path: root/doc/chan.n

'\"
'\" Copyright (c) 2005-2006 Donal K. Fellows
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
.TH chan n 8.5 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
chan \- Read, write and manipulate channels
.SH SYNOPSIS
\fBchan \fIoption\fR ?\fIarg arg ...\fR?
.BE
.SH DESCRIPTION
.PP
This command provides several operations for reading from, writing to
and otherwise manipulating open channels (such as have been created
with the \fBopen\fR and \fBsocket\fR commands, or the default named
channels \fBstdin\fR, \fBstdout\fR or \fBstderr\fR which correspond to
the process's standard input, output and error streams respectively).
\fIOption\fR indicates what to do with the channel; any unique
abbreviation for \fIoption\fR is acceptable. Valid options are:
.\" METHOD: blocked
.TP
\fBchan blocked \fIchannelId\fR
.
This tests whether the last input operation on the channel called
\fIchannelId\fR failed because it would have otherwise caused the
process to block, and returns 1 if that was the case. It returns 0
otherwise. Note that this only ever returns 1 when the channel has
been configured to be non-blocking; all Tcl channels have blocking
turned on by default.
.\" METHOD: close
.TP
\fBchan close \fIchannelId\fR ?\fIdirection\fR?
.
Close and destroy the channel called \fIchannelId\fR. Note that this
deletes all existing file-events registered on the channel.
If the \fIdirection\fR argument (which must be \fBread\fR or \fBwrite\fR or
any unique abbreviation of them) is present, the channel will only be
half-closed, so that it can go from being read-write to write-only or
read-only respectively. If a read-only channel is closed for reading, it is
the same as if the channel is fully closed, and respectively similar for
write-only channels. Without the \fIdirection\fR argument, the channel is
closed for both reading and writing (but only if those directions are
currently open). It is an error to close a read-only channel for writing, or a
write-only channel for reading.
.RS
.PP
As part of closing the channel, all buffered output is flushed to the
channel's output device (only if the channel is ceasing to be writable), any
buffered input is discarded (only if the channel is ceasing to be readable),
the underlying operating system resource is closed and \fIchannelId\fR becomes
unavailable for future use (both only if the channel is being completely
closed).
.PP
If the channel is blocking and the channel is ceasing to be writable, the
command does not return until all output is flushed.  If the channel is
non-blocking and there is unflushed output, the channel remains open and the
command returns immediately; output will be flushed in the background and the
channel will be closed when all the flushing is complete.
.PP
If \fIchannelId\fR is a blocking channel for a command pipeline then
\fBchan close\fR waits for the child processes to complete.
.PP
If the channel is shared between interpreters, then \fBchan close\fR
makes \fIchannelId\fR unavailable in the invoking interpreter but has
no other effect until all of the sharing interpreters have closed the
channel. When the last interpreter in which the channel is registered
invokes \fBchan close\fR (or \fBclose\fR), the cleanup actions
described above occur. With half-closing, the half-close of the channel only
applies to the current interpreter's view of the channel until all channels
have closed it in that direction (or completely).
See the \fBinterp\fR command for a description of channel sharing.
.PP
Channels are automatically fully closed when an interpreter is destroyed and
when the process exits.  Channels are switched to blocking mode, to
ensure that all output is correctly flushed before the process exits.
.PP
The command returns an empty string, and may generate an error if
an error occurs while flushing output.  If a command in a command
pipeline created with \fBopen\fR returns an error, \fBchan close\fR
generates an error (similar to the \fBexec\fR command.)
.PP
Note that half-closes of sockets and command pipelines can have important side
effects because they result in a shutdown() or close() of the underlying
system resource, which can change how other processes or systems respond to
the Tcl program.
.PP
Channels are automatically closed when an interpreter is destroyed and
when the process exits.
From 8.6 on (TIP#398), nonblocking channels are no longer switched to
blocking mode when exiting; this guarantees a timely exit even when the
peer or a communication channel is stalled. To ensure proper flushing of
stalled nonblocking channels on exit, one must now either (a) actively
switch them back to blocking or (b) use the environment variable
\fBTCL_FLUSH_NONBLOCKING_ON_EXIT\fR, which when set and not equal to
.QW \fB0\fR
restores the previous behavior.
.RE
.\" METHOD: configure
.TP
\fBchan configure \fIchannelId\fR ?\fIoptionName\fR? ?\fIvalue\fR? ?\fIoptionName value\fR?...
.
Query or set the configuration options of the channel named
\fIchannelId\fR.
.RS
.PP
If no \fIoptionName\fR or \fIvalue\fR arguments are supplied, the
command returns a list containing alternating option names and values
for the channel.  If \fIoptionName\fR is supplied but no \fIvalue\fR
then the command returns the current value of the given option.  If
one or more pairs of \fIoptionName\fR and \fIvalue\fR are supplied,
the command sets each of the named options to the corresponding
\fIvalue\fR; in this case the return value is an empty string.
.PP
The options described below are supported for all channels. In
addition, each channel type may add options that only it supports. See
the manual entry for the command that creates each type of channel
for the options supported by that specific type of channel. For
example, see the manual entry for the \fBsocket\fR command for additional
options for sockets, and the \fBopen\fR command for additional options for
serial devices.
.RE
.\" OPTION: -blocking
.TP
\fB\-blocking\fI boolean\fR
.
The \fB\-blocking\fR option determines whether I/O operations on the
channel can cause the process to block indefinitely.  The value of the
option must be a proper boolean value.  Channels are normally in
blocking mode; if a channel is placed into non-blocking mode it will
affect the operation of the \fBchan gets\fR, \fBchan read\fR, \fBchan
puts\fR, \fBchan flush\fR, and \fBchan close\fR commands; see the
documentation for those commands for details.  For non-blocking mode to
work correctly, the application must be using the Tcl event loop
(e.g. by calling \fBTcl_DoOneEvent\fR or invoking the \fBvwait\fR
