puts - Write to a channel
Writes the characters given by string
to the channel given by
must be an identifier for an open channel such as a Tcl
standard channel ( stdout
), the return value from an
invocation of open
, 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
normally outputs a newline character after string
, but this
feature may be suppressed by specifying the -nonewline
Newline characters in the output are translated by puts
platform-specific end-of-line sequences according to the current value of the
option for the channel (for example, on PCs newlines are
normally replaced with carriage-return-linefeed sequences. See the
manual entry for a discussion on ways in which
will alter output.
Tcl buffers output internally, so characters written with puts
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
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
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
Print a message in several parts:
puts -nonewline "Hello, "
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!"
file(3tcl), fileevent(3tcl), Tcl_StandardChannels(3tcl)
channel, newline, output, write