Tribblix: manual page: Tcl

Tcl_OpenFileChannel(3) Tcl Library Procedures Tcl_OpenFileChannel(3)

____________________________________________________________________________

NAME

Tcl_OpenFileChannel, Tcl_OpenCommandChannel, Tcl_MakeFileChannel,
Tcl_GetChannel, Tcl_GetChannelNames, Tcl_GetChannelNamesEx,
Tcl_RegisterChannel, Tcl_UnregisterChannel, Tcl_DetachChannel,
Tcl_IsStandardChannel, Tcl_Close, Tcl_ReadChars, Tcl_Read,
Tcl_GetsObj, Tcl_Gets, Tcl_WriteObj, Tcl_WriteChars, Tcl_Write,
Tcl_Flush, Tcl_Seek, Tcl_Tell, Tcl_TruncateChannel,
Tcl_GetChannelOption, Tcl_SetChannelOption, Tcl_Eof,
Tcl_InputBlocked, Tcl_InputBuffered, Tcl_OutputBuffered, Tcl_Ungets,
Tcl_ReadRaw, Tcl_WriteRaw - buffered I/O facilities using channels

SYNOPSIS

#include <tcl.h>

Tcl_Channel
Tcl_OpenFileChannel(interp, fileName, mode, permissions)

Tcl_Channel
Tcl_OpenCommandChannel(interp, argc, argv, flags)

Tcl_Channel
Tcl_MakeFileChannel(handle, readOrWrite)

Tcl_Channel
Tcl_GetChannel(interp, channelName, modePtr)

int
Tcl_GetChannelNames(interp)

int
Tcl_GetChannelNamesEx(interp, pattern)

void
Tcl_RegisterChannel(interp, channel)

int
Tcl_UnregisterChannel(interp, channel)

int
Tcl_DetachChannel(interp, channel)

int
Tcl_IsStandardChannel(channel)

int
Tcl_Close(interp, channel)

int
Tcl_ReadChars(channel, readObjPtr, charsToRead, appendFlag)

int
Tcl_Read(channel, readBuf, bytesToRead)

int
Tcl_GetsObj(channel, lineObjPtr)

int
Tcl_Gets(channel, lineRead)

int
Tcl_Ungets(channel, input, inputLen, addAtEnd)

int
Tcl_WriteObj(channel, writeObjPtr)

int
Tcl_WriteChars(channel, charBuf, bytesToWrite)

int
Tcl_Write(channel, byteBuf, bytesToWrite)

int
Tcl_ReadRaw(channel, readBuf, bytesToRead)

int
Tcl_WriteRaw(channel, byteBuf, bytesToWrite)

int
Tcl_Eof(channel)

int
Tcl_Flush(channel)

int
Tcl_InputBlocked(channel)

int
Tcl_InputBuffered(channel)

int
Tcl_OutputBuffered(channel)

Tcl_WideInt
Tcl_Seek(channel, offset, seekMode)

Tcl_WideInt
Tcl_Tell(channel)

int
Tcl_TruncateChannel(channel, length)

int
Tcl_GetChannelOption(interp, channel, optionName, optionValue)

int
Tcl_SetChannelOption(interp, channel, optionName, newValue)

ARGUMENTS

Tcl_Interp *interp (in) Used for error reporting and
to look up a channel
registered in it.

const char *fileName (in) The name of a local or network
file.

const char *mode (in) Specifies how the file is to
be accessed. May have any of
the values allowed for the
mode argument to the Tcl open
command.

int permissions (in) POSIX-style permission flags
such as 0644. If a new file
is created, these permissions
will be set on the created
file.

int argc (in) The number of elements in
argv.

const char **argv (in) Arguments for constructing a
command pipeline. These
values have the same meaning
as the non-switch arguments to
the Tcl exec command.

int flags (in) Specifies the disposition of
the stdio handles in pipeline:
OR-ed combination of
TCL_STDIN, TCL_STDOUT,
TCL_STDERR, and
TCL_ENFORCE_MODE. If TCL_STDIN
is set, stdin for the first
child in the pipe is the pipe
channel, otherwise it is the
same as the standard input of
the invoking process; likewise
for TCL_STDOUT and TCL_STDERR.
If TCL_ENFORCE_MODE is not
set, then the pipe can
redirect stdio handles to
override the stdio handles for
which TCL_STDIN, TCL_STDOUT
and TCL_STDERR have been set.
If it is set, then such
redirections cause an error.