command).
.\" OPTION: -buffering
.TP
\fB\-buffering\fI newValue\fR
.
If \fInewValue\fR is \fBfull\fR then the I/O system will buffer output
until its internal buffer is full or until the \fBchan flush\fR
command is invoked. If \fInewValue\fR is \fBline\fR, then the I/O
system will automatically flush output for the channel whenever a
newline character is output. If \fInewValue\fR is \fBnone\fR, the I/O
system will flush automatically after every output operation.  The
default is for \fB\-buffering\fR to be set to \fBfull\fR except for
channels that connect to terminal-like devices; for these channels the
initial setting is \fBline\fR.  Additionally, \fBstdin\fR and
\fBstdout\fR are initially set to \fBline\fR, and \fBstderr\fR is set
to \fBnone\fR.
.\" OPTION: -buffersize
.TP
\fB\-buffersize\fI newSize\fR
.
\fInewSize\fR must be an integer; its value is used to set the size
of buffers, in bytes, subsequently allocated for this channel to store
input or output. \fInewSize\fR must be a number of no more than one
million, allowing buffers of up to one million bytes in size.
.\" OPTION: -encoding
.TP
\fB\-encoding\fR \fIname\fR
.
This option is used to specify the encoding of the channel as one of
the named encodings returned by \fBencoding names\fR or the special
value \fBbinary\fR, so that the data can be converted to and from
Unicode for use in Tcl.  For instance, in order for Tcl to read
characters from a Japanese file in \fBshiftjis\fR and properly process
and display the contents, the encoding would be set to \fBshiftjis\fR.
Thereafter, when reading from the channel, the bytes in the Japanese
file would be converted to Unicode as they are read.  Writing is also
supported \- as Tcl strings are written to the channel they will
automatically be converted to the specified encoding on output.
.RS
.PP
If a file contains pure binary data (for instance, a JPEG image), the
encoding for the channel should be configured to be \fBbinary\fR.  Tcl
will then assign no interpretation to the data in the file and simply
read or write raw bytes.  The Tcl \fBbinary\fR command can be used to
manipulate this byte-oriented data.  It is usually better to set the
\fB\-translation\fR option to \fBbinary\fR when you want to transfer
binary data, as this turns off the other automatic interpretations of
the bytes in the stream as well.
.PP
The default encoding for newly opened channels is the same platform-
and locale-dependent system encoding used for interfacing with the
operating system, as returned by \fBencoding system\fR.
.RE
.\" OPTION: -eofchar
.TP
\fB\-eofchar\fI char\fR
.
This option supports DOS file systems that use Control-z (\ex1A) as an
end of file marker.  If \fIchar\fR is not an empty string, then this
character signals end-of-file when it is encountered during input.
Otherwise (the default) there is no special end of file character marker.
The acceptable range for \fB\-eofchar\fR values is \ex01 - \ex7f;
attempting to set \fB\-eofchar\fR to a value outside of this range will
generate an error.
.VS "TCL8.7 TIP656"
.\" OPTION: -profile
.TP
\fB\-profile\fI profile\fR
.
Specifies the encoding profile to be used on the channel. The encoding
transforms in use for the channel's input and output will then be subject to the
rules of that profile. Any failures will result in a channel error. See
\fBPROFILES\fR in the \fBencoding(n)\fR documentation for details about encoding
profiles.
.VE "TCL8.7 TIP656"
.\" OPTION: -translation
.TP
\fB\-translation\fI translation\fR
.TP
\fB\-translation\fR \fB{\fIinTranslation outTranslation\fB}\fR
.
In Tcl scripts the end of a line is always represented using a single
newline character (\en).  However, in actual files and devices the end
of a line may be represented differently on different platforms, or
even for different devices on the same platform.  For example, under
UNIX newlines are used in files, whereas carriage-return-linefeed
sequences are normally used in network connections.  On input (i.e.,
with \fBchan gets\fR and \fBchan read\fR) the Tcl I/O system
automatically translates the external end-of-line representation into
newline characters.  Upon output (i.e., with \fBchan puts\fR), the I/O
system translates newlines to the external end-of-line representation.
The default translation mode, \fBauto\fR, handles all the common cases
automatically, but the \fB\-translation\fR option provides explicit
control over the end of line translations.
.RS
.PP
The value associated with \fB\-translation\fR is a single item for
read-only and write-only channels.  The value is a two-element list for
read-write channels; the read translation mode is the first element of
the list, and the write translation mode is the second element.  As a
convenience, when setting the translation mode for a read-write channel
you can specify a single value that will apply to both reading and
writing.  When querying the translation mode of a read-write channel, a
two-element list will always be returned.  The following values are
currently supported:
.IP \fBauto\fR
As the input translation mode, \fBauto\fR treats any of newline
(\fBlf\fR), carriage return (\fBcr\fR), or carriage return followed by
a newline (\fBcrlf\fR) as the end of line representation.  The end of
line representation can even change from line-to-line, and all cases
are translated to a newline.  As the output translation mode,
\fBauto\fR chooses a platform specific representation; for sockets on
all platforms Tcl chooses \fBcrlf\fR, for all Unix flavors, it chooses
\fBlf\fR, and for the various flavors of Windows it chooses
\fBcrlf\fR.  The default setting for \fB\-translation\fR is \fBauto\fR
for both input and output.
.IP \fBbinary\fR
Like \fBlf\fR, no end-of-line translation is performed, but in addition, sets
\fB\-eofchar\fR to the empty string to disable it, and sets \fB\-encoding\fR
to \fBiso8859-1\fR.  With this one setting, a channel is fully configured
for binary input and output:  Each byte read from the channel
becomes the Unicode character having the same value as that byte, and each
character written to the channel becomes a single byte in the output.  This
makes it possible to work seamlessly with binary data as long as each character
in the data remains in the range of 0 to 255 so that there is no distinction
between binary data and text.  For example, A JPEG image can be read from a
such a channel, manipulated, and then written back to such a channel.
.IP \fBcr\fR
The end of a line in the underlying file or device is represented by a
single carriage return character.  As the input translation mode,
