read(3tcl)
NAME
read - Read from a channel
SYNOPSIS
read ?-nonewline? channelId
read channelId numChars
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DESCRIPTION
In the first form, the read command reads all of the data from
channelId up to the end of the file. If the -nonewline switch is
specified then the last character of the file is discarded if it is a
newline. In the second form, the extra argument specifies how many
characters to read. Exactly that many characters will be read and
returned, unless there are fewer than numChars left in the file; in
this case all the remaining characters are returned. If the channel is
configured to use a multi-byte encoding, then the number of characters
read may not be the same as the number of bytes read.
ChannelId must be an identifier for an open channel such as the Tcl
standard input channel (stdin), the return value from an invocation of
open or socket, or the result of a channel creation command provided by
a Tcl extension. The channel must have been opened for input.
If channelId is in nonblocking mode, the command 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 -nonewline switch is ignored if the command returns
before reaching the end of the file.
Read translates end-of-line sequences in the input into newline
characters according to the -translation option for the channel. See
the fconfigure manual entry for a discussion on ways in which
fconfigure will alter input.
USE WITH SERIAL PORTS
For most applications a channel connected to a serial port should be
configured to be nonblocking: fconfigure channelId -blocking 0. Then
read behaves much like described above. Care must be taken when using
read on blocking serial ports:
read channelId numChars
In this form read blocks until numChars have been received from
the serial port.
read channelId
In this form read blocks until the reception of the end-of-file
character, see fconfigure -eofchar. If there no end-of-file
character has been configured for the channel, then read will
block forever.
EXAMPLE
This example code reads a file all at once, and splits it into a list,
with each line in the file corresponding to an element in the list:
set fl [open /proc/meminfo]
set data [read $fl]
close $fl
set lines [split $data \n]
SEE ALSO
file(3tcl), eof(3tcl), fblocked(3tcl), fconfigure(3tcl),
Tcl_StandardChannels(3tcl)
KEYWORDS
blocking, channel, end of line, end of file, nonblocking, read,
translation, encoding
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