The lower layer provides type-specific channel drivers for each type of device supported on each platform. This manual entry describes the C APIs used to communicate between the generic layer and the type-specific channel drivers. It also explains how new types of channels can be added by providing new channel drivers.
Channel drivers consist of a number of components: First, each channel driver provides a Tcl_ChannelType structure containing pointers to functions implementing the various operations used by the generic layer to communicate with the channel driver. The Tcl_ChannelType structure and the functions referenced by it are described in the section TCL_CHANNELTYPE, below.
Second, channel drivers usually provide a Tcl command to create instances of that type of channel. For example, the Tcl open command creates channels that use the file and command channel drivers, and the Tcl socket command creates channels that use TCP sockets for network communication.
Third, a channel driver optionally provides a C function to open channel instances of that type. For example, Tcl_OpenFileChannel opens a channel that uses the file channel driver, and Tcl_OpenTcpClient opens a channel that uses the TCP network protocol. These creation functions typically use Tcl_CreateChannel internally to open the channel.
To add a new type of channel you must implement a C API or a Tcl command that opens a channel by invoking Tcl_CreateChannel. When your driver calls Tcl_CreateChannel it passes in a Tcl_ChannelType structure describing the driver's I/O procedures. The generic layer will then invoke the functions referenced in that structure to perform operations on the channel.
Tcl_CreateChannel opens a new channel and associates the supplied typePtr and instanceData with it. The channel is opened in the mode indicated by mask. For a discussion of channel drivers, their operations and the Tcl_ChannelType structure, see the section TCL_CHANNELTYPE, below.
Tcl_CreateChannel interacts with the code managing the standard channels. Once a standard channel was initialized either through a call to Tcl_GetStdChannel or a call to Tcl_SetStdChannel closing this standard channel will cause the next call to Tcl_CreateChannel to make the new channel the new standard channel too. See Tcl_StandardChannels for a general treatise about standard channels and the behavior of the Tcl library with regard to them.
Tcl_GetChannelInstanceData returns the instance data associated with the channel in channel. This is the same as the instanceData argument in the call to Tcl_CreateChannel that created this channel.
Tcl_GetChannelType returns a pointer to the Tcl_ChannelType structure used by the channel in the channel argument. This is the same as the typePtr argument in the call to Tcl_CreateChannel that created this channel.
Tcl_GetChannelName returns a string containing the name associated with the channel, or NULL if the channelName argument to Tcl_CreateChannel was NULL.
Tcl_GetChannelHandle places the OS-specific device handle associated with channel for the given direction in the location specified by handlePtr and returns TCL_OK. If the channel does not have a device handle for the specified direction, then TCL_ERROR is returned instead. Different channel drivers will return different types of handle. Refer to the manual entries for each driver to determine what type of handle is returned.
Tcl_GetChannelThread returns the id of the thread currently managing the specified channel. This allows channel drivers to send their file events to the correct event queue even for a multi-threaded core.
Tcl_GetChannelMode returns an OR-ed combination of TCL_READABLE and TCL_WRITABLE, indicating whether the channel is open for input and output.
Tcl_RemoveChannelMode removes an access privilege from the channel, either TCL_READABLE or TCL_WRITABLE, and returns a regular Tcl result code, TCL_OK, or TCL_ERROR. The function throws an error if either an invalid mode is specified or the result of the removal would be an inaccessible channel. In that case an error message is left in the interp argument, if not NULL.
Tcl_GetChannelBufferSize returns the size, in bytes, of buffers allocated to store input or output in channel. If the value was not set by a previous call to Tcl_SetChannelBufferSize, described below, then the default value of 4096 is returned.
Tcl_SetChannelBufferSize sets the size, in bytes, of buffers that will be allocated in subsequent operations on the channel to store input or output. The size argument should be between one and one million, allowing buffers of one byte to one million bytes. If size is outside this range, Tcl_SetChannelBufferSize sets the buffer size to 4096.
Tcl_NotifyChannel is called by a channel driver to indicate to the generic layer that the events specified by mask have occurred on the channel. Channel drivers are responsible for invoking this function whenever the channel handlers need to be called for the channel (or other pending tasks like a write flush should be performed). See WATCHPROC below for more details.
Tcl_BadChannelOption is called from driver specific setOptionProc or getOptionProc to generate a complete error message.
Tcl_ChannelBuffered returns the number of bytes of input currently buffered in the internal buffer (push back area) of the channel itself. It does not report about the data in the overall buffers for the stack of channels the supplied channel is part of.
Tcl_IsChannelShared checks the refcount of the specified channel and returns whether the channel was shared among multiple interpreters (result == 1) or not (result == 0).
Tcl_IsChannelRegistered checks whether the specified channel is registered in the given interpreter (result == 1) or not (result == 0).
