Creating Temporary Files

There are two ways to make temporary files:

Tcl offered file tempfile for creating temporary files.
Tcllib provides commands for tempdir and tempfile:
http://tcllib.sourceforge.net/doc/fileutil.html

However this page is dedicated to ways to create temperory files by adding a bit of Tcl code, for people who targets distributions older than Tcl 8.6 (not having file tempfile) and do not wish to depend on tcllib.

The temp directory is put in to different environment variables on different systems. Examples are:

Mac OS X: TMPDIR (tested on 10.8.5) Windows 2000: TMP and TEMP (tested on Windows 2000 SP3) CygWin: TMP and TEMP (tested on Cygwin 1.7.9)

A scriptlet to do this:

 set tmpdir [pwd]
 if {[file exists "/tmp"]} { set tmpdir "/tmp"}
 catch {set tmpdir $::env(TRASH_FOLDER)} ;# very old Machintosh. Mac OS X doesn't have this.
 catch {set tmpdir $::env(TMP)}
 catch {set tmpdir $::env(TEMP)}

If you just want one temp file:

 set filename [file join $tmpdir [pid]]

If you want multiple tmpfiles, consider appending things like the application name, counter, time stamp, etc. For example:

 set file [file join $tmpdir $appname.[pid].[incr ::globalCounter]]

DKF - Or create in a subdirectory/subfolder

 if {![file exists [file join $tmpdir [pid]]]} {
    file mkdir [file join $tmpdir [pid]]
 }
 set file [file join $tmpdir [pid] [incr ::globalCounter]]

Michael Schlenker - Be aware of potential race conditions while opening temp files. The linux secure programming howto has some nice examples for the linux platform: http://en.tldp.org/HOWTO/Secure-Programs-HOWTO/avoid-race.html

To be sure to get a new tempfile (and not some evil symlink) try something like:

 set access [list RDWR CREAT EXCL TRUNC]
 set perm 0600
 if {[catch {open $file $access $perm} fid ]} {
     # something went wrong 
     error "Could not open tempfile."
 } 
 # ok everything went well

I had some luck with the following procedure on a UNIX box. Maybe, somebody can check if it works for windows.

proc tempfile {prefix suffix} {
    set chars "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    set nrand_chars 10
    set maxtries 10
    set access [list RDWR CREAT EXCL TRUNC]
    set permission 0600
    set channel ""
    set checked_dir_writable 0
    set mypid [pid]
    for {set i 0} {$i < $maxtries} {incr i} {
        set newname $prefix
        for {set j 0} {$j < $nrand_chars} {incr j} {
            append newname [string index $chars \
                    [expr ([clock clicks] ^ $mypid) % 62]]
        }
        append newname $suffix
        if {[file exists $newname]} {
            after 1
        } else {
            if {[catch {open $newname $access $permission} channel]} {
                if {!$checked_dir_writable} {
                    set dirname [file dirname $newname]
                    if {![file writable $dirname]} {
                        error "Directory $dirname is not writable"
                    }
                    set checked_dir_writable 1
                }
            } else {
                # Success
                return [list $newname $channel]
            }
        }
    }
    if {[string compare $channel ""]} {
        error "Failed to open a temporary file: $chanel"
    } else {
        error "Failed to find an unused temporary file name"
    }
}

Igor Volobouev

Stu - My version of mkstemp(3)

 proc randazAZchar {} {
     return [format %c [expr {int(rand() * 26) + [expr {int(rand() * 10) > 5 ? 97 : 65}]}]]
 }
 proc randazAZstring {length} {
     set s {}
     for {set i $length} {$i > 0} {incr i -1} {append s [randazAZchar]}
     return $s
 }
 proc makeTempFile {template} {
     if {[set xi [string first "X" $template]] == -1} {
         set n $template
         set p {}
         set rl 0
     } else {
         set n [string range $template 0 [expr {$xi - 1}]]
         set p [pid]
         set rl [expr {[string length $template] - $xi - [string length $p]}]
         if {$rl < 0} {
             set p [string range $p 0 end-[expr {$rl * -1}]]
             set rl 0
         }
     }
     for {set i [expr {int(pow(26, ($rl <= 6 ? $rl : 6)))}]} {$i > 0} {incr i -1} {
         set fn $n[randazAZstring $rl]$p
         if {![catch {open $fn {CREAT EXCL RDWR} 0600} fd]} {
             file delete $fn
             return $fd
         }
         if {[lindex $::errorCode 1] ne "EEXIST" || [lindex $::errorCode 0] ne "POSIX"} {
             break
         }
     }
     return -code error -errorcode $::errorCode $fd
 }

wld

Have a look at GUID and UUID for generating unique filenames for temp-files.

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