CODEWITHSUNDEEP
error-handlingtcl
Stephen Alter
Stephen Alter

Reputation: 27

Why does TCL CATCH command not return an error?

I am attempting to launch a compiled code to handle the extensive computing needed by a TCL script I am writing. When I launch the code, I am using:

if {[catch {exec $cwd/TaubinSmoother &} error_out]} {
    if {[string first "couldn" $error_out] != -1} {
        puts "TaubinSmoother software could not be located."
        exit
    } else {
        puts "$error_out"
    }
}

When the exec command executes, there appears to be no error, but one is indeed generated because the PATH environment is not properly set for shared libraries that TaubinSmoother needs in order to run. I have tried using:

 proc launch {} {
     set error_catch [catch {exec $cwd/TaubinSmoother &} error_out detail]
     return $detail
 }

 set cwd       [file dirname [info script]]
 set error_out launch
 puts "$error_out"

This too did not work as I only received a "launch" printed to the stdout. I read TCL, get full error message in catch command and more about catch and return, but the answered question did not work in my case and the examples on the man-pages did not help. How do I get the actual error to return to my running script so I can warn future users of the issues encountered?

Upvotes: 2

Views: 987

Answers (1)

mrcalvin
mrcalvin

Reputation: 3434

Problem 1

Your second attempt fails in the sense of "not meeting your expectation" because the line 

 set error_out launch

should read

 set error_out [launch]

Background: in its original form, it is an assignment of a literal string "launch" to a variable error_out. Hence, this is what you see printed.

Problem 2

Your exec runs in background mode (&). Therefore, your Tcl script is not correlated with the backgrounded process any longer. exec will just return without signalling any (well, background) error. Better use a bullet-proven Tcl idiom for checking on a background process, using open and fileevent:

See Detect end of TCL background process in a TCL script

Upvotes: 3

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