Tcl_IsChannelExisting checks whether a channel with the specified name is registered in the (thread)-global list of all channels (result == 1) or not (result == 0).
Tcl_CutChannel removes the specified channel from the (thread)global list of all channels (of the current thread). Application to a channel still registered in some interpreter is not allowed. Also notifies the driver if Tcl_DriverThreadActionProc is defined for it.
Tcl_SpliceChannel adds the specified channel to the (thread)global list of all channels (of the current thread). Application to a channel registered in some interpreter is not allowed. Also notifies the driver if Tcl_DriverThreadActionProc is defined for it.
Tcl_ClearChannelHandlers removes all channel handlers and event scripts associated with the specified channel, thus shutting down all event processing for this channel.
The Tcl_ChannelType structure contains the following fields:
typedef struct {
const char *typeName;
Tcl_ChannelTypeVersion version;
void *closeProc; /* Not used any more*/
Tcl_DriverInputProc *inputProc;
Tcl_DriverOutputProc *outputProc;
void *seekProc; /* Not used any more */
Tcl_DriverSetOptionProc *setOptionProc;
Tcl_DriverGetOptionProc *getOptionProc;
Tcl_DriverWatchProc *watchProc;
Tcl_DriverGetHandleProc *getHandleProc;
Tcl_DriverClose2Proc *close2Proc;
Tcl_DriverBlockModeProc *blockModeProc;
Tcl_DriverFlushProc *flushProc;
Tcl_DriverHandlerProc *handlerProc;
Tcl_DriverWideSeekProc *wideSeekProc;
Tcl_DriverThreadActionProc *threadActionProc;
Tcl_DriverTruncateProc *truncateProc;
} Tcl_ChannelType;
It is not necessary to provide implementations for all channel operations. Those which are not necessary may be set to NULL in the struct: blockModeProc, seekProc, setOptionProc, getOptionProc, getHandleProc, and close2Proc, in addition to flushProc, handlerProc, threadActionProc, and truncateProc. Other functions that cannot be implemented in a meaningful way should return EINVAL when called, to indicate that the operations they represent are not available. Also note that wideSeekProc can be NULL if seekProc is.
The user should only use the above structure for Tcl_ChannelType instantiation. When referencing fields in a Tcl_ChannelType structure, the following functions should be used to obtain the values: Tcl_ChannelName, Tcl_ChannelVersion, Tcl_ChannelBlockModeProc, Tcl_ChannelClose2Proc, Tcl_ChannelInputProc, Tcl_ChannelOutputProc, Tcl_ChannelWideSeekProc, Tcl_ChannelThreadActionProc, Tcl_ChannelTruncateProc, Tcl_ChannelSetOptionProc, Tcl_ChannelGetOptionProc, Tcl_ChannelWatchProc, Tcl_ChannelGetHandleProc, Tcl_ChannelFlushProc, or Tcl_ChannelHandlerProc.
The change to the structures was made in such a way that standard channel types are binary compatible. However, channel types that use stacked channels (i.e. TLS, Trf) have new versions to correspond to the above change since the previous code for stacked channels had problems.
This value can be retrieved with Tcl_ChannelName, which returns a pointer to the string.
This value can be retrieved with Tcl_ChannelVersion.
typedef int Tcl_DriverBlockModeProc(
void *instanceData,
int mode);
The instanceData is the same as the value passed to Tcl_CreateChannel when this channel was created. The mode argument is either TCL_MODE_BLOCKING or TCL_MODE_NONBLOCKING to set the device into blocking or nonblocking mode. The function should return zero if the operation was successful, or a nonzero POSIX error code if the operation failed.
If the operation is successful, the function can modify the supplied instanceData to record that the channel entered blocking or nonblocking mode and to implement the blocking or nonblocking behavior. For some device types, the blocking and nonblocking behavior can be implemented by the underlying operating system; for other device types, the behavior must be emulated in the channel driver.
This value can be retrieved with Tcl_ChannelBlockModeProc, which returns a pointer to the function.
A channel driver not supplying a blockModeProc has to be very, very careful. It has to tell the generic layer exactly which blocking mode is acceptable to it, and should this also document for the user so that the blocking mode of the channel is not changed to an unacceptable value. Any confusion here may lead the interpreter into a (spurious and difficult to find) deadlock.
typedef int Tcl_DriverClose2Proc(
void *instanceData,
Tcl_Interp *interp,
int flags);
If flags is 0, the instanceData argument is the same as the value provided to Tcl_CreateChannel when the channel was created. The function should release any storage maintained by the channel driver for this channel, and close the input and output devices encapsulated by this channel. All queued output will have been flushed to the device before this function is called, and no further driver operations will be invoked on this instance after calling the closeProc. If the close operation is successful, the procedure should return zero; otherwise it should return a nonzero POSIX error code. In addition, if an error occurs and interp is not NULL, the procedure should store an error message in the interpreter's result.
