Secure temporary file

Secure temporary file
You are encouraged to solve this task according to the task description, using any language you may know.

Create a temporary file, securely and exclusively (opening it such that there are no possible race conditions). It's fine assuming local filesystem semantics (NFS or other networking filesystems can have signficantly more complicated semantics for satisfying the "no race conditions" criteria). The function should automatically resolve name collisions and should only fail in cases where permission is denied, the filesystem is read-only or full, or similar conditions exist (returning an error or raising an exception as appropriate to the language/environment).

Ada

Ada creates a temporary file whenever the create procedure is called without file name. That temporary file is automatically deleted at the end of the program creating the file.

This example creates a temporary file, writes to the file, then reads from the file.

with Ada.Text_Io; use Ada.Text_Io;
 
procedure Temp_File is
   Temp : File_Type;
   Contents : String(1..80);
   Length : Natural;
begin
   -- Create a temporary file
   Create(File => Temp);
   Put_Line(File => Temp, Item => "Hello World");
   Reset(File => Temp, Mode => In_File);
   Get_Line(File => Temp, Item => Contents, Last => Length);
   Put_Line(Contents(1..Length));  
end Temp_File;

BBC BASIC

Works with: BBC BASIC for Windows

The file is automatically deleted when closed.

      file% = FNopentempfile
      IF file% = 0 ERROR 100, "Failed to open temp file"
      PRINT #file%, "Hello world!"
      PTR#file% = 0
      INPUT #file%, message$
      CLOSE #file%
      PRINT message$
      END
 
      DEF FNopentempfile
      LOCAL pname%, hfile%, chan%
      OPEN_EXISTING = 3
      FILE_FLAG_DELETE_ON_CLOSE = &4000000
      GENERIC_READ = &80000000
      GENERIC_WRITE = &40000000
      INVALID_HANDLE_VALUE = -1
      DIM pname% LOCAL 260
      FOR chan% = 5 TO 12
        IF @hfile%(chan%) = 0 EXIT FOR
      NEXT
      IF chan% > 12 THEN = 0
      SYS "GetTempFileName", @tmp$, "BBC", 0, pname%
      SYS "CreateFile", $$pname%, GENERIC_READ OR GENERIC_WRITE, 0, 0, \
      \                 OPEN_EXISTING, FILE_FLAG_DELETE_ON_CLOSE, 0 TO hfile%
      IF hfile% = INVALID_HANDLE_VALUE THEN = 0
      @hfile%(chan%) = hfile%
      = chan%

Output:

Hello world!

C

#include <stdlib.h>
#include <stdio.h>
 
int main(void)
{
  FILE *fh = tmpfile(); /* file is automatically deleted when program exits */
  /* do stuff with stream "fh" */
  fclose(fh);
  /* The C standard library also has a tmpnam() function to create a file
    for you to open later. But you should not use it because someone else might
    be able to open the file from the time it is created by this function to the
    time you open it. */
  return 0;
}

The following

Works with: POSIX

#include <stdlib.h>
#include <stdio.h>
 
int main(void)
{
  char filename[] = "/tmp/prefixXXXXXX";
  int fd = mkstemp(filename);
  puts(filename);
  /* do stuff with file descriptor "fd" */
  close(fd);
  return 0;
}

C#

using System;
using System.IO;
 
Console.WriteLine(Path.GetTempFileName());

Clojure

user=> (doto (java.io.File/createTempFile "pre" ".suff") .deleteOnExit)
#<File /tmp/pre8116759964152254766.suff>

D

Works with: Tango

module tempfile ;
import tango.io.TempFile, tango.io.Stdout ;
 
void main(char[][] args) {
 
  // create a temporary file that will be deleted automatically when out of scope
  auto tempTransient = new TempFile(TempFile.Transient) ;
  Stdout(tempTransient.path()).newline ;
 
  // create a temporary file, still persist after the TempFile object has been destroyed
  auto tempPermanent = new TempFile(TempFile.Permanent) ;
  Stdout(tempPermanent.path()).newline ;  
 
