file::temp.3

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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 \f(CW\*(C`sysopen\*(C'\fR
with \f(CW\*(C`O_CREAT | O_EXCL\*(C'\fR if you must open such a filename.
.Sp
.Vb 1
\&  $filename = File::Temp::tempnam( $dir, $prefix );
.Ve
.Sp
Equivalent to running \fImktemp()\fR with \f(CW$dir\fR/$prefixXXXXXXXX
(using unix file convention as an example)
.Sp
Because this function uses \fImktemp()\fR, it can suffer from race conditions.
.Sp
Will \fIcroak()\fR if there is an error.
.SH "UTILITY FUNCTIONS"
.IX Header "UTILITY FUNCTIONS"
Useful functions for dealing with the filehandle and filename.
.IP "\fBunlink0\fR" 4
.IX Item "unlink0"
Given an open filehandle and the associated filename, make a safe
unlink. This is achieved by first checking that the filename and
filehandle initially point to the same file and that the number of
links to the file is 1 (all fields returned by \fIstat()\fR are compared).
Then the filename is unlinked and the filehandle checked once again to
verify that the number of links on that file is now 0.  This is the
closest you can come to making sure that the filename unlinked was the
same as the file whose descriptor you hold.
.Sp
.Vb 2
\&  unlink0($fh, $path)
\&     or die "Error unlinking file $path safely";
.Ve
.Sp
Returns false on error but \fIcroaks()\fR if there is a security
anomaly. The filehandle is not closed since on some occasions this is
not required.
.Sp
On some platforms, for example Windows \s-1NT\s0, it is not possible to
unlink an open file (the file must be closed first). On those
platforms, the actual unlinking is deferred until the program ends and
good status is returned. A check is still performed to make sure that
the filehandle and filename are pointing to the same thing (but not at
the time the end block is executed since the deferred removal may not
have access to the filehandle).
.Sp
Additionally, on Windows \s-1NT\s0 not all the fields returned by \fIstat()\fR can
be compared. For example, the \f(CW\*(C`dev\*(C'\fR and \f(CW\*(C`rdev\*(C'\fR fields seem to be
different.  Also, it seems that the size of the file returned by \fIstat()\fR
does not always agree, with \f(CW\*(C`stat(FH)\*(C'\fR being more accurate than
\&\f(CW\*(C`stat(filename)\*(C'\fR, presumably because of caching issues even when
using autoflush (this is usually overcome by waiting a while after
writing to the tempfile before attempting to \f(CW\*(C`unlink0\*(C'\fR it).
.Sp
Finally, on \s-1NFS\s0 file systems the link count of the file handle does
not always go to zero immediately after unlinking. Currently, this
command is expected to fail on \s-1NFS\s0 disks.
.Sp
This function is disabled if the global variable \f(CW$KEEP_ALL\fR is true
and an unlink on open file is supported. If the unlink is to be deferred
to the \s-1END\s0 block, the file is still registered for removal.
.Sp
This function should not be called if you are using the object oriented
interface since the it will interfere with the object destructor deleting
the file.
.IP "\fBcmpstat\fR" 4
.IX Item "cmpstat"
Compare \f(CW\*(C`stat\*(C'\fR of filehandle with \f(CW\*(C`stat\*(C'\fR 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 \fIstat()\fR are compared).
.Sp
.Vb 2
\&  cmpstat($fh, $path)
\&     or die "Error comparing handle with file";
.Ve
.Sp
Returns false if the stat information differs or if the link count is
greater than 1. Calls croak if there is a security anomaly.
.Sp
On certain platforms, for example Windows, not all the fields returned by \fIstat()\fR
can be compared. For example, the \f(CW\*(C`dev\*(C'\fR and \f(CW\*(C`rdev\*(C'\fR fields seem to be
different in Windows.  Also, it seems that the size of the file
returned by \fIstat()\fR does not always agree, with \f(CW\*(C`stat(FH)\*(C'\fR being more
accurate than \f(CW\*(C`stat(filename)\*(C'\fR, presumably because of caching issues
even when using autoflush (this is usually overcome by waiting a while
after writing to the tempfile before attempting to \f(CW\*(C`unlink0\*(C'\fR it).
.Sp
Not exported by default.
