Tcl8.6.10/Tk8.6.10 Documentation > Tcl Commands > puts

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NAME

puts — Write to a channel

SYNOPSIS

puts ?-nonewline? ?channelId? string

DESCRIPTION

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).

EXAMPLES

Write a short message to the console (or wherever stdout is directed):

puts "Hello, World!"

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

SEE ALSO

file, fileevent, Tcl_StandardChannels

KEYWORDS

channel, newline, output, write