read - Read from a channel
read ?
-nonewline?
channelId
read channelId numChars
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.
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.
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]
file(3tcl),
eof(3tcl),
fblocked(3tcl),
fconfigure(3tcl),
Tcl_StandardChannels(3tcl)
blocking, channel, end of line, end of file, nonblocking, read, translation,
encoding