\fBcr\fR mode converts carriage returns to newline characters.  As the
output translation mode, \fBcr\fR mode translates newline characters
to carriage returns.
.IP \fBcrlf\fR
The end of a line in the underlying file or device is represented by a
carriage return character followed by a linefeed character.  As the
input translation mode, \fBcrlf\fR mode converts
carriage-return-linefeed sequences to newline characters.  As the
output translation mode, \fBcrlf\fR mode translates newline characters
to carriage-return-linefeed sequences.  This mode is typically used on
Windows platforms and for network connections.
.IP \fBlf\fR
The end of a line in the underlying file or device is represented by a
single newline (linefeed) character.  In this mode no translations
occur during either input or output.  This mode is typically used on
UNIX platforms.
.RE
.\" METHOD: copy
.TP
\fBchan copy \fIinputChan outputChan\fR ?\fB\-size \fIsize\fR? ?\fB\-command \fIcallback\fR?
.
Reads characters from \fIinputChan\fR and writes them to \fIoutputChan\fR until
all characters are copied, blocking until the copy is complete and returning
the number of characters copied.  Leverages internal buffers to avoid extra
copies and to avoid buffering too much data in main memory when copying large
files to slow destinations like network sockets.
.RS
.PP
\fB\-size\fR limits the number of characters copied.
.PP
If \fB\-command\fR is given, \fBchan copy\fR returns immediately, works in the
background, and calls \fIcallback\fR when the copy completes, providing as an
additional argument the number of characters written to \fIoutputChan\fR.  If
an error occurs during the background copy, another argument provides message
for the error.  \fIinputChan\fR and \fIoutputChan\fR are automatically
configured for non-blocking mode if needed.  Background copying only works
correctly if events are being processed, e.g. via \fBvwait\fR or Tk.
.PP
During a background copy no other read operation may be performed on
\fIinputChan\fR, and no write operation may be performed on
\fIoutputChan\fR.  However, write operations may by performed on
\fIinputChan\fR and read operations may be performed on \fIoutputChan\fR, as
exhibited by the bidirectional copy example below.
.PP
If either \fIinputChan\fR or \fIoutputChan\fR is closed while the copy is in
progress, copying ceases and \fBno\fR callback is made.  If \fIinputChan\fR is
closed all data already queued is written to \fIoutputChan\fR.
.PP
There should be no event handler established for \fIinputChan\fR  because it
may become readable during a background copy.  An attempt to read or write from
within an event handler results result in the error,  "channel busy".  Any
wrong-sided I/O attempted (by a \fBchan event\fR handler or otherwise) results
in a
.QW "channel busy"
error.
.RE
.\" METHOD: create
.TP
\fBchan create \fImode cmdPrefix\fR
.
This subcommand creates a new script level channel using the command
prefix \fIcmdPrefix\fR as its handler. Any such channel is called a
\fBreflected\fR channel. The specified command prefix, \fBcmdPrefix\fR,
must be a non-empty list, and should provide the API described in the
\fBrefchan\fR manual page. The handle of the new channel is
returned as the result of the \fBchan create\fR command, and the
channel is open. Use either \fBclose\fR or \fBchan close\fR to remove
the channel.
.RS
.PP
The argument \fImode\fR specifies if the new channel is opened for
reading, writing, or both. It has to be a list containing any of the
strings
.QW \fBread\fR
or
.QW \fBwrite\fR ,
The list must have at least one
element, as a channel you can neither write to nor read from makes no
sense. The handler command for the new channel must support the chosen
mode, or an error is thrown.
.PP
The command prefix is executed in the global namespace, at the top of
call stack, following the appending of arguments as described in the
\fBrefchan\fR manual page. Command resolution happens at the
time of the call. Renaming the command, or destroying it means that
the next call of a handler method may fail, causing the channel
command invoking the handler to fail as well. Depending on the
subcommand being invoked, the error message may not be able to explain
the reason for that failure.
.PP
Every channel created with this subcommand knows which interpreter it
was created in, and only ever executes its handler command in that
interpreter, even if the channel was shared with and/or was moved into
a different interpreter. Each reflected channel also knows the thread
it was created in, and executes its handler command only in that
thread, even if the channel was moved into a different thread. To this
end all invocations of the handler are forwarded to the original
thread by posting special events to it. This means that the original
thread (i.e. the thread that executed the \fBchan create\fR command)
must have an active event loop, i.e. it must be able to process such
events. Otherwise the thread sending them will \fIblock
indefinitely\fR. Deadlock may occur.
.PP
Note that this permits the creation of a channel whose two endpoints
live in two different threads, providing a stream-oriented bridge
between these threads. In other words, we can provide a way for
regular stream communication between threads instead of having to send
commands.
.PP
When a thread or interpreter is deleted, all channels created with
this subcommand and using this thread/interpreter as their computing
base are deleted as well, in all interpreters they have been shared
with or moved into, and in whatever thread they have been transferred
to. While this pulls the rug out under the other thread(s) and/or
interpreter(s), this cannot be avoided. Trying to use such a channel
will cause the generation of a regular error about unknown channel
handles.
.PP
This subcommand is \fBsafe\fR and made accessible to safe
interpreters.  While it arranges for the execution of arbitrary Tcl
code the system also makes sure that the code is always executed
within the safe interpreter.
.RE
.\" METHOD: eof
.TP
\fBchan eof \fIchannelId\fR
.
Test whether the last input operation on the channel called
\fIchannelId\fR failed because the end of the data stream was reached,
returning 1 if end-of-file was reached, and 0 otherwise.
.\" METHOD: event
.TP
\fBchan event \fIchannelId event\fR ?\fIscript\fR?
.
Arrange for the Tcl script \fIscript\fR to be installed as a \fIfile
event handler\fR to be called whenever the channel called
\fIchannelId\fR enters the state described by \fIevent\fR (which must
