Tcl_SetChannelError(3) Tcl Library Procedures Tcl_SetChannelError(3)
__________________________________________________________________________________________________________________________________________________
NAME
Tcl_SetChannelError, Tcl_SetChannelErrorInterp, Tcl_GetChannelError, Tcl_GetChannelErrorInterp - functions to create/intercept Tcl errors
by channel drivers.
SYNOPSIS
#include <tcl.h>
void
Tcl_SetChannelError(chan, msg)
void
Tcl_SetChannelErrorInterp(interp, msg)
void
Tcl_GetChannelError(chan, msgPtr)
void
Tcl_GetChannelErrorInterp(interp, msgPtr)
ARGUMENTS
Tcl_Channel chan (in) Refers to the Tcl channel whose bypass area is accessed.
Tcl_Interp* interp (in) Refers to the Tcl interpreter whose bypass area is accessed.
Tcl_Obj* msg (in) Error message put into a bypass area. A list of return options and values, followed by a string message.
Both message and the option/value information are optional.
Tcl_Obj** msgPtr (out) Reference to a place where the message stored in the accessed bypass area can be stored in.
_________________________________________________________________
DESCRIPTION
The current definition of a Tcl channel driver does not permit the direct return of arbitrary error messages, except for the setting and
retrieval of channel options. All other functions are restricted to POSIX error codes.
The functions described here overcome this limitation. Channel drivers are allowed to use Tcl_SetChannelError and Tcl_SetChannelErrorInterp
to place arbitrary error messages in bypass areas defined for channels and interpreters. And the generic I/O layer uses Tcl_GetChannelError
and Tcl_GetChannelErrorInterp to look for messages in the bypass areas and arrange for their return as errors. The posix error codes set by
a driver are used now if and only if no messages are present.
Tcl_SetChannelError stores error information in the bypass area of the specified channel. The number of references to the msg object goes
up by one. Previously stored information will be discarded, by releasing the reference held by the channel. The channel reference must not
be NULL.
Tcl_SetChannelErrorInterp stores error information in the bypass area of the specified interpreter. The number of references to the msg
object goes up by one. Previously stored information will be discarded, by releasing the reference held by the interpreter. The interpreter
reference must not be NULL.
Tcl_GetChannelError places either the error message held in the bypass area of the specified channel into msgPtr, or NULL; and resets the
bypass. I.e. after an invokation all following invokations will return NULL, until an intervening invokation of Tcl_SetChannelError with a
non-NULL message. The msgPtr must not be NULL. The reference count of the message is not touched. The reference previously held by the
channel is now held by the caller of the function and it is its responsibility to release that reference when it is done with the object.
Tcl_GetChannelErrorInterp places either the error message held in the bypass area of the specified interpreter into msgPtr, or NULL; and
resets the bypass. I.e. after an invokation all following invokations will return NULL, until an intervening invokation of Tcl_SetChannel-
ErrorInterp with a non-NULL message. The msgPtr must not be NULL. The reference count of the message is not touched. The reference previ-
ously held by the interpreter is now held by the caller of the function and it is its responsibility to release that reference when it is
done with the object.
Which functions of a channel driver are allowed to use which bypass function is listed below, as is which functions of the public channel
API may leave a messages in the bypass areas.
Tcl_DriverCloseProc
May use Tcl_SetChannelErrorInterp, and only this function.
Tcl_DriverInputProc
May use Tcl_SetChannelError, and only this function.
Tcl_DriverOutputProc
May use Tcl_SetChannelError, and only this function.
Tcl_DriverSeekProc
May use Tcl_SetChannelError, and only this function.
Tcl_DriverWideSeekProc
May use Tcl_SetChannelError, and only this function.
Tcl_DriverSetOptionProc
Has already the ability to pass arbitrary error messages. Must not use any of the new functions.
Tcl_DriverGetOptionProc
Has already the ability to pass arbitrary error messages. Must not use any of the new functions.
Tcl_DriverWatchProc
Must not use any of the new functions. Is internally called and has no ability to return any type of error whatsoever.
Tcl_DriverBlockModeProc
May use Tcl_SetChannelError, and only this function.
Tcl_DriverGetHandleProc
Must not use any of the new functions. It is only a low-level function, and not used by Tcl commands.
Tcl_DriverHandlerProc
Must not use any of the new functions. Is internally called and has no ability to return any type of error whatsoever.
Given the information above the following public functions of the Tcl C API are affected by these changes. I.e. when these functions are
called the channel may now contain a stored arbitrary error message requiring processing by the caller.
Tcl_StackChannel
Tcl_Seek
Tcl_Tell
Tcl_ReadRaw
Tcl_Read
Tcl_ReadChars
Tcl_Gets
Tcl_GetsObj
Tcl_Flush
Tcl_WriteRaw
Tcl_WriteObj
Tcl_Write
Tcl_WriteChars
All other API functions are unchanged. Especially the functions below leave all their error information in the interpreter result.
Tcl_Close
Tcl_UnregisterChannel
Tcl_UnstackChannel
SEE ALSO
Tcl_Close(3), Tcl_OpenFileChannel(3), Tcl_SetErrno(3)
KEYWORDS
channel driver, error messages, channel type
Tcl 8.5 Tcl_SetChannelError(3)