Version 10 of Dangerous constructs

Updated 2004-12-20 19:29:59

Arjen Markus (12 november 2003) In response to a recent discussion on the c.l.t. about a problem that arose in the context of regular expressions, I have started this page. Its sole purpose: document dangerous constructs in Tcl


Using the subst command on arbitrary data:

   set a "Hello,"
   set b "world!"
   set string "$a $b"
   puts [subst $string]

gives:

 Hello, world!

but:

   set string "\[exit\]"
   puts [subst $string]

stops your program!

The subst command allows you to suppress the execution of commands:

   puts [subst -nocommands $string]

gives:

  [exit]

Unexpected 0ctals

Remember that if Tcl is expecting an integer, and the value begins with a 0 (zero), it will interpret it as an octal number. This bug is triggered most often when trying to do arithmetic on date and time values obtained from [clock format]. This is especially dangerous since you can (e.g..) write a program in January that works just fine for months, then crashes in August and September.


RS: A simple error that will appear only at runtime is not protecting a switch command with --:

 switch $input {...}

The error will occur if $input starts with a minus (-) sign. So best always use

 switch -- $input {...}

LV There are a number of other tcl commands which also support -- ; if the command supports it, and you are using random input from users or input files, you probably should use it.


TV opening any server socket, expecting a certain other party to connect. For instance a file transfer �� la ftp where a control connection triggers a file transfer over a seperate socket pair.


Please: the next!


See also the Frequently Made Mistakes FMM page.


[Mention un-braced expr use.] What's dangerous about unbraced expr?

Short answer:

      # Uh-oh; what if it's "exec rm -rf ..." rather than "exec touch ..."?
    set a {[exec touch /tmp/77]}
    set b {[exec touch /tmp/78]}
    catch {expr $a + 4}
    catch {expr {$b + 4}}

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