Tcl_CreateChannel(3)

NAME
Tcl_CreateChannel, Tcl_GetChannelInstanceData, Tcl_GetChannelType, Tcl_GetChannelName, Tcl_GetChannelFile, Tcl_GetChannelBufferSize, Tcl_SetDefaultTranslation, Tcl_SetChannelBufferSize - procedures for creating and manipulating channels

SYNOPSIS
#include <tcl.h>

Tcl_Channel
Tcl_CreateChannel(typePtr, channelName, inFile, outFile, instanceData)

ClientData
Tcl_GetChannelInstanceData(channel)

Tcl_ChannelType *
Tcl_GetChannelType(channel)

char *
Tcl_GetChannelName(channel)

Tcl_File
Tcl_GetChannelFile(channel, direction)

void
Tcl_SetDefaultTranslation(channel, transMode)

int
Tcl_GetChannelBufferSize(channel)

void
Tcl_SetChannelBufferSize(channel, size)

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. Tcl file for the input device to associate with this channel. If NULL, input will not be allowed on the channel. Tcl file for the output device to associate with this channel. If NULL, output will not be allowed on the channel. Arbitrary one-word value to be associated with this channel. This value is passed to procedures in typePtr when they are invoked. The channel to operate on. TCL_READABLE means the input file is wanted; TCL_WRITABLE means the output file is wanted. 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.

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 file, socket and device supported on each platform. This manual entry describes the C APIs used by the generic layer to communicate with type-specific channel drivers to perform the input and output operations. 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, inFile, outFile and instanceData with it. 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_GetChannelFile returns the inFile associated with channel if direction is TCL_READABLE, or the outFile if direction is TCL_WRITABLE. The operation returns NULL if the respective value was specified as NULL in the call to Tcl_CreateChannel that created channel.

Tcl_SetDefaultTranslation sets the default end of line translation mode. This mode will be installed as the translation mode for the channel if an attempt is made to output on the channel while it is still in TCL_TRANSLATE_AUTO mode. For a description of end of line translation modes, see the manual entry for fconfigure.

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_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_ChannelType;

The driver must provide implementations for all functions except blockModeProc, seekProc, setOptionProc, and getOptionProc, which may be specified as NULL to indicate that the channel does not support seeking. 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,
    Tcl_File inFile,
    Tcl_File outFile,
    int mode);

The instanceData, inFile and outFile arguments are the same as the values 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 modify inFile and outFile 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

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,
    Tcl_File inFile,
    Tcl_File outFile);

The instanceData, inFile, and outFile arguments are the same as the respective values 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 identified by inFile and outFile. 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.

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,
    Tcl_File inFile,
    char *buf,
    int bufSize,
    int *errorCodePtr);

InstanceData and InFile are the same as the values 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 identified by inFile and store it at buf. On success, the function should return a positive 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,
    Tcl_File outFile,
    char *buf,
    int toWrite,
    int *errorCodePtr);

InstanceData and OutFile are the same as the values 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 identified by outFile. On success, the function should return a positive 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,
    Tcl_File inFile,
    Tcl_File outFile,
    long offset,
    int seekMode,
    int *errorCodePtr);

The instanceData, inFile and outFile arguments are the same as the values given to Tcl_CreateChannel when this channel was created. Offset and seekMode have the same meaning as for the Tcl_SeekChannel 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 returns TCL_ERROR if the optionName is unrecognized or if optionValue specifies a value for the option that is not supported. In this case, the function leaves 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,
    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. If an error occurs, the function returns TCL_ERROR and calls 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.

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

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