fileevent(n) Tcl Built-In Commands fileevent(n)______________________________________________________________________________NAMEfileevent - Execute a script when a channel becomes readable or
writable
SYNOPSISfileevent channelId readable ?script?
fileevent channelId writable ?script?
_________________________________________________________________DESCRIPTION
This command is used to create file event handlers. A file event han‐
dler is a binding between a channel and a script, such that the script
is evaluated whenever the channel becomes readable or writable. File
event handlers are most commonly used to allow data to be received from
another process on an event-driven basis, so that the receiver can con‐
tinue to interact with the user while waiting for the data to arrive.
If an application invokes gets or read on a blocking channel when there
is no input data available, the process will block; until the input
data arrives, it will not be able to service other events, so it will
appear to the user to “freeze up”. With fileevent, the process can
tell when data is present and only invoke gets or read when they will
not block.
The channelId argument to fileevent refers to an open channel such as a
Tcl standard channel (stdin, 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.
If the script argument is specified, then fileevent creates a new event
handler: script will be evaluated whenever the channel becomes read‐
able or writable (depending on the second argument to fileevent). In
this case fileevent returns an empty string. The readable and writable
event handlers for a file are independent, and may be created and
deleted separately. However, there may be at most one readable and one
writable handler for a file at a given time in a given interpreter. If
fileevent is called when the specified handler already exists in the
invoking interpreter, the new script replaces the old one.
If the script argument is not specified, fileevent returns the current
script for channelId, or an empty string if there is none. If the
script argument is specified as an empty string then the event handler
is deleted, so that no script will be invoked. A file event handler is
also deleted automatically whenever its channel is closed or its inter‐
preter is deleted.
A channel is considered to be readable if there is unread data avail‐
able on the underlying device. A channel is also considered to be
readable if there is unread data in an input buffer, except in the spe‐
cial case where the most recent attempt to read from the channel was a
gets call that could not find a complete line in the input buffer.
This feature allows a file to be read a line at a time in nonblocking
mode using events. A channel is also considered to be readable if an
end of file or error condition is present on the underlying file or
device. It is important for script to check for these conditions and
handle them appropriately; for example, if there is no special check
for end of file, an infinite loop may occur where script reads no data,
returns, and is immediately invoked again.
A channel is considered to be writable if at least one byte of data can
be written to the underlying file or device without blocking, or if an
error condition is present on the underlying file or device.
Event-driven I/O works best for channels that have been placed into
nonblocking mode with the fconfigure command. In blocking mode, a puts
command may block if you give it more data than the underlying file or
device can accept, and a gets or read command will block if you attempt
to read more data than is ready; a readable underlying file or device
may not even guarantee that a blocking [read 1] will succeed (counter-
examples being multi-byte encodings, compression or encryption trans‐
forms ). In all such cases, no events will be processed while the com‐
mands block.
In nonblocking mode puts, read, and gets never block. See the documen‐
tation for the individual commands for information on how they handle
blocking and nonblocking channels.
Testing for the end of file condition should be done after any attempts
read the channel data. The eof flag is set once an attempt to read the
end of data has occurred and testing before this read will require an
additional event to be fired.
The script for a file event is executed at global level (outside the
context of any Tcl procedure) in the interpreter in which the fileevent
command was invoked. If an error occurs while executing the script
then the command registered with interp bgerror is used to report the
error. In addition, the file event handler is deleted if it ever
returns an error; this is done in order to prevent infinite loops due
to buggy handlers.
EXAMPLE
In this setup GetData will be called with the channel as an argument
whenever $chan becomes readable. The read call will read whatever
binary data is currently available without blocking. Here the channel
has the fileevent removed when an end of file occurs to avoid being
continually called (see above). Alternatively the channel may be closed
on this condition.
proc GetData {chan} {
set data [read $chan]
puts "[string length $data] $data"
if {[eof $chan]} {
fileevent $chan readable {}
}
}
fconfigure $chan -blocking 0 -encoding binary
fileevent $chan readable [list GetData $chan]
The next example demonstrates use of gets to read line-oriented data.
proc GetData {chan} {
if {[gets $chan line] >= 0} {
puts $line
}
if {[eof $chan]} {
close $chan
}
}
fconfigure $chan -blocking 0 -buffering line -translation crlf
fileevent $chan readable [list GetData $chan]
CREDITSfileevent is based on the addinput command created by Mark Diekhans.
SEE ALSOfconfigure(n), gets(n), interp(n), puts(n), read(n), Tcl_StandardChan‐
nels(3)KEYWORDS
asynchronous I/O, blocking, channel, event handler, nonblocking, read‐
able, script, writable.
Tcl 7.5 fileevent(n)