  // both can only be accessed by the current user (the program?). 
}

Go

import "io/ioutil"
 
file_obj, err := ioutil.TempFile("", "foo")

Groovy

Follows technique of Java example

def file = File.createTempFile( "xxx", ".txt" )
file.deleteOnExit()
println file

Output:

C:\DOCUME~1\ROGER~1.GLO\LOCALS~1\Temp\xxx8918700806071036530.txt

Haskell

import System.IO
 
main = do (pathOfTempFile, h) <- openTempFile "." "prefix.suffix" -- first argument is path to directory where you want to put it
          -- do stuff with it here; "h" is the Handle to the opened file
          return ()

HicEst

! The "scratch" option opens a file exclusively for the current process.
! A scratch file is automatically deleted upon process termination.
 
OPEN( FIle='TemporaryAndExclusive', SCRatch, IOStat=ErrNr)
WRITE(FIle='TemporaryAndExclusive') "something"
WRITE(FIle='TemporaryAndExclusive', CLoSe=1) ! explicit "close" deletes file
 
! Without "scratch" access can be controlled by "denyread", "denywrite", "denyreadwrite" options.
 
OPEN( FIle='DenyForOthers', DenyREAdWRIte, IOStat=ErrNr)
WRITE(FIle='DenyForOthers') "something"
WRITE(FIle='DenyForOthers', DELETE=1)

Java

import java.io.File;
 
try {
    // Create temp file
    File filename = File.createTempFile("prefix", ".suffix");
 
    // Delete temp file when program exits
    filename.deleteOnExit();
 
    System.out.println(filename);
 
} catch (IOException e) {
}

Lua

fp = io.tmpfile()
 
-- do some file operations
 
fp:close()

Mathematica

tmp = OpenWrite[]
Close[tmp]

OCaml

From the module Filename, one can use the functions temp_file or open_temp_file

# Filename.temp_file "prefix." ".suffix" ;;
- : string = "/home/blue_prawn/tmp/prefix.301f82.suffix"

Octave

Octave has several related functions

  [FID, MSG] = tmpfile();    % Return the file ID corresponding to a new temporary 
  filename = tmpnam (...);   % generates temporary file name, but does not open file
  [FID, NAME, MSG] = mkstemp (TEMPLATE, DELETE);    % Return the file ID corresponding to a new temporary file with a unique name created from TEMPLATE.

Pascal

Works with: Free_Pascal

Library: SysUtils

Program TempFileDemo;
 
uses
  SysUtils;
 
var
  tempFile: text;
 
begin
  assign (Tempfile, GetTempFileName);
  rewrite (tempFile);
  writeln (tempFile, 5);
  close (tempFile);
end.

Perl

function interface:

use File::Temp qw(tempfile);
$fh = tempfile();
($fh2, $filename) = tempfile(); # this file stays around by default
print "$filename\n";
close $fh;
close $fh2;

object-oriented interface:

use File::Temp;
$fh = new File::Temp;
print $fh->filename, "\n";
close $fh;

PHP

$fh = tmpfile();
// do stuff with $fh
fclose($fh);
// file removed when closed
 
// or:
$filename = tempnam('/tmp', 'prefix');
echo "$filename\n";
// open $filename and do stuff with it

PicoLisp

The 'tmp' function returns temporary file names which are exclusively for the current process (based on the process ID). These files are automatically deleted upon process termination. Background tasks within a single PicoLisp process is always non-preemptive, therefore dedicated locks are usually not necessary. If they are (e.g. because such a file name is passed to a child process), explicit locks with the 'ctl' functions are possible.