ClientData handle (in) Operating system specific
handle for I/O to a file. For
Unix this is a file
descriptor, for Windows it is
a HANDLE.

int readOrWrite (in) OR-ed combination of
TCL_READABLE and TCL_WRITABLE
to indicate what operations
are valid on handle.

const char *channelName (in) The name of the channel.

int *modePtr (out) Points at an integer variable
that will receive an OR-ed
combination of TCL_READABLE
and TCL_WRITABLE denoting
whether the channel is open
for reading and writing.

const char *pattern (in) The pattern to match on,
passed to Tcl_StringMatch, or
NULL.

Tcl_Channel channel (in) A Tcl channel for input or
output. Must have been the
return value from a procedure
such as Tcl_OpenFileChannel.

Tcl_Obj *readObjPtr (in/out) A pointer to a Tcl value in
which to store the characters
read from the channel.

int charsToRead (in) The number of characters to
read from the channel. If the
channel's encoding is binary,
this is equivalent to the
number of bytes to read from
the channel.

int appendFlag (in) If non-zero, data read from
the channel will be appended
to the value. Otherwise, the
data will replace the existing
contents of the value.

char *readBuf (out) A buffer in which to store the
bytes read from the channel.

int bytesToRead (in) The number of bytes to read
from the channel. The buffer
readBuf must be large enough
to hold this many bytes.

Tcl_Obj *lineObjPtr (in/out) A pointer to a Tcl value in
which to store the line read
from the channel. The line
read will be appended to the
current value of the value.

Tcl_DString *lineRead (in/out) A pointer to a Tcl dynamic
string in which to store the
line read from the channel.
Must have been initialized by
the caller. The line read
will be appended to any data
already in the dynamic string.

const char *input (in) The input to add to a channel
buffer.

int inputLen (in) Length of the input

int addAtEnd (in) Flag indicating whether the
input should be added to the
end or beginning of the
channel buffer.

Tcl_Obj *writeObjPtr (in) A pointer to a Tcl value whose
contents will be output to the
channel.

const char *charBuf (in) A buffer containing the
characters to output to the
channel.

const char *byteBuf (in) A buffer containing the bytes
to output to the channel.

int bytesToWrite (in) The number of bytes to consume
from charBuf or byteBuf and
output to the channel.

Tcl_WideInt offset (in) How far to move the access
point in the channel at which
the next input or output
operation will be applied,
measured in bytes from the
position given by seekMode.
May be either positive or
negative.

int seekMode (in) Relative to which point to
seek; used with offset to
calculate the new access point
for the channel. Legal values
are SEEK_SET, SEEK_CUR, and
SEEK_END.

Tcl_WideInt length (in) The (non-negative) length to
truncate the channel the
channel to.

const char *optionName (in) The name of an option
applicable to this channel,
such as -blocking. May have
any of the values accepted by
the fconfigure command.

Tcl_DString *optionValue (in) Where to store the value of an
option or a list of all
options and their values. Must
have been initialized by the
caller.

const char *newValue (in) New value for the option given
by optionName.
____________________________________________________________________________

DESCRIPTION

The Tcl channel mechanism provides a device-independent and platform-
independent mechanism for performing buffered input and output
operations on a variety of file, socket, and device types. The
channel mechanism is extensible to new channel types, by providing a
low-level channel driver for the new type; the channel driver
interface is described in the manual entry for Tcl_CreateChannel. The
channel mechanism provides a buffering scheme modeled after Unix's
standard I/O, and it also allows for nonblocking I/O on channels.

The procedures described in this manual entry comprise the C APIs of
the generic layer of the channel architecture. For a description of
the channel driver architecture and how to implement channel drivers
for new types of channels, see the manual entry for
Tcl_CreateChannel.

TCL_OPENFILECHANNEL
Tcl_OpenFileChannel opens a file specified by fileName and returns a
channel handle that can be used to perform input and output on the
file. This API is modeled after the fopen procedure of the Unix
standard I/O library. The syntax and meaning of all arguments is
similar to those given in the Tcl open command when opening a file.
If an error occurs while opening the channel, Tcl_OpenFileChannel
returns NULL and records a POSIX error code that can be retrieved
with Tcl_GetErrno. In addition, if interp is non-NULL,
Tcl_OpenFileChannel leaves an error message in interp's result after
any error. As of Tcl 8.4, the value-based API Tcl_FSOpenFileChannel
should be used in preference to Tcl_OpenFileChannel wherever
possible.

