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Tcl_AddErrorInfo(3)	    Tcl Library Procedures	   Tcl_AddErrorInfo(3)

______________________________________________________________________________

NAME
       Tcl_GetReturnOptions,	  Tcl_SetReturnOptions,	     Tcl_AddErrorInfo,
       Tcl_AppendObjToErrorInfo,   Tcl_AddObjErrorInfo,	  Tcl_SetObjErrorCode,
       Tcl_SetErrorCode,  Tcl_SetErrorCodeVA,  Tcl_PosixError, Tcl_LogCommand‐
       Info - retrieve or record information about  errors  and	 other	return
       options

SYNOPSIS
       #include <tcl.h>

       Tcl_Obj *							       │
       Tcl_GetReturnOptions(interp, code)				       │

       int								       │
       Tcl_SetReturnOptions(interp, options)				       │

       Tcl_AddErrorInfo(interp, message)

       Tcl_AppendObjToErrorInfo(interp, objPtr)				       │

       Tcl_AddObjErrorInfo(interp, message, length)

       Tcl_SetObjErrorCode(interp, errorObjPtr)

       Tcl_SetErrorCode(interp, element, element, ... (char *) NULL)

       Tcl_SetErrorCodeVA(interp, argList)

       const char *
       Tcl_PosixError(interp)

       void
       Tcl_LogCommandInfo(interp, script, command, commandLength)

ARGUMENTS
       Tcl_Interp *interp (in)		      Interpreter  in  which to record
					      information.

       int	    code		      The code	returned  from	script
					      evaluation.

       Tcl_Obj	    *options		      A dictionary of return options.

       char *message (in)		      For  Tcl_AddErrorInfo, this is a
					      conventional C string to	append
					      to the -errorinfo return option.
					      For  Tcl_AddObjErrorInfo,	  this
					      points  to  the first byte of an
					      array of length bytes containing
					      a	  string   to  append  to  the
					      -errorinfo return option.	  This
					      byte  array may contain embedded
					      null bytes unless length is neg‐
					      ative.			       │

       Tcl_Obj *objPtr (in)						       │
					      A	 message to be appended to the │
					      -errorinfo return option in  the │
					      form of a Tcl_Obj value.

       int length (in)			      The number of bytes to copy from
					      message when  appending  to  the
					      -errorinfo  return  option.   If
					      negative, all bytes  up  to  the
					      first null byte are used.

       Tcl_Obj *errorObjPtr (in)	      The   -errorcode	return	option
					      will be set to this value.

       char *element (in)		      String to record as one  element
					      of the -errorcode return option.
					      Last element  argument  must  be
					      NULL.

       va_list argList (in)		      An argument list which must have
					      been initialized using va_start,
					      and cleared using va_end.

       const char *script (in)		      Pointer  to  first  character in
					      script containing command	 (must
					      be <= command)

       const char *command (in)		      Pointer  to  first  character in
					      command that generated the error

       int commandLength (in)		      Number of bytes in  command;  -1
					      means  use all bytes up to first
					      null byte
_________________________________________________________________

DESCRIPTION
       The Tcl_SetReturnOptions and Tcl_GetReturnOptions routines  expose  the │
       same  capabilities  as  the return and catch commands, respectively, in │
       the form of a C interface.					       │

       Tcl_GetReturnOptions retrieves the dictionary of return options from an │
       interpreter  following  a script evaluation.  Routines such as Tcl_Eval │
       are called to evaluate a script	in  an	interpreter.   These  routines │
       return  an  integer  completion code.  These routines also leave in the │
       interpreter both a result and a dictionary of return options  generated │
       by  script  evaluation.	Just as Tcl_GetObjResult retrieves the result, │
       Tcl_GetReturnOptions retrieves the dictionary of return	options.   The │
       integer	completion  code  should  be  passed  as  the code argument to │
       Tcl_GetReturnOptions so that all required options will  be  present  in │
       the  dictionary.	  Specifically,	 a code value of TCL_ERROR will ensure │
       that entries for the keys -errorinfo, -errorcode, and  -errorline  will │
       appear  in  the	dictionary.   Also, the entries for the keys -code and │
       -level will be adjusted if necessary to agree with the value  of	 code. │
       The  (Tcl_Obj *) returned by Tcl_GetReturnOptions points to an unshared │
       Tcl_Obj with reference count of zero.  The dictionary  may  be  written │
       to, either adding, removing, or overwriting any entries in it, with the │
       need to check for a shared object.				       │