.IP "\fBunlink1\fR" 4
.IX Item "unlink1"
Similar to \f(CW\*(C`unlink0\*(C'\fR except after file comparison using cmpstat, the
filehandle is closed prior to attempting to unlink the file. This
allows the file to be removed without using an \s-1END\s0 block, but does
mean that the post-unlink comparison of the filehandle state provided
by \f(CW\*(C`unlink0\*(C'\fR is not available.
.Sp
.Vb 2
\&  unlink1($fh, $path)
\&     or die "Error closing and unlinking file";
.Ve
.Sp
Usually called from the object destructor when using the \s-1OO\s0 interface.
.Sp
Not exported by default.
.Sp
This function is disabled if the global variable \f(CW$KEEP_ALL\fR is true.
.Sp
Can call \fIcroak()\fR if there is a security anomaly during the \fIstat()\fR
comparison.
.IP "\fBcleanup\fR" 4
.IX Item "cleanup"
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.
.Sp
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).
.Sp
.Vb 1
\&  File::Temp::cleanup();
.Ve
.Sp
Not exported by default.
.SH "PACKAGE VARIABLES"
.IX Header "PACKAGE VARIABLES"
These functions control the global state of the package.
.IP "\fBsafe_level\fR" 4
.IX Item "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:
.RS 4
.IP "\s-1STANDARD\s0" 8
.IX Item "STANDARD"
Do the basic security measures to ensure the directory exists and is
writable, 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.
.IP "\s-1MEDIUM\s0" 8
.IX Item "MEDIUM"
In addition to the \s-1STANDARD\s0 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.
.Sp
Will not work on platforms that do not support the \f(CW\*(C`\-k\*(C'\fR test
for sticky bit.
.IP "\s-1HIGH\s0" 8
.IX Item "HIGH"
In addition to the \s-1MEDIUM\s0 security checks, also check for the
possibility of ``\fIchown()\fR giveaway'' using the \s-1POSIX\s0
\&\fIsysconf()\fR function. If this is a possibility, each directory in the
path is checked in turn for safeness, recursively walking back to the
root directory.
.Sp
For platforms that do not support the \s-1POSIX\s0
\&\f(CW\*(C`_PC_CHOWN_RESTRICTED\*(C'\fR symbol (for example, Windows \s-1NT\s0) it is
assumed that ``\fIchown()\fR giveaway'' is possible and the recursive test
is performed.
.RE
.RS 4
.Sp
The level can be changed as follows:
.Sp
.Vb 1
\&  File::Temp\->safe_level( File::Temp::HIGH );
.Ve
.Sp
The level constants are not exported by the module.
.Sp
Currently, you must be running at least perl v5.6.0 in order to
run with \s-1MEDIUM\s0 or \s-1HIGH\s0 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.
.Sp
On systems that do not support the \s-1HIGH\s0 or \s-1MEDIUM\s0 safety levels
(for example Win \s-1NT\s0 or \s-1OS/2\s0) 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.
.Sp
If you really need to see whether the change has been accepted
simply examine the return value of \f(CW\*(C`safe_level\*(C'\fR.
.Sp
.Vb 3
\&  $newlevel = File::Temp\->safe_level( File::Temp::HIGH );
\&  die "Could not change to high security"
\&      if $newlevel != File::Temp::HIGH;
.Ve
.RE
.IP "TopSystemUID" 4
.IX Item "TopSystemUID"
This is the highest \s-1UID\s0 on the current system that refers to a root
\&\s-1UID\s0. This is used to make sure that the temporary directory is
owned by a system \s-1UID\s0 (\f(CW\*(C`root\*(C'\fR, \f(CW\*(C`bin\*(C'\fR, \f(CW\*(C`sys\*(C'\fR etc) rather than
simply by root.
.Sp
This is required since on many unix systems \f(CW\*(C`/tmp\*(C'\fR is not owned
by root.
.Sp
Default is to assume that any \s-1UID\s0 less than or equal to 10 is a root
\&\s-1UID\s0.
.Sp
.Vb 2
\&  File::Temp\->top_system_uid(10);
\&  my $topid = File::Temp\->top_system_uid;
.Ve
.Sp
This value can be adjusted to reduce security checking if required.