Alternatively, channels that support closing the read and write sides independently may accept other flag values than 0. Then close2Proc will be called with flags set to an OR'ed combination of TCL_CLOSE_READ or TCL_CLOSE_WRITE to indicate that the driver should close the read and/or write side of the channel. The channel driver may be invoked to perform additional operations on the channel after close2Proc is called to close one or both sides of the channel. In all cases, the close2Proc function should return zero if the close operation was successful; otherwise it should return a nonzero POSIX error code. In addition, if an error occurs and interp is not NULL, the procedure should store an error message in the interpreter's result.
The close2Proc value can be retrieved with Tcl_ChannelClose2Proc, which returns a pointer to the function.
typedef int Tcl_DriverInputProc(
void *instanceData,
char *buf,
int bufSize,
int *errorCodePtr);
InstanceData is the same as the value passed to Tcl_CreateChannel when the channel was created. The buf argument points to an array of bytes in which to store input from the device, and the bufSize argument indicates how many bytes are available at buf.
The errorCodePtr argument points to an integer variable provided by the generic layer. If an error occurs, the function should set the variable to a POSIX error code that identifies the error that occurred.
The function should read data from the input device encapsulated by the channel and store it at buf. On success, the function should return a nonnegative integer indicating how many bytes were read from the input device and stored at buf. On error, the function should return -1. If an error occurs after some data has been read from the device, that data is lost.
If inputProc can determine that the input device has some data available but less than requested by the bufSize argument, the function should only attempt to read as much data as is available and return without blocking. If the input device has no data available whatsoever and the channel is in nonblocking mode, the function should return an EAGAIN error. If the input device has no data available whatsoever and the channel is in blocking mode, the function should block for the shortest possible time until at least one byte of data can be read from the device; then, it should return as much data as it can read without blocking.
This value can be retrieved with Tcl_ChannelInputProc, which returns a pointer to the function.
typedef int Tcl_DriverOutputProc(
void *instanceData,
const char *buf,
int toWrite,
int *errorCodePtr);
InstanceData is the same as the value passed to Tcl_CreateChannel when the channel was created. The buf argument contains an array of bytes to be written to the device, and the toWrite argument indicates how many bytes are to be written from the buf argument.
The errorCodePtr argument points to an integer variable provided by the generic layer. If an error occurs, the function should set this variable to a POSIX error code that identifies the error.
The function should write the data at buf to the output device encapsulated by the channel. On success, the function should return a nonnegative integer indicating how many bytes were written to the output device. The return value is normally the same as toWrite, but may be less in some cases such as if the output operation is interrupted by a signal. If an error occurs the function should return -1. In case of error, some data may have been written to the device.
If the channel is nonblocking and the output device is unable to absorb any data whatsoever, the function should return -1 with an EAGAIN error without writing any data.
This value can be retrieved with Tcl_ChannelOutputProc, which returns a pointer to the function.
typedef long long Tcl_DriverWideSeekProc(
void *instanceData,
long long offset,
int seekMode,
int *errorCodePtr);
The instanceData argument is the same as the value given to Tcl_CreateChannel when this channel was created. Offset and seekMode have the same meaning as for the Tcl_Seek procedure (described in the manual entry for Tcl_OpenFileChannel).
The errorCodePtr argument points to an integer variable provided by the generic layer for returning errno values from the function. The function should set this variable to a POSIX error code if an error occurs. The function should store an EINVAL error code if the channel type does not implement seeking.
The return value is the new access point or -1 in case of error. If an error occurred, the function should not move the access point.
The wideSseekProc value can be retrieved with Tcl_ChannelWideSeekProc, which returns a pointer to the function.
typedef int Tcl_DriverSetOptionProc(
void *instanceData,
Tcl_Interp *interp,
const char *optionName,
const char *newValue);
optionName is the name of an option to set, and newValue is the new value for that option, as a string. The instanceData is the same as the value given to Tcl_CreateChannel when this channel was created. The function should do whatever channel type specific action is required to implement the new value of the option.
Some options are handled by the generic code and this function is never called to set them, e.g. -blockmode. Other options are specific to each channel type and the setOptionProc procedure of the channel driver will get called to implement them. The setOptionProc field can be NULL, which indicates that this channel type supports no type specific options.
If the option value is successfully modified to the new value, the function returns TCL_OK. It should call Tcl_BadChannelOption which itself returns TCL_ERROR if the optionName is unrecognized. If newValue specifies a value for the option that is not supported or if a system call error occurs, the function should leave an error message in the result of interp if interp is not NULL. The function should also call Tcl_SetErrno to store an appropriate POSIX error code.
