puts - Write to a channel
puts ?
-nonewline? ?
channelId?
string
Writes the characters given by
string to the channel given by
channelId.
ChannelId must be an identifier for an open channel such as a Tcl
standard channel (
stdout or
stderr), 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 output.
If no
channelId is specified then it defaults to
stdout.
Puts normally outputs a newline character after
string, but this
feature may be suppressed by specifying the
-nonewline switch.
Newline characters in the output are translated by
puts to
platform-specific end-of-line sequences according to the current value of the
-translation option for the channel (for example, on PCs newlines are
normally replaced with carriage-return-linefeed sequences. See the
fconfigure manual entry for a discussion on ways in which
fconfigure will alter output.
Tcl buffers output internally, so characters written with
puts 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
flush command.
When the output buffer fills up, the
puts command will normally block
until all the buffered data has been accepted for output by the operating
system. If
channelId is in nonblocking mode then the
puts
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 nonblocking 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
nonblocking mode, which could consume a large amount of memory. To avoid
wasting memory, nonblocking I/O should normally be used in an event-driven
fashion with the
fileevent command (do not invoke
puts unless
you have recently been notified via a file event that the channel is ready for
more output data).
Write a short message to the console (or wherever
stdout is directed):
Print a message in several parts:
puts -nonewline "Hello, "
puts "World!"
Print a message to the standard error channel:
puts stderr "Hello, World!"
Append a log message to a file:
set chan [open my.log a]
set timestamp [clock format [clock seconds]]
puts $chan "$timestamp - Hello, World!"
close $chan
file(3tcl),
fileevent(3tcl),
Tcl_StandardChannels(3tcl)
channel, newline, output, write