Tcl_AddErrorInfo(3) Tcl Library Procedures Tcl_AddErrorInfo(3)______________________________________________________________________________NAME
Tcl_AddObjErrorInfo, Tcl_AddErrorInfo, Tcl_SetErrorCode, Tcl_PosixError
- record information about errors
Tcl_AddObjErrorInfo(interp, message, length)
Tcl_SetErrorCode(interp, element, element, ... (char *) NULL)
Tcl_Interp *interp (in) Interpreter in which to record infor‐
char *message (in) For Tcl_AddObjErrorInfo, this points
to the first byte of an array of
bytes containing a string to record
in the errorInfo variable. This byte
array may contain embedded null bytes
unless length is negative. For
Tcl_AddErrorInfo, this is a conven‐
tional C string to record in the
int length (in) The number of bytes to copy from mes‐
sage when setting the errorInfo vari‐
able. If negative, all bytes up to
the first null byte are used.
Tcl_Obj *errorObjPtr(in) This variable errorCode will be set
to this value.
char *element (in) String to record as one element of
errorCode variable. Last element
argument must be NULL.
These procedures are used to manipulate two Tcl global variables that
hold information about errors. The variable errorInfo holds a stack
trace of the operations that were in progress when an error occurred,
and is intended to be human-readable. The variable errorCode holds a
list of items that are intended to be machine-readable. The first item
in errorCode identifies the class of error that occurred (e.g. POSIX
means an error occurred in a POSIX system call) and additional elements
in errorCode hold additional pieces of information that depend on the
class. See the Tcl overview manual entry for details on the various
formats for errorCode.
The errorInfo variable is gradually built up as an error unwinds
through the nested operations. Each time an error code is returned to
Tcl_EvalObj (or Tcl_Eval, which calls Tcl_EvalObj) it calls the proce‐
dure Tcl_AddObjErrorInfo to add additional text to errorInfo 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 activity in progress when the
It is sometimes useful to add additional information to errorInfo
beyond what can be supplied automatically by Tcl_EvalObj. Tcl_AddOb‐
jErrorInfo may be used for this purpose: its message and length argu‐
ments describe an additional string to be appended to errorInfo. For
example, the source command calls Tcl_AddObjErrorInfo to record the
name of the file being processed and the line number on which the error
occurred; for Tcl procedures, the procedure name and line number within
the procedure are recorded, and so on. The best time to call
Tcl_AddObjErrorInfo is just after Tcl_EvalObj has returned TCL_ERROR.
In calling Tcl_AddObjErrorInfo, you may find it useful to use the
errorLine field of the interpreter (see the Tcl_Interp manual entry for
Tcl_AddErrorInfo resembles Tcl_AddObjErrorInfo but differs in initial‐
izing errorInfo from the string value of the interpreter's result if
the error is just starting to be logged. It does not use the result as
a Tcl object so any embedded null characters in the result will cause
information to be lost. It also takes a conventional C string in mes‐
sage instead of Tcl_AddObjErrorInfo's counted string.
The procedure Tcl_SetObjErrorCode is used to set the errorCode vari‐
able. errorObjPtr contains a list object built up by the caller. error‐
Code is set to this value. Tcl_SetObjErrorCode is typically invoked
just before returning an error in an object command. If an error is
returned without calling Tcl_SetObjErrorCode or Tcl_SetErrorCode the
Tcl interpreter automatically sets errorCode to NONE.
The procedure Tcl_SetErrorCode is also used to set the errorCode vari‐
able. However, it takes one or more strings to record instead of an
object. Otherwise, it is similar to Tcl_SetObjErrorCode in behavior.
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 errorCode in the POSIX format. 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
errorCode). It may be convenient to include this string as part of the
error message returned to the application in the interpreter's result.
It is important to call the procedures described here rather than set‐
ting errorInfo or errorCode directly with Tcl_ObjSetVar2. The reason
for this is that the Tcl interpreter keeps information about whether
these procedures have been called. For example, the first time
Tcl_AddObjErrorInfo is called for an error, it clears the existing
value of errorInfo and adds the error message in the interpreter's
result to the variable before appending message; in subsequent calls,
it just appends the new message. When Tcl_SetErrorCode is called, it
sets a flag indicating that errorCode has been set; this allows the Tcl
interpreter to set errorCode to NONE if it receives an error return
when Tcl_SetErrorCode hasn't been called.
If the procedure Tcl_ResetResult is called, it clears all of the state
associated with errorInfo and errorCode (but it doesn't actually modify
the variables). If an error had occurred, this will clear the error
state to make it appear as if no error had occurred after all.
Tcl_DecrRefCount, Tcl_IncrRefCount, Tcl_Interp, Tcl_ResetResult,
error, object, object result, stack, trace, variable
Tcl 7.5 Tcl_AddErrorInfo(3)