: (out (tmp "foo") (println 123))         # Write tempfile
-> 123
 
: (in (tmp "foo") (read))                 # Read tempfile
-> 123
 
: (let F (tmp "foo")
   (ctl F                                 # Get exclusive lock
      (let N (in F (read))                # Atomic increment
         (out F (println (inc N))) ) ) )
-> 124

PureBasic

Procedure.s TempFile()
  Protected a, Result$
 
  For a = 0 To 9999
    Result$ = GetTemporaryDirectory() + StringField(GetFilePart(ProgramFilename()),1,".")
    Result$ + "_" + Str(ElapsedMilliseconds()) + "_(" + RSet(Str(a),4,"0") + ").tmp"
    If FileSize(Result$) = -1                                      ;  -1 = File not found
      ProcedureReturn Result$
    EndIf
  Next
 
  ProcedureReturn ""
EndProcedure
 
 
Define File, File$
 
File$ = TempFile()
If File$ <> ""
  File = CreateFile(#PB_Any, File$)
  If File <> 0
    WriteString(File, "Some temporary data here...")
    CloseFile(File)
  EndIf
EndIf

Python

Works with: Python version 2.3+

In both cases, the temporary file will be deleted automatically when the file is closed. The invisible file will not be accessible on UNIX-like systems. You can use os.link to preserve the visible temporary file contents.

>>> import tempfile
>>> invisible = tempfile.TemporaryFile()
>>> invisible.name
'<fdopen>'
>>> visible = tempfile.NamedTemporaryFile()
>>> visible.name
'/tmp/tmpZNfc_s'
>>> visible.close()
>>> invisible.close()

More low level way, if you have special needs. This was the only option before Python 2.3:

fd, path = tempfile.mkstemp()
try:
    # use the path or the file descriptor
finally:
    os.close(fd)

Racket

 
#lang racket
(make-temporary-file)
 

Ruby

irb(main):001:0> require 'tempfile'
=> true
irb(main):002:0> f = Tempfile.new('foo')
=> #<File:/tmp/foo20081226-307-10p746n-0>
irb(main):003:0> f.path
=> "/tmp/foo20081226-307-10p746n-0"
irb(main):004:0> f.close
=> nil

Scala

Basically same as Java version.

import java.io.File
 
try {
    // Create temp file
    val filename = File.createTempFile("prefix", ".suffix")
 
    // Delete temp file when program exits
    filename.deleteOnExit
 
    System.out.println(filename)
 
} catch {
  case _: java.io.IOException =>
}
 

Standard ML

val filename = OS.FileSys.tmpName ();

Tcl

Works with: Tcl version 8.6

Will store the name of the file in the variable filenameVar and an open read-write channel on the file in the variable chan.

set chan [file tempfile filenameVar]

Note that because we're asking for the filename in the script, Tcl does not automatically clean the file. (There are cases where auto-cleaning would be really unwanted.) If we hadn't asked for it, the file would be automatically deleted (at a time that depends on platform constraints).

TUSCRIPT

 
$$ MODE TUSCRIPT
tmpfile="test.txt"
ERROR/STOP CREATE (tmpfile,seq-E,-std-)
text="hello world"
FILE $tmpfile = text
- tmpfile "test.txt" can only be accessed by one user an will be deleted upon programm termination
 

UNIX Shell

UNIX shell scripts cannot guarantee secure, race-free, exclusive access to an open file descriptor. The best approach to working around this limitation is to create a directory (the mkdir command is a wrapper around the atomic mkdir() system call) and then perform all temporary file operations thereunder.

RESTOREUMASK=$(umask)
TRY=0
while :; do
   TRY=$(( TRY + 1 ))
   umask 0077
   MYTMP=${TMPDIR:-/tmp}/$(basename $0).$$.$(date +%s).$TRY
   trap "rm -fr $MYTMP" EXIT
   mkdir "$MYTMP" 2>/dev/null && break
done
umask "$RESTOREUMASK"
cd "$MYTMP" || {
   echo "Temporary directory failure on $MYTMP" >&2
   exit 1; }

Note that the shell special variable $$ (the PID of the currently exec()-ed shell) is unique at any given moment on a UNIX system, and $(date +%s) gives the time represented as a number of seconds since the UNIX epoch (GNU date or any other with the %s extension). So any name collision here is most likely "enemy action." This code will loop, picking new names and resetting the trap (clean-up command) until it succeeds in making a directory. (Simple changes to the code could limit the number of attempts or implement a timeout).