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, eof, fblocked, fconfigure, Tcl_StandardChannels
blocking, channel, end of line, end of file, nonblocking, read, translation, encoding
Copyright © 1995-1997 Roger E. Critchlow Jr.
Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.