The newly created channel is not registered in the supplied
interpreter; to register it, use Tcl_RegisterChannel, described
below. If one of the standard channels, stdin, stdout or stderr was
previously closed, the act of creating the new channel also assigns
it as a replacement for the standard channel.

TCL_OPENCOMMANDCHANNEL
Tcl_OpenCommandChannel provides a C-level interface to the functions
of the exec and open commands. It creates a sequence of subprocesses
specified by the argv and argc arguments and returns a channel that
can be used to communicate with these subprocesses. The flags
argument indicates what sort of communication will exist with the
command pipeline.

If the TCL_STDIN flag is set then the standard input for the first
subprocess will be tied to the channel: writing to the channel will
provide input to the subprocess. If TCL_STDIN is not set, then
standard input for the first subprocess will be the same as this
application's standard input. If TCL_STDOUT is set then standard
output from the last subprocess can be read from the channel;
otherwise it goes to this application's standard output. If
TCL_STDERR is set, standard error output for all subprocesses is
returned to the channel and results in an error when the channel is
closed; otherwise it goes to this application's standard error. If
TCL_ENFORCE_MODE is not set, then argc and argv can redirect the
stdio handles to override TCL_STDIN, TCL_STDOUT, and TCL_STDERR; if
it is set, then it is an error for argc and argv to override stdio
channels for which TCL_STDIN, TCL_STDOUT, and TCL_STDERR have been
set.

If an error occurs while opening the channel, Tcl_OpenCommandChannel
returns NULL and records a POSIX error code that can be retrieved
with Tcl_GetErrno. In addition, Tcl_OpenCommandChannel leaves an
error message in the interpreter's result. interp cannot be NULL.

The newly created channel is not registered in the supplied
interpreter; to register it, use Tcl_RegisterChannel, described
below. If one of the standard channels, stdin, stdout or stderr was
previously closed, the act of creating the new channel also assigns
it as a replacement for the standard channel.

TCL_MAKEFILECHANNEL
Tcl_MakeFileChannel makes a Tcl_Channel from an existing, platform-
specific, file handle. The newly created channel is not registered
in the supplied interpreter; to register it, use Tcl_RegisterChannel,
described below. If one of the standard channels, stdin, stdout or
stderr was previously closed, the act of creating the new channel
also assigns it as a replacement for the standard channel.

TCL_GETCHANNEL
Tcl_GetChannel returns a channel given the channelName used to create
it with Tcl_CreateChannel and a pointer to a Tcl interpreter in
interp. If a channel by that name is not registered in that
interpreter, the procedure returns NULL. If the modePtr argument is
not NULL, it points at an integer variable that will receive an OR-ed
combination of TCL_READABLE and TCL_WRITABLE describing whether the
channel is open for reading and writing.

Tcl_GetChannelNames and Tcl_GetChannelNamesEx write the names of the
registered channels to the interpreter's result as a list value.
Tcl_GetChannelNamesEx will filter these names according to the
pattern. If pattern is NULL, then it will not do any filtering. The
return value is TCL_OK if no errors occurred writing to the result,
otherwise it is TCL_ERROR, and the error message is left in the
interpreter's result.

TCL_REGISTERCHANNEL
Tcl_RegisterChannel adds a channel to the set of channels accessible
in interp. After this call, Tcl programs executing in that
interpreter can refer to the channel in input or output operations
using the name given in the call to Tcl_CreateChannel. After this
call, the channel becomes the property of the interpreter, and the
caller should not call Tcl_Close for the channel; the channel will be
closed automatically when it is unregistered from the interpreter.

Code executing outside of any Tcl interpreter can call
Tcl_RegisterChannel with interp as NULL, to indicate that it wishes
to hold a reference to this channel. Subsequently, the channel can be
registered in a Tcl interpreter and it will only be closed when the
matching number of calls to Tcl_UnregisterChannel have been made.
This allows code executing outside of any interpreter to safely hold
a reference to a channel that is also registered in a Tcl
interpreter.

This procedure interacts with the code managing the standard
channels. If no standard channels were initialized before the first
call to Tcl_RegisterChannel, they will get initialized by that call.
See Tcl_StandardChannels for a general treatise about standard
channels and the behavior of the Tcl library with regard to them.

TCL_UNREGISTERCHANNEL
Tcl_UnregisterChannel removes a channel from the set of channels
accessible in interp. After this call, Tcl programs will no longer be
able to use the channel's name to refer to the channel in that
interpreter. If this operation removed the last registration of the
channel in any interpreter, the channel is also closed and destroyed.