be either \fBreadable\fR or \fBwritable\fR); only one such handler may
be installed per event per channel at a time.  If \fIscript\fR is the
empty string, the current handler is deleted (this also happens if the
channel is closed or the interpreter deleted).  If \fIscript\fR is
omitted, the currently installed script is returned (or an empty
string if no such handler is installed).  The callback is only
performed if the event loop is being serviced (e.g. via \fBvwait\fR or
\fBupdate\fR).
.RS
.PP
A file event handler is a binding between a channel and a script, such
that the script is evaluated whenever the channel becomes readable or
writable.  File event handlers are most commonly used to allow data to
be received from another process on an event-driven basis, so that the
receiver can continue to interact with the user or with other channels
while waiting for the data to arrive.  If an application invokes
\fBchan gets\fR or \fBchan read\fR on a blocking channel when there is
no input data available, the process will block; until the input data
arrives, it will not be able to service other events, so it will
appear to the user to
.QW "freeze up"
\&.
With \fBchan event\fR, the
process can tell when data is present and only invoke \fBchan gets\fR
or \fBchan read\fR when they will not block.
.PP
A channel is considered to be readable if there is unread data
available on the underlying device.  A channel is also considered to
be readable if there is unread data in an input buffer, except in the
special case where the most recent attempt to read from the channel
was a \fBchan gets\fR call that could not find a complete line in the
input buffer.  This feature allows a file to be read a line at a time
in non-blocking mode using events.  A channel is also considered to be
readable if an end of file or error condition is present on the
underlying file or device.  It is important for \fIscript\fR to check
for these conditions and handle them appropriately; for example, if
there is no special check for end of file, an infinite loop may occur
where \fIscript\fR reads no data, returns, and is immediately invoked
again.
.PP
A channel is considered to be writable if at least one byte of data
can be written to the underlying file or device without blocking, or
if an error condition is present on the underlying file or device.
Note that client sockets opened in asynchronous mode become writable
when they become connected or if the connection fails.
.PP
Event-driven I/O works best for channels that have been placed into
non-blocking mode with the \fBchan configure\fR command.  In blocking
mode, a \fBchan puts\fR command may block if you give it more data
than the underlying file or device can accept, and a \fBchan gets\fR
or \fBchan read\fR command will block if you attempt to read more data
than is ready; no events will be processed while the commands block.
In non-blocking mode \fBchan puts\fR, \fBchan read\fR, and \fBchan
gets\fR never block.
.PP
The script for a file event is executed at global level (outside the
context of any Tcl procedure) in the interpreter in which the \fBchan
event\fR command was invoked.  If an error occurs while executing the
script then the command registered with \fBinterp bgerror\fR is used
to report the error.  In addition, the file event handler is deleted
if it ever returns an error; this is done in order to prevent infinite
loops due to buggy handlers.
.RE
.\" METHOD: flush
.TP
\fBchan flush \fIchannelId\fR
.
Ensures that all pending output for the channel called \fIchannelId\fR
is written.
.RS
.PP
If the channel is in blocking mode the command does not return until
all the buffered output has been flushed to the channel. If the
channel is in non-blocking mode, the command may return before all
buffered output has been flushed; the remainder will be flushed in the
background as fast as the underlying file or device is able to absorb
it.
.RE
.\" METHOD: gets
.TP
\fBchan gets \fIchannelId\fR ?\fIvarName\fR?
.
Reads a line from the channel consisting of all characters up to the next
end-of-line sequence or until end of file is seen. The line feed character
corresponding to end-of-line sequence is not included as part of the line.
If the \fIvarName\fR argument is specified, the line is stored in the variable
of that name and the command returns the length of the line. If \fIvarName\fR
is not specified, the command returns the line itself as the result of the command.
.RS
.PP
If a complete line is not available and the channel is not at EOF, the command
will block in the case of a blocking channel. For non-blocking channels, the
command will return the empty string as the result in the case of \fIvarName\fR
not specified and -1 if it is.
.RE
.RS
.PP
If a blocking channel is already at EOF, the command returns an empty string if
\fIvarName\fR is not specified. Note an empty string result can also be returned
when a blank line (no characters before the next end of line sequence). The two
cases can be distinguished by calling the \fBchan eof\fR command to check for
end of file. If \fIvarName\fR is specified, the command returns -1 on end of file.
There is no ambiguity in this case because blank lines result in 0 being returned.
.RE
.RS
.PP
If a non-blocking channel is already at EOF, the command returns an empty line
if \fIvarName\fR is not specified. This can be distinguished from an empty line
being returned by either a blank line being read or a full line not being available
through the use of the \fBchan eof\fR and \fBchan blocked\fR commands. If
\fBchan eof\fR returns true, the channel is at EOF. If \fBchan blocked\fR returns
true, a full line was not available. If both commands return false, an empty
line was read. If \fIvarName\fR was specified for a non-bocking channel at EOF,
the command returns -1. This can be distinguished from full line not being
available either by \fBchan eof\fR or \fBchan blocked\fR as above. Note that
when \fIvarName\fR is specified, there is no need to distinguish between eof
and blank lines as the latter will result in the command returning 0.
.PP
If the encoding profile \fBstrict\fR is in effect for the channel, the command
will raise an exception with the POSIX error code \fBEILSEQ\fR if any encoding
errors are encountered in the channel input data. The file pointer remains
unchanged and it is possible to introspect, and in some cases recover, by
changing the encoding in use. See \fBENCODING ERROR EXAMPLES\fR later.
.RE
.\" METHOD: names
.TP
\fBchan names\fR ?\fIpattern\fR?
.
Produces a list of all channel names. If \fIpattern\fR is specified,