The value is only relevant when \f(CW\*(C`safe_level\*(C'\fR is set to \s-1MEDIUM\s0 or higher.
.ie n .IP "\fB\fB$KEEP_ALL\fB\fR" 4
.el .IP "\fB\f(CB$KEEP_ALL\fB\fR" 4
.IX Item "$KEEP_ALL"
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.
.Sp
.Vb 1
\&  $File::Temp::KEEP_ALL = 1;
.Ve
.Sp
Default is for files to be removed as requested by the caller.
.Sp
In some cases, files will only be retained if this variable is true
when the file is created. This means that you can not create a temporary
file, set this variable and expect the temp file to still be around
when the program exits.
.ie n .IP "\fB\fB$DEBUG\fB\fR" 4
.el .IP "\fB\f(CB$DEBUG\fB\fR" 4
.IX Item "$DEBUG"
Controls whether debugging messages should be enabled.
.Sp
.Vb 1
\&  $File::Temp::DEBUG = 1;
.Ve
.Sp
Default is for debugging mode to be disabled.
.SH "WARNING"
.IX Header "WARNING"
For maximum security, endeavour always to avoid ever looking at,
touching, or even imputing the existence of the filename.  You do not
know that that filename is connected to the same file as the handle
you have, and attempts to check this can only trigger more race
conditions.  It's far more secure to use the filehandle alone and
dispense with the filename altogether.
.PP
If you need to pass the handle to something that expects a filename
then, on a unix system, use \f(CW\*(C`"/dev/fd/" . fileno($fh)\*(C'\fR for arbitrary
programs, or more generally \f(CW\*(C`"+<=&" . fileno($fh)\*(C'\fR for Perl
programs.  You will have to clear the close-on-exec bit on that file
descriptor before passing it to another process.
.PP
.Vb 3
\&    use Fcntl qw/F_SETFD F_GETFD/;
\&    fcntl($tmpfh, F_SETFD, 0)
\&        or die "Can\*(Aqt clear close\-on\-exec flag on temp fh: $!\en";
.Ve
.Sh "Temporary files and \s-1NFS\s0"
.IX Subsection "Temporary files and NFS"
Some problems are associated with using temporary files that reside
on \s-1NFS\s0 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 \s-1NFS\s0 will not be as good as for
a local disk.
.Sh "Forking"
.IX Subsection "Forking"
In some cases files created by File::Temp are removed from within an
\&\s-1END\s0 block. Since \s-1END\s0 blocks are triggered when a child process exits
(unless \f(CW\*(C`POSIX::_exit()\*(C'\fR is used by the child) File::Temp takes care
to only remove those temp files created by a particular process \s-1ID\s0. This
means that a child will not attempt to remove temp files created by the
parent process.
.PP
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(\s-1EXPR\s0) 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.
.Sh "\s-1BINMODE\s0"
.IX Subsection "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 \f(CW\*(C`binmode()\*(C'\fR
function to change the mode of the filehandle.
.PP
Note that you can modify the encoding of a file opened by File::Temp
also by using \f(CW\*(C`binmode()\*(C'\fR.
.SH "HISTORY"
.IX Header "HISTORY"
Originally began life in May 1999 as an \s-1XS\s0 interface to the system
\&\fImkstemp()\fR function. In March 2000, the OpenBSD \fImkstemp()\fR 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.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\*(L"tmpnam\*(R" in \s-1POSIX\s0, \*(L"tmpfile\*(R" in \s-1POSIX\s0, File::Spec, File::Path
.PP
See IO::File and File::MkTemp, Apache::TempFile for
different implementations of temporary file handling.
.PP
See File::Tempdir for an alternative object-oriented wrapper for
the \f(CW\*(C`tempdir\*(C'\fR function.
.SH "AUTHOR"
.IX Header "AUTHOR"
Tim Jenness <[e-mail]>
.PP
Copyright (C) 2007 Tim Jenness.
Copyright (C) 1999\-2007 Tim Jenness and the \s-1UK\s0 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.
.PP
Original Perl implementation loosely based on the OpenBSD C code for
\&\fImkstemp()\fR. Thanks to Tom Christiansen for suggesting that this module
should be written and providing ideas for code improvements and
security enhancements.