Code not associated with a Tcl interpreter can call
Tcl_UnregisterChannel with interp as NULL, to indicate to Tcl that it
no longer holds a reference to that channel. If this is the last
reference to the channel, it will now be closed.
Tcl_UnregisterChannel is very similar to Tcl_DetachChannel except
that it will also close the channel if no further references to it
exist.

TCL_DETACHCHANNEL
Tcl_DetachChannel removes a channel from the set of channels
accessible in interp. After this call, Tcl programs will no longer be
able to use the channel's name to refer to the channel in that
interpreter. Beyond that, this command has no further effect. It
cannot be used on the standard channels (stdout, stderr, stdin), and
will return TCL_ERROR if passed one of those channels.

Code not associated with a Tcl interpreter can call Tcl_DetachChannel
with interp as NULL, to indicate to Tcl that it no longer holds a
reference to that channel. If this is the last reference to the
channel, unlike Tcl_UnregisterChannel, it will not be closed.

TCL_ISSTANDARDCHANNEL
Tcl_IsStandardChannel tests whether a channel is one of the three
standard channels, stdin, stdout or stderr. If so, it returns 1,
otherwise 0.

No attempt is made to check whether the given channel or the standard
channels are initialized or otherwise valid.

TCL_CLOSE
Tcl_Close destroys the channel channel, which must denote a currently
open channel. The channel should not be registered in any interpreter
when Tcl_Close is called. Buffered output is flushed to the channel's
output device prior to destroying the channel, and any buffered input
is discarded. If this is a blocking channel, the call does not
return until all buffered data is successfully sent to the channel's
output device. If this is a nonblocking channel and there is
buffered output that cannot be written without blocking, the call
returns immediately; output is flushed in the background and the
channel will be closed once all of the buffered data has been output.
In this case errors during flushing are not reported.

If the channel was closed successfully, Tcl_Close returns TCL_OK. If
an error occurs, Tcl_Close returns TCL_ERROR and records a POSIX
error code that can be retrieved with Tcl_GetErrno. If the channel
is being closed synchronously and an error occurs during closing of
the channel and interp is not NULL, an error message is left in the
interpreter's result.

Note: it is not safe to call Tcl_Close on a channel that has been
registered using Tcl_RegisterChannel; see the documentation for
Tcl_RegisterChannel, above, for details. If the channel has ever been
given as the chan argument in a call to Tcl_RegisterChannel, you
should instead use Tcl_UnregisterChannel, which will internally call
Tcl_Close when all calls to Tcl_RegisterChannel have been matched by
corresponding calls to Tcl_UnregisterChannel.

TCL_READCHARS AND TCL_READ
Tcl_ReadChars consumes bytes from channel, converting the bytes to
UTF-8 based on the channel's encoding and storing the produced data
in readObjPtr's string representation. The return value of
Tcl_ReadChars is the number of characters, up to charsToRead, that
were stored in readObjPtr. If an error occurs while reading, the
return value is -1 and Tcl_ReadChars records a POSIX error code that
can be retrieved with Tcl_GetErrno.

Setting charsToRead to -1 will cause the command to read all
characters currently available (non-blocking) or everything until eof
(blocking mode).

The return value may be smaller than the value to read, indicating
that less data than requested was available. This is called a short
read. In blocking mode, this can only happen on an end-of-file. In
nonblocking mode, a short read can also occur if there is not enough
input currently available: Tcl_ReadChars returns a short count
rather than waiting for more data.

If the channel is in blocking mode, a return value of zero indicates
an end-of-file condition. If the channel is in nonblocking mode, a
return value of zero indicates either that no input is currently
available or an end-of-file condition. Use Tcl_Eof and
Tcl_InputBlocked to tell which of these conditions actually occurred.

Tcl_ReadChars translates the various end-of-line representations into
the canonical \n internal representation according to the current
end-of-line recognition mode. End-of-line recognition and the
various platform-specific modes are described in the manual entry for
the Tcl fconfigure command.

As a performance optimization, when reading from a channel with the
encoding binary, the bytes are not converted to UTF-8 as they are
read. Instead, they are stored in readObjPtr's internal
representation as a byte-array value. The string representation of
this value will only be constructed if it is needed (e.g., because of
a call to Tcl_GetStringFromObj). In this way, byte-oriented data can
be read from a channel, manipulated by calling
Tcl_GetByteArrayFromObj and related functions, and then written to a
channel without the expense of ever converting to or from UTF-8.

