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 invocation all following invocations will return NULL, until an intervening invocation 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 invocation all following invocations will return NULL, until an intervening invocation 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)