only those channel names that match it (according to the rules of
\fBstring match\fR) will be returned.
.\" METHOD: pending
.TP
\fBchan pending \fImode channelId\fR
.
Depending on whether \fImode\fR is \fBinput\fR or \fBoutput\fR,
returns the number of
bytes of input or output (respectively) currently buffered
internally for \fIchannelId\fR (especially useful in a readable event
callback to impose application-specific limits on input line lengths to avoid
a potential denial-of-service attack where a hostile user crafts
an extremely long line that exceeds the available memory to buffer it).
Returns -1 if the channel was not opened for the mode in question.
.\" METHOD: pipe
.TP
\fBchan pipe\fR
.
Creates a standalone pipe whose read- and write-side channels are
returned as a 2-element list, the first element being the read side and
the second the write side. Can be useful e.g. to redirect
separately \fBstderr\fR and \fBstdout\fR from a subprocess. To do
this, spawn with "2>@" or
">@" redirection operators onto the write side of a pipe, and then
immediately close it in the parent. This is necessary to get an EOF on
the read side once the child has exited or otherwise closed its output.
.RS
.PP
Note that the pipe buffering semantics can vary at the operating system level
substantially; it is not safe to assume that a write performed on the output
side of the pipe will appear instantly to the input side. This is a
fundamental difference and Tcl cannot conceal it. The overall stream semantics
\fIare\fR compatible, so blocking reads and writes will not see most of the
differences, but the details of what exactly gets written when are not. This
is most likely to show up when using pipelines for testing; care should be
taken to ensure that deadlocks do not occur and that potential short reads are
allowed for.
.RE
.\" METHOD: pop
.TP
\fBchan pop \fIchannelId\fR
.
Removes the topmost transformation from the channel \fIchannelId\fR, if there
is any. If there are no transformations added to \fIchannelId\fR, this is
equivalent to \fBchan close\fR of that channel. The result is normally the
empty string, but can be an error in some situations (i.e. where the
underlying system stream is closed and that results in an error).
.\" METHOD: postevent
.TP
\fBchan postevent \fIchannelId eventSpec\fR
.
This subcommand is used by command handlers specified with \fBchan
create\fR. It notifies the channel represented by the handle
\fIchannelId\fR that the event(s) listed in the \fIeventSpec\fR have
occurred. The argument has to be a list containing any of the strings
\fBread\fR and \fBwrite\fR. The list must contain at least one
element as it does not make sense to invoke the command if there are
no events to post.
.RS
.PP
Note that this subcommand can only be used with channel handles that
were created/opened by \fBchan create\fR. All other channels will
cause this subcommand to report an error.
.PP
As only the Tcl level of a channel, i.e. its command handler, should
post events to it we also restrict the usage of this command to the
interpreter that created the channel. In other words, posting events
to a reflected channel from an interpreter that does not contain it's
implementation is not allowed. Attempting to post an event from any
other interpreter will cause this subcommand to report an error.
.PP
Another restriction is that it is not possible to post events that the
I/O core has not registered an interest in. Trying to do so will cause
the method to throw an error. See the command handler method
\fBwatch\fR described in \fBrefchan\fR, the document specifying
the API of command handlers for reflected channels.
.PP
This command is \fBsafe\fR and made accessible to safe interpreters.
It can trigger the execution of \fBchan event\fR handlers, whether in the
current interpreter or in other interpreters or other threads, even
where the event is posted from a safe interpreter and listened for by
a trusted interpreter. \fBChan event\fR handlers are \fIalways\fR
executed in the interpreter that set them up.
.RE
.\" METHOD: push
.TP
\fBchan push \fIchannelId cmdPrefix\fR
.
Adds a new transformation on top of the channel \fIchannelId\fR. The
\fIcmdPrefix\fR argument describes a list of one or more words which represent
a handler that will be used to implement the transformation. The command
prefix must provide the API described in the \fBtranschan\fR manual page.
The result of this subcommand is a handle to the transformation. Note that it
is important to make sure that the transformation is capable of supporting the
channel mode that it is used with or this can make the channel neither
readable nor writable.
.\" METHOD: puts
.TP
\fBchan puts\fR ?\fB\-nonewline\fR? ?\fIchannelId\fR? \fIstring\fR
.
Writes \fIstring\fR to the channel named \fIchannelId\fR followed by a
newline character. A trailing newline character is written unless the
optional flag \fB\-nonewline\fR is given. If \fIchannelId\fR is
omitted, the string is written to the standard output channel,
\fBstdout\fR.
.RS
.PP
Newline characters in the output are translated by \fBchan puts\fR to
platform-specific end-of-line sequences according to the currently
configured value of the \fB\-translation\fR option for the channel
(for example, on PCs newlines are normally replaced with
carriage-return-linefeed sequences; see \fBchan configure\fR above for
details).
.PP
Tcl buffers output internally, so characters written with \fBchan
puts\fR may not appear immediately on the output file or device; Tcl
will normally delay output until the buffer is full or the channel is
closed.  You can force output to appear immediately with the \fBchan
flush\fR command.
.PP
When the output buffer fills up, the \fBchan puts\fR command will
normally block until all the buffered data has been accepted for
output by the operating system.  If \fIchannelId\fR is in non-blocking
mode then the \fBchan puts\fR command will not block even if the
operating system cannot accept the data.  Instead, Tcl continues to
buffer the data and writes it in the background as fast as the
underlying file or device can accept it.  The application must use the
Tcl event loop for non-blocking output to work; otherwise Tcl never
finds out that the file or device is ready for more output data.  It
is possible for an arbitrarily large amount of data to be buffered for
a channel in non-blocking mode, which could consume a large amount of
memory.  To avoid wasting memory, non-blocking I/O should normally be
used in an event-driven fashion with the \fBchan event\fR command