Tcl_Read is similar to Tcl_ReadChars, except that it does not do
encoding conversions, regardless of the channel's encoding. It is
deprecated and exists for backwards compatibility with non-
internationalized Tcl extensions. It consumes bytes from channel and
stores them in readBuf, performing end-of-line translations on the
way. The return value of Tcl_Read is the number of bytes, up to
bytesToRead, written in readBuf. The buffer produced by Tcl_Read is
not null-terminated. Its contents are valid from the zeroth position
up to and excluding the position indicated by the return value.

Tcl_ReadRaw is the same as Tcl_Read but does not compensate for
stacking. While Tcl_Read (and the other functions in the API) always
get their data from the topmost channel in the stack the supplied
channel is part of, Tcl_ReadRaw does not. Thus this function is only
usable for transformational channel drivers, i.e. drivers used in the
middle of a stack of channels, to move data from the channel below
into the transformation.

TCL_GETSOBJ AND TCL_GETS
Tcl_GetsObj consumes bytes from channel, converting the bytes to
UTF-8 based on the channel's encoding, until a full line of input has
been seen. If the channel's encoding is binary, each byte read from
the channel is treated as an individual Unicode character. All of
the characters of the line except for the terminating end-of-line
character(s) are appended to lineObjPtr's string representation. The
end-of-line character(s) are read and discarded.

If a line was successfully read, the return value is greater than or
equal to zero and indicates the number of bytes stored in lineObjPtr.
If an error occurs, Tcl_GetsObj returns -1 and records a POSIX error
code that can be retrieved with Tcl_GetErrno. Tcl_GetsObj also
returns -1 if the end of the file is reached; the Tcl_Eof procedure
can be used to distinguish an error from an end-of-file condition.

If the channel is in nonblocking mode, the return value can also be
-1 if no data was available or the data that was available did not
contain an end-of-line character. When -1 is returned, the
Tcl_InputBlocked procedure may be invoked to determine if the channel
is blocked because of input unavailability.

Tcl_Gets is the same as Tcl_GetsObj except the resulting characters
are appended to the dynamic string given by lineRead rather than a
Tcl value.

TCL_UNGETS
Tcl_Ungets is used to add data to the input queue of a channel, at
either the head or tail of the queue. The pointer input points to
the data that is to be added. The length of the input to add is
given by inputLen. A non-zero value of addAtEnd indicates that the
data is to be added at the end of queue; otherwise it will be added
at the head of the queue. If channel has a "sticky" EOF set, no data
will be added to the input queue. Tcl_Ungets returns inputLen or -1
if an error occurs.

TCL_WRITECHARS, TCL_WRITEOBJ, AND TCL_WRITE
Tcl_WriteChars accepts bytesToWrite bytes of character data at
charBuf. The UTF-8 characters in the buffer are converted to the
channel's encoding and queued for output to channel. If bytesToWrite
is negative, Tcl_WriteChars expects charBuf to be null-terminated and
it outputs everything up to the null.

Data queued for output may not appear on the output device
immediately, due to internal buffering. If the data should appear
immediately, call Tcl_Flush after the call to Tcl_WriteChars, or set
the -buffering option on the channel to none. If you wish the data
to appear as soon as a complete line is accepted for output, set the
-buffering option on the channel to line mode.

The return value of Tcl_WriteChars is a count of how many bytes were
accepted for output to the channel. This is either greater than zero
to indicate success or -1 to indicate that an error occurred. If an
error occurs, Tcl_WriteChars records a POSIX error code that may be
retrieved with Tcl_GetErrno.

Newline characters in the output data are translated to platform-
specific end-of-line sequences according to the -translation option
for the channel. This is done even if the channel has no encoding.

Tcl_WriteObj is similar to Tcl_WriteChars except it accepts a Tcl
value whose contents will be output to the channel. The UTF-8
characters in writeObjPtr's string representation are converted to
the channel's encoding and queued for output to channel. As a
performance optimization, when writing to a channel with the encoding
binary, UTF-8 characters are not converted as they are written.
Instead, the bytes in writeObjPtr's internal representation as a
byte-array value are written to the channel. The byte-array
representation of the value will be constructed if it is needed. In
this way, byte-oriented data can be read from a channel, manipulated
by calling Tcl_GetByteArrayFromObj and related functions, and then
written to a channel without the expense of ever converting to or
from UTF-8.