       A typical usage for Tcl_GetReturnOptions is to retrieve the stack trace │
       when script evaluation returns TCL_ERROR, like so:		       │
	      int code = Tcl_Eval(interp, script);			       │
	      if (code == TCL_ERROR) {					       │
		  Tcl_Obj *options = Tcl_GetReturnOptions(interp, code);       │
		  Tcl_Obj *key = Tcl_NewStringObj("-errorinfo", -1);	       │
		  Tcl_Obj *stackTrace;					       │
		  Tcl_IncrRefCount(key);				       │
		  Tcl_DictObjGet(NULL, options, key, &stackTrace);	       │
		  Tcl_DecrRefCount(key);				       │
		  /* Do something with stackTrace */			       │
	      }								       │

       Tcl_SetReturnOptions  sets  the return options of interp to be options. │
       If options contains any invalid value for any key,  TCL_ERROR  will  be │
       returned,  and  the  interp  result will be set to an appropriate error │
       message.	 Otherwise, a completion code in agreement with the -code  and │
       -level keys in options will be returned.				       │

       As  an  example,	 Tcl's	return	command itself could be implemented in │
       terms of Tcl_SetReturnOptions like so:				       │
	      if ((objc % 2) == 0) { /* explicit result argument */	       │
		  objc--;						       │
		  Tcl_SetObjResult(interp, objv[objc]);			       │
	      }								       │
	      return Tcl_SetReturnOptions(interp, Tcl_NewListObj(objc-1, objv+1));│
       (It is not really implemented that  way.	  Internal  access  privileges │
       allow  for  a  more  efficient  alternative that meshes better with the │
       bytecode compiler.)						       │

       Note that a newly created Tcl_Obj may be passed in as the options argu‐ │
       ment without the need to tend to any reference counting.	 This is anal‐ │
       ogous to Tcl_SetObjResult.					       │

       While Tcl_SetReturnOptions provides a general interface to set any col‐ │
       lection	of  return options, there are a handful of return options that │
       are very frequently used.  Most notably the -errorinfo  and  -errorcode │
       return  options	should be set properly when the command procedure of a │
       command returns TCL_ERROR.  Tcl provides several simpler interfaces  to │
       more directly set these return options.

       The  -errorinfo	option holds a stack trace of the operations that were
       in progress when an error occurred, and is intended to  be  human-read‐
       able.  The -errorcode option holds a list of items that are intended to
       be machine-readable.  The first item in the -errorcode value identifies
       the class of error that occurred (e.g. POSIX means an error occurred in
       a POSIX system call) and additional elements hold additional pieces  of
       information that depend on the class.  See the tclvars manual entry for
       details on the various formats for the -errorcode option used by	 Tcl's
       built-in commands.

       The  -errorinfo	option value is gradually built up as an error unwinds
       through the nested operations.  Each time an error code is returned  to
       Tcl_Eval,  or  any of the routines that performs script evaluation, the
       procedure Tcl_AddErrorInfo is called to	add  additional	 text  to  the
       -errorinfo  value  describing  the command that was being executed when
       the error occurred.  By the time the error has been passed all the  way
       back to the application, it will contain a complete trace of the activ‐
       ity in progress when the error occurred.

       It is sometimes useful to add additional information to the  -errorinfo
       value  beyond  what can be supplied automatically by the script evalua‐
       tion routines.  Tcl_AddErrorInfo may be used for this purpose: its mes‐
       sage  argument is an additional string to be appended to the -errorinfo
       option.	For example, when an error arises during the  source  command,
       the procedure Tcl_AddErrorInfo is called to record the name of the file
       being processed and the line number on which the error occurred.	 Like‐
       wise,  when  an error arises during evaluation of a Tcl procedures, the
       procedure name and line number within the procedure are	recorded,  and
       so  on.	 The best time to call Tcl_AddErrorInfo is just after a script
       evaluation routine has returned TCL_ERROR.  The value of the -errorline
       return  option  (retrieved  via	a  call to Tcl_GetReturnOptions) often
       makes up a useful part of the message passed to Tcl_AddErrorInfo.