(do not invoke \fBchan puts\fR unless you have recently been notified
via a file event that the channel is ready for more output data).
.PP
The command will raise an error exception with POSIX error code \fBEILSEQ\fR if
the encoding profile \fBstrict\fR is in effect for the channel and the output
data cannot be encoded in the encoding configured for the channel. Data
may be partially written to the channel in this case.
.RE
.\" METHOD: read
.TP
\fBchan read \fIchannelId\fR ?\fInumChars\fR?
.TP
\fBchan read \fR?\fB\-nonewline\fR? \fIchannelId\fR
.
In the first form, the result will be the next \fInumChars\fR
characters read from the channel named \fIchannelId\fR; if
\fInumChars\fR is omitted, all characters up to the point when the
channel would signal a failure (whether an end-of-file, blocked or
other error condition) are read. In the second form (i.e. when
\fInumChars\fR has been omitted) the flag \fB\-nonewline\fR may be
given to indicate that any trailing newline in the string that has
been read should be trimmed.
.RS
.PP
If \fIchannelId\fR is in non-blocking mode, \fBchan read\fR may not
read as many characters as requested: once all available input has
been read, the command will return the data that is available rather
than blocking for more input.  If the channel is configured to use a
multi-byte encoding, then there may actually be some bytes remaining
in the internal buffers that do not form a complete character.  These
bytes will not be returned until a complete character is available or
end-of-file is reached.  The \fB\-nonewline\fR switch is ignored if
the command returns before reaching the end of the file.
.PP
\fBChan read\fR translates end-of-line sequences in the input into
newline characters according to the \fB\-translation\fR option for the
channel (see \fBchan configure\fR above for a discussion on the ways
in which \fBchan configure\fR will alter input).
.PP
When reading from a serial port, most applications should configure
the serial port channel to be non-blocking, like this:
.PP
.CS
\fBchan configure \fIchannelId \fB\-blocking \fI0\fR.
.CE
.PP
Then \fBchan read\fR behaves much like described above.  Note that
most serial ports are comparatively slow; it is entirely possible to
get a \fBreadable\fR event for each character read from them. Care
must be taken when using \fBchan read\fR on blocking serial ports:
.TP
\fBchan read \fIchannelId numChars\fR
.
In this form \fBchan read\fR blocks until \fInumChars\fR have been
received from the serial port.
.TP
\fBchan read \fIchannelId\fR
.
In this form \fBchan read\fR blocks until the reception of the
end-of-file character, see \fBchan configure -eofchar\fR. If there no
end-of-file character has been configured for the channel, then
\fBchan read\fR will block forever.
.PP
If the encoding profile \fBstrict\fR is in effect for the channel, the command
will raise an exception with the POSIX error code \fBEILSEQ\fR if any encoding
errors are encountered in the channel input data. If the channel is in blocking
mode, the error is thrown after advancing the file pointer to the beginning of
the invalid data. The successfully decoded leading portion of the data prior to
the error location is returned as the value of the \fB\-data\fR key of the error
option dictionary. If the channel is in non-blocking mode, the successfully
decoded portion of data is returned by the command without an error
exception being raised. A subsequent read will start at the invalid data
and immediately raise a \fBEILSEQ\fR POSIX error exception. Unlike the
blocking channel case, the \fB\-data\fR key is not present in the
error option dictionary. In the case of exception thrown due to encoding
errors, it is possible to introspect, and in some cases recover, by
changing the encoding in use. See \fBENCODING ERROR EXAMPLES\fR later.
.RE
.\" METHOD: seek
.TP
\fBchan seek \fIchannelId offset\fR ?\fIorigin\fR?
.
Sets the current access position within the underlying data stream for
the channel named \fIchannelId\fR to be \fIoffset\fR bytes relative to
\fIorigin\fR. \fIOffset\fR must be an integer (which may be negative)
and \fIorigin\fR must be one of the following:
.RS
.IP \fBstart\fR
The new access position will be \fIoffset\fR bytes from the start
of the underlying file or device.
.IP \fBcurrent\fR
The new access position will be \fIoffset\fR bytes from the current
access position; a negative \fIoffset\fR moves the access position
backwards in the underlying file or device.
.IP \fBend\fR
The new access position will be \fIoffset\fR bytes from the end of the
file or device.  A negative \fIoffset\fR places the access position
before the end of file, and a positive \fIoffset\fR places the access
position after the end of file.
.PP
The \fIorigin\fR argument defaults to \fBstart\fR.
.PP
\fBChan seek\fR flushes all buffered output for the channel before the
command returns, even if the channel is in non-blocking mode.  It also
discards any buffered and unread input.  This command returns an empty
string.  An error occurs if this command is applied to channels whose
underlying file or device does not support seeking.
.PP
Note that \fIoffset\fR values are byte offsets, not character offsets.
Both \fBchan seek\fR and \fBchan tell\fR operate in terms of bytes,
not characters, unlike \fBchan read\fR.
.RE
.\" METHOD: tell
.TP
\fBchan tell \fIchannelId\fR
.
Returns a number giving the current access position within the
underlying data stream for the channel named \fIchannelId\fR. This
value returned is a byte offset that can be passed to \fBchan seek\fR
in order to set the channel to a particular position.  Note that this
value is in terms of bytes, not characters like \fBchan read\fR.  The
value returned is -1 for channels that do not support seeking.
.\" METHOD: truncate
.TP
\fBchan truncate \fIchannelId\fR ?\fIlength\fR?
.
Sets the byte length of the underlying data stream for the channel
named \fIchannelId\fR to be \fIlength\fR (or to the current byte
offset within the underlying data stream if \fIlength\fR is
omitted). The channel is flushed before truncation.
.
.SH EXAMPLES
.SS "SIMPLE CHANNEL OPERATION EXAMPLES"
.PP
Instruct Tcl to always send output to \fBstdout\fR immediately,
whether or not it is to a terminal:
.PP
.CS
\fBfconfigure\fR stdout -buffering none
.CE
.PP
In the following example a file is opened using the encoding CP1252, which is
common on Windows, searches for a string, rewrites that part, and truncates the
file two lines later.
.PP
.CS
set f [open somefile.txt r+]
\fBchan configure\fR $f -encoding cp1252
set offset 0

