On Mar 27, 2011, at 11:14 PM, Ken Thomases wrote:
> On Mar 27, 2011, at 1:27 PM, Jason Harris wrote:
>
>> - (void) doWorkerLaunches
>> {
>> for (int i = 1; i <50; i++)
>> {
>> dispatch_async(dispatch_get_global_queue(0,0), ^{
>> NSTask* task = [[NSTask alloc] init];
>> [task setLaunchPath: @"/bin/ls"];
>>
>> NSArray* arguments = [NSArray arrayWithObjects: @"-l", @"-a",
>> @"-t", nil];
>> [task setArguments: arguments];
>>
>> NSPipe* pipe = [NSPipe pipe];
>> [task setStandardOutput: pipe];
>>
>> NSFileHandle* file = [pipe fileHandleForReading];
>>
>> [task launch];
>> [task waitUntilExit];
>>
>> NSData* data = [file readDataToEndOfFile];
>> });
>> }
>> }
>
> Possibly unrelated to your issue with the dispatch threads (although possibly
> related), the above use of NSTask is somewhat broken. You've made a common
> mistake. It is not OK to block waiting for the task to exit when you haven't
> established an ongoing asynchronous read of its output (when you're capturing
> the output rather than letting it go to file, /dev/console, or /dev/null,
> etc.).
>
> The problem is that pipes have a fixed buffer in the kernel. If a task
> writes more than that amount to the pipe when no reader is draining the pipe,
> the writer blocks. You don't read from the pipe until after the task has
> exited, but the task may be prevented from exiting because you're not reading
> from the pipe. Classic deadlock.
>
> Put another way, have you confirmed that your tasks are really completing?
> Maybe the dispatch threads are still alive because of this deadlock I'm
> describing. (I guess it depends on whether your "ls -l -a -t" command is
> producing more output than the size of a pipe's buffer, which in turn depends
> on the current working directory and its contents.)
Thanks! Just to try this out I'll rework into the notification form of NSTask
and then retest... More soon...
Thanks,
Jas_______________________________________________
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