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>

Tcl_SetChannelError(chan, msg)

Tcl_SetChannelErrorInterp(interp, msg)

Tcl_GetChannelError(chan, msgPtr)

Tcl_GetChannelErrorInterp(interp, msgPtr)

ARGUMENTS
Refers to the Tcl channel whose bypass area is accessed. Refers to the Tcl interpreter whose bypass area is accessed. 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. This must be a well-formed list. Reference to a place where the message stored in the accessed bypass area can be stored in.

DESCRIPTION

The standard 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 value 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 value 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, that is, 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 value.

Tcl_GetChannelErrorInterp places either the error message held in the bypass area of the specified interpreter into msgPtr, or NULL; and resets the bypass, that is, after an invocation all following invocations will return NULL, until an intervening invocation of Tcl_SetChannelErrorInterp 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 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 value.

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_DriverInputProc
    May use Tcl_SetChannelError, and only this function.

Tcl_DriverOutputProc
    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; when these functions are called, the channel may now contain a stored arbitrary error message requiring processing by the caller.
Tcl_Flush    Tcl_GetsObj    Tcl_Gets
Tcl_ReadChars    Tcl_ReadRaw    Tcl_Read
Tcl_Seek    Tcl_StackChannel    Tcl_Tell
Tcl_WriteChars    Tcl_WriteObj    Tcl_WriteRaw
Tcl_Write

All other API functions are unchanged. In particular, the functions below leave all their error information in the interpreter result.
Tcl_Close    Tcl_UnstackChannel    Tcl_UnregisterChannel

REFERENCE COUNT MANAGEMENT

The msg argument to Tcl_SetChannelError and Tcl_SetChannelErrorInterp, if not NULL, may have any reference count; these functions will copy.

Tcl_GetChannelError and Tcl_GetChannelErrorInterp write a value reference into their msgPtr, but do not manipulate its reference count. The reference count will be at least 1 (unless the reference is NULL).

SEE ALSO
Tcl_Close(3), Tcl_OpenFileChannel(3), Tcl_SetErrno(3)

KEYWORDS
channel driver, error messages, channel type