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bgexec(n) BLT Built-In Commands bgexec(n)
______________________________________________________________________________
NAME
bgexec - Run programs in the background while handling Tk events.
SYNOPSIS
blt::bgexec varName ?option value?... program ?arg?...
_________________________________________________________________
DESCRIPTION
The bgexec command executes programs in the background, allowing Tk to
handle events. A global Tcl variable varName is set when the program
has completed.
INTRODUCTION
Tcl's exec command is very useful for gathering information from the
operating system. It runs a program and returns the output as its
result. This works well for Tcl-only applications. But for Tk applica-
tions, a problem occurs when the program takes time to process. Let's
say we want the get the disk usage of a directory. We'll use the Unix
program du to get the summary.
set out [exec du -s $dir]
puts "Disk usage for $dir is $out"
While du is running, scrollbars won't respond. None of the Tk widgets
will be redrawn properly. The send command won't work. And the worst
part is that the application appears hung up or dead. The problem is
that while exec is waiting for du to finish, Tk is not able to handle X
events.
The bgexec command performs the same functions as exec, but also allows
Tk to handle events. You can execute a long-running program and the Tk
widgets will behave normally. When the program finishes, its output
and the exit status are written to Tcl variables. This makes it easy
to monitor and save the output of a program.
EXAMPLE
Here is the disk usage example again, this time using bgexec. The syn-
tax to invoke "du" is exactly the same as the previous example, when we
used exec.
global myStatus myOutput
blt::bgexec myStatus -output myOutput du -s $dir
puts "Disk usage for $dir is $myOutput"
Two global variables, myStatus and myOutput, will be set by bgexec when
du has completed. MyStatus will contain the program's exit status.
MyOutput, specified by the -output option, will store the output of the
program.
You can also terminate the program by setting the variable myStatus.
If myStatus is set before du has completed, the process is killed.
Under Unix, this is done sending by a configurable signal (by default
it's SIGKILL). Under Win32, this is done by calling TerminateProcess.
It makes no difference what myStatus is set to.
set myStatus {}
There are several bgexec options to collect different types of informa-
tion.
global myStatus myOutput myErrs
blt::bgexec myStatus -output myOutput -error myErrs du -s $dir
The -error option is similar to -output. It sets a global variable
when the program completes. The variable will contain any data written
to stderr by the program.
The -output and -error variables are set only after the program com-
pletes. But if the program takes a long time, to run you may want to
receive its partial output. You can gather data as it becomes avail-
able using the -onoutput option. It specifies a Tcl command prefix.
Whenever new data is available, this command is executed, with the data
appended as an argument to the command.
proc GetInfo { data } {
puts $data
}
blt::bgexec myStatus -onoutput GetInfo du -s $dir
When output is available, the procedure GetInfo is called. The
-onerror option performs a similar function for the stderr data stream.
Like exec, bgexec returns an error if the exit code of the program is
not zero. If you think you may get a non-zero exit code, you might
want to invoke bgexec from within a catch.
catch { blt::bgexec myStatus -output myOutput du -s $dir }
By default, bgexec will wait for the program to finish. But you can
detach the program making ampersand (&) the last argument on the com-
mand line.
global myStatus myOutput
blt::bgexec myStatus -output myOutput du -s $dir &
Bgexec will return immediately and its result will be a list of the
spawned process ids. If at some point you need to wait for the program
to finish up, you can use tkwait. When the program finishes, the vari-
able myStatus will be written to, breaking out the tkwait command.
global myStatus myOutput
blt::bgexec myStatus -output myOutput du -s $dir &
...
tkwait variable myStatus
SYNTAX
The bgexec command takes the following form:
blt::bgexec varName ?option value?... program ?arg?...
VarName is the name of a global variable which is set when program has
finished executing. The exit status of will be stored in varName. The
exit status is a list of a status token, the process-id of the program,
the exit code, and a status message. You can also prematurely termi-
nate the program by setting varName. Under Unix, the program will be
sent a signal to terminate it (by default the signal is a SIGKILL; see
the -killsignal option).
Program is the name of the program to be executed and args are any
extra arguments for program. The syntax of program and args is the
same as the exec command. So you can redirect I/O, execute pipelines,
etc. (see the exec manual for further information) just like exec. If
the last argument is an ampersand (&), the program will be run
detached, and bgexec will return immediately. VarName will still be
set with the return status when program completes.
OPTIONS
Option refers to the switch name always beginning with a dash (-).