Tcl_Write is similar to Tcl_WriteChars except that it does not do
encoding conversions, regardless of the channel's encoding. It is
deprecated and exists for backwards compatibility with non-
internationalized Tcl extensions. It accepts bytesToWrite bytes of
data at byteBuf and queues them for output to channel. If
bytesToWrite is negative, Tcl_Write expects byteBuf to be null-
terminated and it outputs everything up to the null.

Tcl_WriteRaw is the same as Tcl_Write but does not compensate for
stacking. While Tcl_Write (and the other functions in the API) always
feed their input to the topmost channel in the stack the supplied
channel is part of, Tcl_WriteRaw does not. Thus this function is only
usable for transformational channel drivers, i.e. drivers used in the
middle of a stack of channels, to move data from the transformation
into the channel below it.

TCL_FLUSH
Tcl_Flush causes all of the buffered output data for channel to be
written to its underlying file or device as soon as possible. If the
channel is in blocking mode, the call does not return until all the
buffered data has been sent to the channel or some error occurred.
The call returns immediately if the channel is nonblocking; it starts
a background flush that will write the buffered data to the channel
eventually, as fast as the channel is able to absorb it.

The return value is normally TCL_OK. If an error occurs, Tcl_Flush
returns TCL_ERROR and records a POSIX error code that can be
retrieved with Tcl_GetErrno.

TCL_SEEK
Tcl_Seek moves the access point in channel where subsequent data will
be read or written. Buffered output is flushed to the channel and
buffered input is discarded, prior to the seek operation.

Tcl_Seek normally returns the new access point. If an error occurs,
Tcl_Seek returns -1 and records a POSIX error code that can be
retrieved with Tcl_GetErrno. After an error, the access point may or
may not have been moved.

TCL_TELL
Tcl_Tell returns the current access point for a channel. The returned
value is -1 if the channel does not support seeking.

TCL_TRUNCATECHANNEL
Tcl_TruncateChannel truncates the file underlying channel to a given
length of bytes. It returns TCL_OK if the operation succeeded, and
TCL_ERROR otherwise.

TCL_GETCHANNELOPTION
Tcl_GetChannelOption retrieves, in optionValue, the value of one of
the options currently in effect for a channel, or a list of all
options and their values. The channel argument identifies the
channel for which to query an option or retrieve all options and
their values. If optionName is not NULL, it is the name of the
option to query; the option's value is copied to the Tcl dynamic
string denoted by optionValue. If optionName is NULL, the function
stores an alternating list of option names and their values in
optionValue, using a series of calls to Tcl_DStringAppendElement. The
various preexisting options and their possible values are described
in the manual entry for the Tcl fconfigure command. Other options can
be added by each channel type. These channel type specific options
are described in the manual entry for the Tcl command that creates a
channel of that type; for example, the additional options for TCP-
based channels are described in the manual entry for the Tcl socket
command. The procedure normally returns TCL_OK. If an error occurs,
it returns TCL_ERROR and calls Tcl_SetErrno to store an appropriate
POSIX error code.

TCL_SETCHANNELOPTION
Tcl_SetChannelOption sets a new value newValue for an option
optionName on channel. The procedure normally returns TCL_OK. If an
error occurs, it returns TCL_ERROR; in addition, if interp is non-
NULL, Tcl_SetChannelOption leaves an error message in the
interpreter's result.

TCL_EOF
Tcl_Eof returns a nonzero value if channel encountered an end of file
during the last input operation.

TCL_INPUTBLOCKED
Tcl_InputBlocked returns a nonzero value if channel is in nonblocking
mode and the last input operation returned less data than requested
because there was insufficient data available. The call always
returns zero if the channel is in blocking mode.

TCL_INPUTBUFFERED
Tcl_InputBuffered returns the number of bytes of input currently
buffered in the internal buffers for a channel. If the channel is not
open for reading, this function always returns zero.

TCL_OUTPUTBUFFERED
Tcl_OutputBuffered returns the number of bytes of output currently
buffered in the internal buffers for a channel. If the channel is not
open for writing, this function always returns zero.

PLATFORM ISSUES

The handles returned from Tcl_GetChannelHandle depend on the platform
and the channel type. On Unix platforms, the handle is always a Unix
file descriptor as returned from the open system call. On Windows
platforms, the handle is a file HANDLE when the channel was created
with Tcl_OpenFileChannel, Tcl_OpenCommandChannel, or
Tcl_MakeFileChannel. Other channel types may return a different type
of handle on Windows platforms.

KEYWORDS