path: root/doc/fconfigure.n

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.TH fconfigure n 8.3 Tcl "Tcl Built-In Commands"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
fconfigure \- Set and get options on a channel
.SH SYNOPSIS
.nf
\fBfconfigure \fIchannelId\fR
\fBfconfigure \fIchannelId\fR \fIname\fR
\fBfconfigure \fIchannelId\fR \fIname value \fR?\fIname value ...\fR?
.fi
.BE
.SH DESCRIPTION
.PP
The \fBfconfigure\fR command sets and retrieves options for channels.
.PP
\fIChannelId\fR identifies the channel for which to set or query an
option and must refer to an open channel such as a Tcl standard
channel (\fBstdin\fR, \fBstdout\fR, or \fBstderr\fR), the return
value from an invocation of \fBopen\fR or \fBsocket\fR, or the result
of a channel creation command provided by a Tcl extension.
.PP
If no \fIname\fR or \fIvalue\fR arguments are supplied, the command
returns a list containing alternating option names and values for the channel.
If \fIname\fR is supplied but no \fIvalue\fR then the command returns
the current value of the given option.
If one or more pairs of \fIname\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 channels for the options
that that specific type of channel supports. 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.
.TP
\fB\-blocking\fR \fIboolean\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
nonblocking mode it will affect the operation of the \fBgets\fR,
\fBread\fR, \fBputs\fR, \fBflush\fR, and \fBclose\fR commands by
allowing them to operate asynchronously;
see the documentation for those commands for details.
For nonblocking 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).
.TP
\fB\-buffering\fR \fInewValue\fR
.
If \fInewValue\fR is \fBfull\fR then the I/O system will buffer output
until its internal buffer is full or until the \fBflush\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.
.TP
\fB\-buffersize\fR \fInewSize\fR
.
\fINewvalue\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. \fINewvalue\fR must be between one and one million, allowing
buffers of one to one million bytes in size.
.TP
\fB\-encoding\fR \fIname\fR
.
This option is used to specify the encoding of the channel, 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
.TP
\fB\-eofchar\fR \fIchar\fR
.TP
\fB\-eofchar\fR \fB{\fIinChar outChar\fB}\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.  For
output, the end-of-file character is output when the channel is closed.
If \fIchar\fR is the empty string, then there is no special end of file
character marker.  For read-write channels, a two-element list specifies
the end of file marker for input and output, respectively.  As a
convenience, when setting the end-of-file character for a read-write
channel you can specify a single value that will apply to both reading
and writing.  When querying the end-of-file character of a read-write
channel, a two-element list will always be returned.  The default value
for \fB\-eofchar\fR is the empty string in all cases except for files
under Windows.  In that case the \fB\-eofchar\fR is Control-z (\ex1a) for
reading and the empty string for writing.
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.
.TP
\fB\-translation\fR \fImode\fR
.TP
\fB\-translation\fR \fB{\fIinMode outMode\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 \fBgets\fR
and \fBread\fR) the Tcl I/O system automatically translates the external
end-of-line representation into newline characters.  Upon output (i.e.,
with \fBputs\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:
.TP
\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.
.TP
\fBbinary\fR
.
No end-of-line translations are performed.  This is nearly identical to
\fBlf\fR mode, except that in addition \fBbinary\fR mode also sets the
end-of-file character to the empty string (which disables it) and sets the
encoding to \fBbinary\fR (which disables encoding filtering).  See the
description of \fB\-eofchar\fR and \fB\-encoding\fR for more information.
.RS
.PP
Internally, i.e. when it comes to the actual behaviour of the
translator this value \fBis\fR identical to \fBlf\fR and is therefore
reported as such when queried. Even if \fBbinary\fR was used to set
the translation.
.RE
.TP
\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.
.TP
\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.
.TP
\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
.PP
.SH "STANDARD CHANNELS"
.PP
The Tcl standard channels (\fBstdin\fR, \fBstdout\fR, and \fBstderr\fR)
can be configured through this command like every other channel opened
by the Tcl library. Beyond the standard options described above they
will also support any special option according to their current type.
If, for example, a Tcl application is started by the \fBinet\fR
super-server common on Unix system its Tcl standard channels will be
sockets and thus support the socket options.
.SH 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
Open a socket and read lines from it without ever blocking the
processing of other events:
.PP
.CS
set s [socket some.where.com 12345]
\fBfconfigure\fR $s -blocking 0
fileevent $s readable "readMe $s"
proc readMe chan {
    if {[gets $chan line] < 0} {
        if {[eof $chan]} {
            close $chan
            return
        }
        # Could not read a complete line this time; Tcl's
        # internal buffering will hold the partial line for us
        # until some more data is available over the socket.
    } else {
        puts stdout $line
    }
}
.CE
.PP
Read a PPM-format image from a file:
.PP
.CS
# Open the file and put it into Unix ASCII mode
set f [open teapot.ppm]
\fBfconfigure\fR $f \-encoding ascii \-translation lf

# Get the header
if {[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} {
    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.
\fBfconfigure\fR $f \-translation binary
set numDataBytes [expr {3 * $xSize * $ySize}]
set data [read $f $numDataBytes]

close $f
.CE
.SH "SEE ALSO"
close(n), flush(n), gets(n), open(n), puts(n), read(n), socket(n),
Tcl_StandardChannels(3)
.SH KEYWORDS
blocking, buffering, carriage return, end of line, flushing, linemode,
newline, nonblocking, platform, translation, encoding, filter, byte array,
binary
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