Tcl_CreateChannel(3)

NAME
Tcl_CreateChannel, Tcl_GetChannelInstanceData, Tcl_GetChannelType, Tcl_GetChannelName, Tcl_GetChannelHandle, Tcl_GetChannelMode, Tcl_GetChannelBufferSize, Tcl_SetChannelBufferSize, Tcl_NotifyChannel, Tcl_BadChannelOption - procedures for creating and manipulating channels

SYNOPSIS
#include <tcl.h>

Tcl_Channel
Tcl_CreateChannel(typePtr, channelName, instanceData, mask)

ClientData
Tcl_GetChannelInstanceData(channel)

Tcl_ChannelType *
Tcl_GetChannelType(channel)

char *
Tcl_GetChannelName(channel)

int
Tcl_GetChannelHandle(channel, direction, handlePtr)

int
Tcl_GetChannelBufferSize(channel)

Tcl_SetChannelBufferSize(channel, size)

Tcl_NotifyChannel(channel, mask)

int
Tcl_BadChannelOption(interp, optionName, optionList)

ARGUMENTS
Points to a structure containing the addresses of procedures that can be called to perform I/O and other functions on the channel. The name of this channel, such as file3; must not be in use by any other channel. Can be NULL, in which case the channel is created without a name. Arbitrary one-word value to be associated with this channel. This value is passed to procedures in typePtr when they are invoked. OR-ed combination of TCL_READABLE and TCL_WRITABLE to indicate whether a channel is readable and writable. The channel to operate on. TCL_READABLE means the input handle is wanted; TCL_WRITABLE means the output handle is wanted. Points to the location where the desired OS-specific handle should be stored. The translation mode; one of the constants TCL_TRANSLATE_AUTO, TCL_TRANSLATE_CR, TCL_TRANSLATE_LF and TCL_TRANSLATE_CRLF. The size, in bytes, of buffers to allocate in this channel. An OR-ed combination of TCL_READABLE, TCL_WRITABLE and TCL_EXCEPTION that indicates events that have occurred on this channel. Current interpreter. (can be NULL) Name of the invalid option. Specific options list (space separated words, without "-") to append to the standard generic options list. Can be NULL for generic options error message only.

DESCRIPTION

Tcl uses a two-layered channel architecture. It provides a generic upper layer to enable C and Tcl programs to perform input and output using the same APIs for a variety of files, devices, sockets etc. The generic C APIs are described in the manual entry for Tcl_OpenFileChannel.

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_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_GetChannelMode returns an OR-ed combination of TCL_READABLE and TCL_WRITABLE, indicating whether the channel is open for input and output.

 Tcl_GetChannelBufferSize returns the size, in bytes, of buffers allocated to store input or output in chan. 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 ten and one million, allowing buffers of ten bytes 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. See WATCHPROC below for more details.

Tcl_BadChannelOption is called from driver specific set or get option procs to generate a complete error message.

TCL_CHANNELTYPE

A channel driver provides a Tcl_ChannelType structure that contains pointers to functions that implement the various operations on a channel; these operations are invoked as needed by the generic layer. The Tcl_ChannelType structure contains the following fields:

typedef struct Tcl_ChannelType {
    char *typeName;
    Tcl_DriverBlockModeProc *blockModeProc;
    Tcl_DriverCloseProc *closeProc;
    Tcl_DriverInputProc *inputProc;
    Tcl_DriverOutputProc *outputProc;
    Tcl_DriverSeekProc *seekProc;
    Tcl_DriverSetOptionProc *setOptionProc;
    Tcl_DriverGetOptionProc *getOptionProc;
    Tcl_DriverWatchProc *watchProc;
    Tcl_DriverGetHandleProc *getHandleProc;
    Tcl_DriverClose2Proc *close2Proc; } Tcl_ChannelType;

The driver must provide implementations for all functions except blockModeProc, seekProc, setOptionProc, getOptionProc, and close2Proc, which may be specified as NULL. Other functions that can not be implemented for this type of device should return EINVAL when invoked to indicate that they are not implemented.