\fI# Search for string "FOOBAR" in the file\fR
while {[\fBchan gets\fR $f line] >= 0} {
    set idx [string first FOOBAR $line]
    if {$idx >= 0} {
        \fI# Found it; rewrite line\fR

        \fBchan seek\fR $f [expr {$offset + $idx}]
        \fBchan puts\fR -nonewline $f BARFOO

        \fI# Skip to end of following line, and truncate\fR
        \fBchan gets\fR $f
        \fBchan gets\fR $f
        \fBchan truncate\fR $f

        \fI# Stop searching the file now\fR
        break
    }

    \fI# Save offset of start of next line for later\fR
    set offset [\fBchan tell\fR $f]
}
\fBchan close\fR $f
.CE
.PP
This example illustrates flushing of a channel. The user is
prompted for some information. Because the standard input channel
is line buffered, it must be flushed for the user to see the prompt.
.PP
.CS
chan puts -nonewline "Please type your name: "
\fBchan flush\fR stdout
chan gets stdin name
chan puts "Hello there, $name!"
.CE
.PP
This example reads a file one line at a time and prints it out with
the current line number attached to the start of each line.
.PP
.CS
set chan [open "some.file.txt"]
set lineNumber 0
while {[\fBchan gets\fR $chan line] >= 0} {
    chan puts "[incr lineNumber]: $line"
}
chan close $chan
.CE
.PP
In this example illustrating event driven reads,
\fBGetData\fR will be called with the channel as an
argument whenever $chan becomes readable. The \fBread\fR call will
read whatever binary data is currently available without blocking.
Here the channel has the fileevent removed when an end of file
occurs to avoid being continually called (see above). Alternatively
the channel may be closed on this condition.
.PP
.CS
proc GetData {chan} {
    set data [chan read $chan]
    chan puts "[string length $data] $data"
    if {[chan eof $chan]} {
        chan event $chan readable {}
    }
}

chan configure $chan -blocking 0 -encoding binary
\fBchan event\fR $chan readable [list GetData $chan]
.CE
.PP
The next example is similar but uses \fBchan gets\fR to read
line-oriented data.
.PP
.CS
proc GetData {chan} {
    if {[chan gets $chan line] >= 0} {
        chan puts $line
    }
    if {[chan eof $chan]} {
        chan close $chan
    }
}

chan configure $chan -blocking 0 -buffering line -translation crlf
\fBchan event\fR $chan readable [list GetData $chan]
.CE
.PP
A network server that echoes its input line-by-line without
preventing servicing of other connections at the same time:
.PP
.CS
# This is a very simple logger...
proc log {message} {
    \fBchan puts\fR stdout $message
}

# This is called whenever a new client connects to the server
proc connect {chan host port} {
    set clientName [format <%s:%d> $host $port]
    log "connection from $clientName"
    \fBchan configure\fR $chan -blocking 0 -buffering line
    \fBchan event\fR $chan readable [list echoLine $chan $clientName]
}

# This is called whenever either at least one byte of input
# data is available, or the channel was closed by the client.
proc echoLine {chan clientName} {
    \fBchan gets\fR $chan line
    if {[\fBchan eof\fR $chan]} {
        log "finishing connection from $clientName"
        \fBchan close\fR $chan
    } elseif {![\fBchan blocked\fR $chan]} {
        # Didn't block waiting for end-of-line
        log "$clientName - $line"
        \fBchan puts\fR $chan $line
    }
}

# Create the server socket and enter the event-loop to wait
# for incoming connections...
socket -server connect 12345
vwait forever
.CE
.PP
The following example reads a PPM-format image from a file
combining ASCII and binary content.
.PP
.CS
# Open the file and put it into Unix ASCII mode
set f [open teapot.ppm]
\fBchan configure\fR $f -encoding ascii -translation lf

# Get the header
if {[chan gets $f] ne "P6"} {
    error "not a raw\-bits PPM"
}

# Read lines until we have got non-comment lines
# that supply us with three decimal values.
set words {}
while {[llength $words] < 3} {
    chan gets $f line
    if {[string match "#*" $line]} continue
    lappend words {*}[join [scan $line %d%d%d]]
}

# Those words supply the size of the image and its
# overall depth per channel. Assign to variables.
lassign $words xSize ySize depth