Value is the value of the option. Option-value pairs are terminated
either by the program name, or double dashes (--). The following
options are available for bgexec:
-decodeerror encodingName
Specifies the encoding of the stderr channel. This affects only
data returned to the Tcl interpreter. No translation is done on
file redirection.
For example if data is to be converted from Unicode for use in
Tcl, you would use the "unicode" encoding. The default is that
no tranlation is performed.
-decodeoutput encodingName
Specifies the encoding of the stdout channels. This affects
only data returned to the Tcl interpreter. No translation is
done on file redirection.
For example if data is to be converted from Unicode for use in
Tcl, you would use the "unicode" encoding. The default is that
no tranlation is performed.
-error varName
Specifies that a global variable varName is to be set with the
contents of stderr after the program has completed.
-keepnewline boolean
Specifies that a trailing newline should be retained in the out-
put. If boolean is true, the trailing newline is truncated from
the output of the -onoutput and -output variables. The default
value is true.
-killsignal signal
Specifies the signal to be sent to the program when terminating.
This is available only under Unix. Signal can either be a num-
ber (typically 1-32) or a mnemonic (such as SIGINT). If signal
is the empty string, then no signal is sent. The default signal
is 9 (SIGKILL).
-lasterror varName
Specifies a variable varName that is updated whenever data
becomes available from standard error of the program. VarName
is a global variable. Unlike the -error option, data is avail-
able as soon as it arrives.
-lastoutput varName
Specifies a variable varName that is updated whenever data
becomes available from standard output of the program. VarName
is a global variable. Unlike the -output option, data is avail-
able as soon as it arrives.
-linebuffered boolean
Specifies that updates should be made on a line-by-line basis.
Normally when new data is available bgexec will set the variable
(-lastoutput and -lasterror options) or invoke the command
(-onoutput and -onerror options) delivering all the new data
currently available. If boolean is true, only one line at a
time will be delivered. This can be useful when you want to
process the output on a line-by-line basis. The default value
is false.
-output varName
Specifies that a global variable varName is to be set with the
output of the program, once it has completed. If this option is
not set, no output will be accumulated.
-onerror command
Specifies the start of a Tcl command that will be executed when-
ever new data is available from standard error. The data is
appended to the command as an extra argument before it is exe-
cuted.
-onoutput command
Specifies the start of a Tcl command that will be executed when-
ever new data is available from standard output. The data is
appended to the command as an extra argument before it is exe-
cuted.
-update varName
Deprecated. This option is replaced by -lasterror.
-- This marks the end of the options. The following argument will
be considered the name of a program even if it starts with a
dash (-).
PREEMPTION
Because bgexec allows Tk to handle events while a program is running,
it's possible for an application to preempt itself with further user-
interactions. Let's say your application has a button that runs the
disk usage example. And while the du program is running, the user
accidently presses the button again. A second bgexec program will pre-
empt the first. What this means is that the first program can not fin-
ish until the second program has completed.
Care must be taken to prevent an application from preempting itself by
blocking further user-interactions (such as button clicks). The BLT
busy command is very useful for just these situations. See the busy
manual for details.
DIFFERENCES WITH FILEEVENT
Since Tk 4.0, a subset of bgexec can be also achieved using the
fileevent command. The steps for running a program in the background
are:
Execute the program with the open command (using the "|" syntax) and
save the file handle.
global fileId
set fileId [open "|du -s $dir" r]
Next register a Tcl code snippet with fileevent to be run whenever out-
put is available on the file handle. The code snippet will read from
the file handle and save the output in a variable.
fileevent fileId readable {
if { [gets $fileId line] < 0 } {
close $fileId
set output $temp
unset fileId temp
} else {
append temp $line
}
}
The biggest advantage of bgexec is that, unlike fileevent, it requires
no additional Tcl code to run a program. It's simpler and less error
prone. You don't have to worry about non-blocking I/O. It's handled
tranparently for you.
Bgexec runs programs that fileevent can not. Fileevent assumes that
the when stdout is closed the program has completed. But some pro-
grams, like the Unix compress program, reopen stdout, fooling fileevent
into thinking the program has terminated. In the example above, we
assume that the program will write and flush its output line-by-line.
However running another program, your application may block in the gets
command reading a partial line.
Bgexec lets you get back the exit status of the program. It also allows
you to collect data from both stdout and stderr simultaneously.
Finally, since data collection is handled in C code, bgexec is faster.
You get back to the Tk event loop more quickly, making your application
seem more responsive.
SEE ALSO
busy, exec, tkwait
KEYWORDS
exec, background, busy
BLT 2.4 bgexec(n)
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This page was generated on Sat Sep 8 16:41:06 GMT 2007
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