TYPENAME

The typeName field contains a null-terminated string that identifies the type of the device implemented by this driver, e.g. file or socket.

BLOCKMODEPROC

The blockModeProc field contains the address of a function called by the generic layer to set blocking and nonblocking mode on the device. BlockModeProc should match the following prototype:

typedef int Tcl_DriverBlockModeProc(
    ClientData 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.

CLOSEPROC AND CLOSE2PROC

The closeProc field contains the address of a function called by the generic layer to clean up driver-related information when the channel is closed. CloseProc must match the following prototype:

typedef int Tcl_DriverCloseProc(
    ClientData instanceData,
    Tcl_Interp *interp);

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 interp->result.

Alternatively, channels that support closing the read and write sides independently may set closeProc to TCL_CLOSE2PROC and set close2Proc to the address of a function that matches the following prototype:

typedef int Tcl_DriverClose2Proc(
    ClientData instanceData,
    Tcl_Interp *interp,
    int flags);

The 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. If flags is 0 (zero), the driver should close the channel in the manner described above for closeProc. No further operations will be invoked on this instance after close2Proc is called with all flags cleared. 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 interp->result.

INPUTPROC

The inputProc field contains the address of a function called by the generic layer to read data from the file or device and store it in an internal buffer. InputProc must match the following prototype:

typedef int Tcl_DriverInputProc(
    ClientData 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.

OUTPUTPROC

The outputProc field contains the address of a function called by the generic layer to transfer data from an internal buffer to the output device. OutputProc must match the following prototype:

typedef int Tcl_DriverOutputProc(
    ClientData instanceData,
    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.

SEEKPROC

The seekProc field contains the address of a function called by the generic layer to move the access point at which subsequent input or output operations will be applied. SeekProc must match the following prototype:

typedef int Tcl_DriverSeekProc(
    ClientData instanceData,
    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.

SETOPTIONPROC

The setOptionProc field contains the address of a function called by the generic layer to set a channel type specific option on a channel. setOptionProc must match the following prototype:

typedef int Tcl_DriverSetOptionProc(
    ClientData instanceData,
    Tcl_Interp *interp,
    char *optionName,
    char *optionValue);

optionName is the name of an option to set, and optionValue 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 optionValue 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 field of interp if interp is not NULL. The function should also call Tcl_SetErrno to store an appropriate POSIX error code.

GETOPTIONPROC

The getOptionProc field contains the address of a function called by the generic layer to get the value of a channel type specific option on a channel. getOptionProc must match the following prototype:

typedef int Tcl_DriverGetOptionProc(
    ClientData instanceData,
    Tcl_Interp *interp,
    char *optionName,
    Tcl_DString *dsPtr);

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 dsPtr. If optionName is NULL, the function stores in dsPtr 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 field 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.

WATCHPROC

The watchProc field contains the address of a function called by the generic layer to initialize the event notification mechanism to notice events of interest on this channel. WatchProc should match the following prototype:

typedef void Tcl_DriverWatchProc(
    ClientData 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.

GETHANDLEPROC

The getHandleProc field contains the address of a function called by the generic layer to retrieve a device-specific handle from the channel. GetHandleProc should match the following prototype:

typedef int Tcl_DriverGetHandleProc(
    ClientData instanceData,
    int direction,
    ClientData *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.

TCL_BADCHANNELOPTION

This procedure generates a "bad option" error message in an (optional) interpreter. It is used by channel drivers when a invalid Set/Get option is requested. Its purpose is to concatenate the generic options list to the specific ones and factorize the generic options error message string.

It always return TCL_ERROR

An error message is generated in interp's result object to indicate that a command was invoked with the 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="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.

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

KEYWORDS
blocking, channel driver, channel registration, channel type, nonblocking