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catch(3tcl)


NAME

   catch - Evaluate script and trap exceptional returns

SYNOPSIS

   catch script ?resultVarName? ?optionsVarName?
______________________________________________________________________________

DESCRIPTION

   The  catch  command may be used to prevent errors from aborting command
   interpretation.   The  catch  command   calls   the   Tcl   interpreter
   recursively  to  execute  script, and always returns without raising an
   error, regardless of  any  errors  that  might  occur  while  executing
   script.

   If  script  raises an error, catch will return a non-zero integer value
   corresponding to the exceptional return code returned by evaluation  of
   script.   Tcl  defines the normal return code from script evaluation to
   be zero (0), or TCL_OK.   Tcl  also  defines  four  exceptional  return
   codes:   1   (TCL_ERROR),   2   (TCL_RETURN),   3  (TCL_BREAK),  and  4
   (TCL_CONTINUE).  Errors during evaluation of a script are indicated  by
   a  return  code  of  TCL_ERROR.  The other exceptional return codes are
   returned by the return, break,  and  continue  commands  and  in  other
   special situations as documented.  Tcl packages can define new commands
   that return other integer values as return codes as well,  and  scripts
   that  make  use  of the return -code command can also have return codes
   other than the five defined by Tcl.

   If the resultVarName argument is given, then the variable it  names  is
   set  to the result of the script evaluation.  When the return code from
   the script is 1 (TCL_ERROR), the value stored in  resultVarName  is  an
   error message.  When the return code from the script is 0 (TCL_OK), the
   value stored in resultVarName is the value returned from script.

   If the optionsVarName argument is given, then the variable it names  is 
   set to a dictionary of return options returned by evaluation of script. 
   Tcl specifies two entries that are always defined  in  the  dictionary: 
   -code  and  -level.   When the return code from evaluation of script is 
   not TCL_RETURN, the value of the -level entry will be 0, and the  value 
   of  the -code entry will be the same as the return code.  Only when the 
   return code is TCL_RETURN will the  values  of  the  -level  and  -code 
   entries  be  something  else, as further described in the documentation 
   for the return command.                                                 

   When the return code from evaluation  of  script  is  TCL_ERROR,  three 
   additional  entries  are  defined  in  the dictionary of return options 
   stored in optionsVarName: -errorinfo, -errorcode, and -errorline.   The 
   value  of  the  -errorinfo  entry is a formatted stack trace containing 
   more information about the context in which the  error  happened.   The 
   formatted  stack  trace  is meant to be read by a person.  The value of 
   the -errorcode entry is additional information about the  error  stored 
   as  a  list.   The -errorcode value is meant to be further processed by 
   programs, and may not be particularly readable by people.  The value of 
   the  -errorline entry is an integer indicating which line of script was 
   being evaluated when the error occurred.  The values of the  -errorinfo 
   and  -errorcode  entries of the most recent error are also available as 
   values  of   the   global   variables   ::errorInfo   and   ::errorCode 
   respectively.                                                           

   Tcl  packages  may  provide  commands  that  set  other  entries in the 
   dictionary of return options, and the return command  may  be  used  by 
   scripts to set return options in addition to those defined above.

EXAMPLES

   The  catch  command may be used in an if to branch based on the success
   of a script.
          if { [catch {open $someFile w} fid] } {
              puts stderr "Could not open $someFile for writing\n$fid"
              exit 1
          }

   There are more complex examples of catch usage in the documentation for
   the return command.

SEE ALSO

   break(3tcl),  continue(3tcl),  dict(3tcl),  error(3tcl),  return(3tcl),
   tclvars(3tcl)

KEYWORDS

   catch, error



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