This value can be retrieved with Tcl_ChannelSetOptionProc, which returns a pointer to the function.
typedef int Tcl_DriverGetOptionProc(
void *instanceData,
Tcl_Interp *interp,
const char *optionName,
Tcl_DString *optionValue);
OptionName is the name of an option supported by this type of channel. If the option name is not NULL, the function stores its current value, as a string, in the Tcl dynamic string optionValue. If optionName is NULL, the function stores in optionValue an alternating list of all supported options and their current values. On success, the function returns TCL_OK. It should call Tcl_BadChannelOption which itself returns TCL_ERROR if the optionName is unrecognized. If a system call error occurs, the function should leave an error message in the result of interp if interp is not NULL. The function should also call Tcl_SetErrno to store an appropriate POSIX error code.
Some options are handled by the generic code and this function is never called to retrieve their value, e.g. -blockmode. Other options are specific to each channel type and the getOptionProc procedure of the channel driver will get called to implement them. The getOptionProc field can be NULL, which indicates that this channel type supports no type specific options.
This value can be retrieved with Tcl_ChannelGetOptionProc, which returns a pointer to the function.
typedef void Tcl_DriverWatchProc(
void *instanceData,
int mask);
The instanceData is the same as the value passed to Tcl_CreateChannel when this channel was created. The mask argument is an OR-ed combination of TCL_READABLE, TCL_WRITABLE and TCL_EXCEPTION; it indicates events the caller is interested in noticing on this channel.
The function should initialize device type specific mechanisms to notice when an event of interest is present on the channel. When one or more of the designated events occurs on the channel, the channel driver is responsible for calling Tcl_NotifyChannel to inform the generic channel module. The driver should take care not to starve other channel drivers or sources of callbacks by invoking Tcl_NotifyChannel too frequently. Fairness can be insured by using the Tcl event queue to allow the channel event to be scheduled in sequence with other events. See the description of Tcl_QueueEvent for details on how to queue an event.
This value can be retrieved with Tcl_ChannelWatchProc, which returns a pointer to the function.
typedef int Tcl_DriverGetHandleProc(
void *instanceData,
int direction,
void **handlePtr);
InstanceData is the same as the value passed to Tcl_CreateChannel when this channel was created. The direction argument is either TCL_READABLE to retrieve the handle used for input, or TCL_WRITABLE to retrieve the handle used for output.
If the channel implementation has device-specific handles, the function should retrieve the appropriate handle associated with the channel, according the direction argument. The handle should be stored in the location referred to by handlePtr, and TCL_OK should be returned. If the channel is not open for the specified direction, or if the channel implementation does not use device handles, the function should return TCL_ERROR.
This value can be retrieved with Tcl_ChannelGetHandleProc, which returns a pointer to the function.
typedef int Tcl_DriverFlushProc(
void *instanceData);
This value can be retrieved with Tcl_ChannelFlushProc, which returns a pointer to the function.
typedef int Tcl_DriverHandlerProc(
void *instanceData,
int interestMask);
InstanceData is the same as the value passed to Tcl_CreateChannel when this channel was created. The interestMask is an OR-ed combination of TCL_READABLE or TCL_WRITABLE; it indicates what type of event occurred on this channel.
This value can be retrieved with Tcl_ChannelHandlerProc, which returns a pointer to the function.
The action TCL_CHANNEL_THREAD_INSERT is used to notify the driver that it should update or initialize any thread-specific data it might be maintaining using the calling thread as the associate. See Tcl_CutChannel and Tcl_SpliceChannel for more detail.
typedef void Tcl_DriverThreadActionProc(
void *instanceData,
int action);
InstanceData is the same as the value passed to Tcl_CreateChannel when this channel was created.
These values can be retrieved with Tcl_ChannelThreadActionProc, which returns a pointer to the function.
typedef int Tcl_DriverTruncateProc(
void *instanceData,
long long length);
InstanceData is the same as the value passed to Tcl_CreateChannel when this channel was created, and length is the new length of the underlying file, which should not be negative. The result should be 0 on success or an errno code (suitable for use with Tcl_SetErrno) on failure.
These values can be retrieved with Tcl_ChannelTruncateProc, which returns a pointer to the function.
It always returns TCL_ERROR
An error message is generated in interp's result value to indicate that a command was invoked with a bad option. The message has the form
bad option "blah": should be one of
<...generic options...>+<...specific options...>
so you get for instance:
bad option "-blah": should be one of -blocking,
-buffering, -buffersize, -eofchar, -translation,
-peername, or -sockname
when called with optionList equal to
“peername sockname”
“blah” is the optionName argument and “<specific options>” is a space separated list of specific option words. The function takes good care of inserting minus signs before each option, commas after, and an “or” before the last option.