# Now switch to binary mode to pull in the data,
# one byte per channel (red,green,blue) per pixel.
\fBchan configure\fR $f -translation binary
set numDataBytes [expr {3 * $xSize * $ySize}]
set data [chan read $f $numDataBytes]

close $f
.CE
.SS "FILE SEEK EXAMPLES"
.PP
Read a file twice:
.PP
.CS
set f [open file.txt]
set data1 [chan read $f]
\fBchan seek\fR $f 0
set data2 [chan read $f]
chan close $f
# $data1 eq $data2 if the file wasn't updated
.CE
.PP
Read the last 10 bytes from a file:
.PP
.CS
set f [open file.data]
# This is guaranteed to work with binary data but
# may fail with other encodings...
chan configure $f -translation binary
\fBchan seek\fR $f -10 end
set data [chan read $f 10]
chan close $f
.CE
.PP
Read a line from a file channel only if it starts with \fBfoobar\fR:
.PP
.CS
# Save the offset in case we need to undo the read...
set offset [\fBtell\fR $chan]
if {[read $chan 6] eq "foobar"} {
    gets $chan line
} else {
    set line {}
    # Undo the read...
    seek $chan $offset
}
.CE
.SS "ENCODING ERROR EXAMPLES"
.PP
The example below illustrates handling of an encoding error encountered
during channel input. First, creation of a test file containing
the invalid UTF-8 sequence (\fBA \\xC3 B\fR):
.PP
.CS
% set f [open test_A_195_B.txt wb]; chan puts -nonewline $f A\\xC3B; chan close $f
.CE
.PP
An attempt to read the file will result in an encoding error which is
then introspected by switching the channel to binary mode. Note in the
example that when the error is reported the file position remains
unchanged so that the \fBchan gets\fR during recovery returns the
full line.
.PP
.CS
% set f [open test_A_195_B.txt r]
file384b6a8
% chan configure $f -encoding utf-8
% catch {chan gets $f} e d
1
% set d
-code 1 -level 0
-errorstack {INNER {invokeStk1 gets file384b6a8}}
-errorcode {POSIX EILSEQ {invalid or incomplete multibyte or wide character}}
-errorinfo {...} -errorline 1
% chan tell $f
0
% chan configure $f -encoding binary
% chan gets $f
AÃB
.CE
.PP
The following example is similar to the above but demonstrates recovery after a
blocking read. The successfully decoded data "A" is returned in the error options
dictionary key \fB\-data\fR. The file position is advanced on the encoding error
position 1. The data at the error position is thus recovered by the next
\fBchan read\fR command.
.PP
.CS
% set f [open test_A_195_B.txt r]
file35a65a0
% chan configure $f -encoding utf-8 -blocking 1
% catch {chan read $f} e d
1
% set d
-data A -code 1 -level 0
-errorstack {INNER {invokeStk1 read file35a65a0}}
-errorcode {POSIX EILSEQ {invalid or incomplete multibyte or wide character}}
-errorinfo {...} -errorline 1
% chan tell $f
1
% chan configure $f -encoding binary
% chan read $f
ÃB
% chan close $f
.CE
.PP
Finally the same example, but this time with a non-blocking channel.
.PP
.CS
% set f [open test_A_195_B.txt r]
file35a65a0
% chan configure $f -encoding utf-8 -blocking 0
% chan read $f
A
% chan tell $f
1
% catch {chan read $f} e d
1
% set d
-code 1 -level 0
-errorstack {INNER {invokeStk1 read file384b228}}
-errorcode {POSIX EILSEQ {invalid or incomplete multibyte or wide character}}
-errorinfo {...} -errorline 1
.CE

.SS "CHANNEL COPY EXAMPLES"
.PP
The first example transfers the contents of one channel exactly to
another. Note that when copying one file to another, it is better to
use \fBfile copy\fR which also copies file metadata (e.g. the file
access permissions) where possible.
.PP
.CS
\fBchan configure\fR $in -translation binary
\fBchan configure\fR $out -translation binary
\fBchan copy\fR $in $out
.CE
.PP
This second example shows how the callback gets
passed the number of bytes transferred.
It also uses vwait to put the application into the event loop.
Of course, this simplified example could be done without the command
callback.
.PP
.CS
proc Cleanup {in out bytes {error {}}} {
    global total
    set total $bytes
    \fBchan close\fR $in
    \fBchan close\fR $out
    if {$error ne ""} {
        # error occurred during the copy
    }
}

set in [open $file1]
set out [socket $server $port]
\fBchan copy\fR $in $out -command [list Cleanup $in $out]
vwait total
.CE
.PP
The third example copies in chunks and tests for end of file
in the command callback.
.PP
.CS
proc CopyMore {in out chunk bytes {error {}}} {
    global total done
    incr total $bytes
    if {($error ne "") || [\fBchan eof\fR $in]} {
        set done $total
        \fBchan close\fR $in
        \fBchan close\fR $out
    } else {
        \fBchan copy\fR $in $out -size $chunk \e
            -command [list CopyMore $in $out $chunk]
    }
}

set in [open $file1]
set out [socket $server $port]
set chunk 1024
set total 0
\fBchan copy\fR $in $out -size $chunk \e
    -command [list CopyMore $in $out $chunk]
vwait done
.CE
.PP
The fourth example starts an asynchronous, bidirectional copy between
two sockets. Those could also be pipes from two bidirectional pipelines
(e.g., \fI[open "|hal 9000" r+]\fR); the conversation will remain
essentially secret to the script, since all four \fBchan event\fR slots
are busy, though any transforms that are \fBchan push\fRed on the
channels will be able to observe the passing traffic.
.PP
.CS
proc Done {dir args} {
    global flows done
    \fBchan puts\fR "$dir is over."
    incr flows -1
    if {$flows <= 0} {
        set done 1
    }
}

set flows 2
\fBchan copy\fR $sok1 $sok2 -command [list Done UP]
\fBchan copy\fR $sok2 $sok1 -command [list Done DOWN]
vwait done
.CE
.SH "SEE ALSO"
close(n), eof(n), fblocked(n), fconfigure(n), fcopy(n), file(n),
fileevent(n), flush(n), gets(n), open(n), puts(n), read(n), seek(n),
socket(n), tell(n), refchan(n), transchan(n),
Tcl_StandardChannels(3)
.SH KEYWORDS
blocking, channel, end of file, events, input, non-blocking,
offset, output, readable, seek, stdio, tell, writable
'\" Local Variables:
'\" mode: nroff
'\" End: