File::Temp - return name and handle of a temporary file safely
This section is at the top in order to provide easier access to porters. It is not expected to be rendered by a standard pod formatting tool. Please skip straight to the SYNOPSIS section if you are not trying to port this module to a new platform.
This module is designed to be portable across operating systems and it currently supports Unix, VMS, DOS, OS/2, Windows and Mac OS (Classic). When porting to a new OS there are generally three main issues that have to be solved:
Can the OS unlink an open file? If it can not then the C<_can_unlink_opened_file> method should be modified.
Are the return values from C
Security. Systems that can not support a test for the sticky bit on a directory can not use the MEDIUM and HIGH security tests. The C<_can_do_level> method should be modified accordingly.
- 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 => $dir );
The following interfaces are provided for compatibility with existing APIs. They should not be used in new code.
- use File::Temp qw/ :mktemp /;
- ($fh, $file) = mkstemp( "tmpfileXXXXX" );
- ($fh, $file) = mkstemps( "tmpfileXXXXXX", $suffix);
- $tmpdir = mkdtemp( $template );
- $unopened_file = mktemp( $template );
- use File::Temp qw/ :POSIX /;
- $file = tmpnam();
- $fh = tmpfile();
- ($fh, $file) = tmpnam();
- $unopened_file = File::Temp::tempnam( $dir, $pfx );
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 L<"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 L
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.
This is the primary interface for interacting with
Note that there is no method to obtain the filehandle from the
Create a temporary file object.
- my $tmp = new File::Temp();
by default the object is constructed as if C
Supported arguments are the same as for C
- $tmp = new File::Temp( TEMPLATE => 'tempXXXXX',
- DIR => 'mydir',
- SUFFIX => '.dat');
Arguments are case insensitive.
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.
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.
When the object goes out of scope, the destructor is called. This
destructor will attempt to unlink the file (using C
No error is given if the unlink fails.
If the global variable $KEEP_ALL is true, the file will not be removed.
This section describes the recommended interface for generating temporary files and directories.
This is the basic function to generate temporary files. The behaviour of the file can be changed using various options:
- $fh = tempfile();
- ($fh, $filename) = tempfile();
Create a temporary file in the directory specified for temporary
files, as specified by the tmpdir() function in L
- ($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 => $suffix)
Same as previously, except that a suffix is added to the template after the `X' translation. Useful for ensuring that a temporary filename has a particular extension when needed by other applications. But see the WARNING at the end.
- ($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 (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 created).
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
automatically generated. This temporary file is placed in tmpdir()
- $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() 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 C
- (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.
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 L
- $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, 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 C
- $tempdir = tempdir( $template, CLEANUP => 1);
Create a temporary directory using the supplied template, but
attempt to remove it (and all files inside it) when the program
exits. Note that an attempt will be made to remove all files from
the directory even if they were not created by this module (otherwise
why ask to clean it up?). The directory removal is made with
the rmtree() function from the L
The following functions are Perl implementations of the mktemp() family of temp file generation system calls.
Given a template, returns a filehandle to the temporary file and the name of the file.
- ($fh, $name) = mkstemp( $template );
In scalar context, just the filehandle is returned.
The template may be any filename with some number of X's appended to it, for example F. The trailing X's are replaced with unique alphanumeric combinations.
Similar to mkstemp(), except that an extra argument can be supplied with a suffix to be appended to the template.
- ($fh, $name) = mkstemps( $template, $suffix );
For example a template of C
Returns just the filehandle alone when called in scalar context.
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.
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().
This section describes the re-implementation of the tmpnam()
and tmpfile() functions described in L
Unlike the L
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.
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.
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.
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 C
- $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.
Useful functions for dealing with the filehandle and filename.
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 C
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 $KEEP_ALL is true and an unlink on open file is supported. If the unlink is to be deferred to the END block, the file is still registered for removal.
- cmpstat($fh, $path)
- or die "Error comparing handle with file";
Returns false if the stat information differs or if the link count is greater than 1.
On certain platofms, eg Windows, not all the fields returned by stat()
can be compared. For example, the C
Not exported by default.
Similar to C
- unlink1($fh, $path)
- or die "Error closing and unlinking file";
Usually called from the object destructor when using the OO interface.
Not exported by default.
This function is disabled if the global variable $KEEP_ALL is true.
Calling this function will cause any temp files or temp directories that are registered for removal to be removed. This happens automatically 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).
Not exported by default.
These functions control the global state of the package.
Controls the lengths to which the module will go to check the safety of the temporary file or directory before proceeding. Options are:
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 L
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 C<-k> test for sticky bit.
In addition to the MEDIUM security checks, also check for the
possibility of ``chown() giveaway'' using the L
For platforms that do not support the L
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 L
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 C
- $newlevel = File::Temp->safe_level( File::Temp::HIGH );
- die "Could not change to high security"
- if $newlevel != File::Temp::HIGH;
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 (C
This is required since on many unix systems C is not owned by root.
Default is to assume that any UID less than or equal to 10 is a root UID.
- my $topid = File::Temp->top_system_uid;
This value can be adjusted to reduce security checking if required.
The value is only relevant when C
Controls whether temporary files and directories should be retained regardless of any instructions in the program to remove them automatically. 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 temporary file, set this variable and expect the temp file to still be around when the program exits.
Controls whether debugging messages should be enabled.
- $File::Temp::DEBUG = 1;
Default is for debugging mode to be disabled.
For maximum security, endeavour always to avoid ever looking at, touching, or even imputing the existence of the filename. You do not know that that filename is connected to the same file as the handle you have, and attempts to check this can only trigger more race conditions. It's far more secure to use the filehandle alone and dispense with the filename altogether.
If you need to pass the handle to something that expects a filename then, on a unix system, use C<"/dev/fd/" . fileno($fh)> for arbitrary programs, or more generally C<< "+<=&" . 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.
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.
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
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.
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.
Tim Jenness E
Copyright (C) 1999-2005 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.