename is connected to the same file as the handle you
have, and attempts to check this can only trigger more race conditions.
It's far more secure to use the filehandle alone and dispense with the
filename altogether.
If you need to pass the handle to something that expects a filename
then, on a unix system, use ""/dev/fd/" . fileno($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 per-
formance of I/O operations over NFS will not be as good as for a local
disk.
Forking
In some cases files created by File::Temp are removed from within an
END block. Since END blocks are triggered when a child process exits
(unless "POSIX::_exit()" is used by the child) File::Temp takes care to
only remove those temp files created by a particular process ID. This
means that a child will not attempt to remove temp files created by the
parent process.
If you are forking many processes in parallel that are all creating
temporary files, you may need to reset the random number seed using
srand(EXPR) in each child else all the children will attempt to walk
through the same set of random file names and may well cause themselves
to give up if they exceed the number of retry attempts.
BINMODE
The file returned by File::Temp will have been opened in binary mode if
such a mode is available. If that is not correct, use the binmode()
function to change the mode of the filehandle.
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. The module was shipped as a standard part
of perl from v5.6.1.
SEE ALSO
"tmpnam" in POSIX, "tmpfile" in POSIX, File::Spec, File::Path
See IO::File and File::MkTemp, Apache::TempFile for different implemen-
tations of temporary file handling.
AUTHOR
Tim Jenness <tjenness@cpan.org>
Copyright (C) 1999-2007 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 secu-
rity enhancements.
perl v5.8.6 2007-01-21 Temp(3)