       Tcl_AppendObjToErrorInfo is an alternative interface to the same	 func‐ │
       tionality as Tcl_AddErrorInfo.  Tcl_AppendObjToErrorInfo is called when │
       the string value to be appended to the -errorinfo option	 is  available │
       as a Tcl_Obj instead of as a char array.

       Tcl_AddObjErrorInfo  is	nearly	identical  to Tcl_AddErrorInfo, except
       that it has an additional length argument.   This  allows  the  message
       string  to  contain  embedded  null bytes.  This is essentially never a
       good idea.  If the message needs to contain the null character  U+0000,
       Tcl's  usual  internal  encoding rules should be used to avoid the need
       for a null byte.	 If the Tcl_AddObjErrorInfo interface is used at  all,
       it should be with a negative length value.

       The  procedure Tcl_SetObjErrorCode is used to set the -errorcode return
       option  to  the	list  object  errorObjPtr  built  up  by  the  caller.
       Tcl_SetObjErrorCode  is	typically  invoked  just  before  returning an
       error. If an error is returned without calling  Tcl_SetObjErrorCode  or
       Tcl_SetErrorCode	 the Tcl interpreter automatically sets the -errorcode
       return option to NONE.

       The procedure Tcl_SetErrorCode is  also	used  to  set  the  -errorcode
       return  option. However, it takes one or more strings to record instead
       of an object. Otherwise, it is similar to Tcl_SetObjErrorCode in behav‐
       ior.

       Tcl_SetErrorCodeVA  is the same as Tcl_SetErrorCode except that instead
       of taking a variable number of arguments it takes an argument list.

       Tcl_PosixError sets the -errorcode variable after an error in  a	 POSIX
       kernel  call.   It  reads  the  value of the errno C variable and calls
       Tcl_SetErrorCode to set the -errorcode return option in the POSIX  for‐
       mat.  The caller must previously have called Tcl_SetErrno to set errno;
       this is necessary on some platforms (e.g. Windows) where Tcl is	linked
       into  an application as a shared library, or when the error occurs in a
       dynamically loaded extension. See the manual entry for Tcl_SetErrno for
       more information.

       Tcl_PosixError  returns	a  human-readable  diagnostic  message for the
       error (this is the same value that will appear as the third element  in
       the  -errorcode value).	It may be convenient to include this string as
       part of the error message returned to the  application  in  the	inter‐
       preter's result.

       Tcl_LogCommandInfo  is invoked after an error occurs in an interpreter.
       It adds information about the command that was being executed when  the
       error  occurred	to  the	 -errorinfo  value, and the line number stored
       internally in the interpreter is set.

       In older releases of Tcl, there was  no	Tcl_GetReturnOptions  routine.
       In its place, the global Tcl variables errorInfo and errorCode were the
       only place to retrieve the error information.  Much existing code writ‐
       ten  for	 older	Tcl  releases  still access this information via those
       global variables.

       It is important to realize that while reading from those	 global	 vari‐
       ables  remains a supported way to access these return option values, it
       is important not to assume that writing to those global variables  will
       properly set the corresponding return options.  It has long been empha‐
       sized in this manual page that it is important to call  the  procedures
       described here rather than setting errorInfo or errorCode directly with
       Tcl_ObjSetVar2.

       If the procedure Tcl_ResetResult is called, it clears all of the	 state
       of  the	interpreter  associated	 with script evaluation, including the
       entire return options dictionary.  In particular,  the  -errorinfo  and
       -errorcode  options are reset.  If an error had occurred, the Tcl_Rese‐
       tResult call will clear the error state to make	it  appear  as	if  no
       error  had  occurred  after  all.   The	global variables errorInfo and
       errorCode are not modified by Tcl_ResetResult so they continue to  hold
       a  record  of information about the most recent error seen in an inter‐
       preter.

SEE ALSO
       Tcl_DecrRefCount,   Tcl_IncrRefCount,   Tcl_Interp,    Tcl_ResetResult,
       Tcl_SetErrno

KEYWORDS
       error, object, object result, stack, trace, variable

Tcl				      8.5		   Tcl_AddErrorInfo(3)
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