> 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 again Ken for you suggestion on this bug and on the last one to do with
NSTask.
Right, I have changed the example a bit so that the NSTask in this case is
using notifications. I also get it to print the output. Actually now I can see
this is reproducing the old problem I had of some of the output results being
just simply dropped.
Eg Here is the new code (header):
@interface ShellTask : NSObject
{
NSString* generatingCmd_; // The command that was
executed
NSArray* generatingArgs_; // The arguments used
to the command
int result_; // The
result of executing the command
NSTask* task_; // The NSTask
we are trying to perform
NSFileHandle* outHandle_;
NSFileHandle* errHandle_;
NSMutableData* outputData_;
NSMutableData* errorData_;
}
@property (nonatomic, assign) NSTask* task;
+ (void) execute:(NSString*)cmd withArgs:(NSArray*)args;
@end
Here is the new code (body):
-------------------------------
- (void) doWorkerLaunches
{
for (int i = 1; i <50; i++)
{
dispatch_async(dispatch_get_global_queue(0,0), ^{
[ShellTask execute:@"/bin/ls" withArgs:[NSArray
arrayWithObjects: @"-l", @"-a", @"-t", nil]];
});
}
}
@implementation ShellTask
@synthesize task = task_;
- (void) gotOutput:(NSNotification*)notification
{
NSData* data = [notification.userInfo
objectForKey:NSFileHandleNotificationDataItem];
if (notification.object == outHandle_)
{
if (data.length > 0)
{
[outputData_ appendData:data];
[outHandle_ readInBackgroundAndNotify];
}
else
outHandle_ = nil;
}
}
- (void) gotError:(NSNotification*)notification
{
if (notification.object == errHandle_)
{
NSData* data = [notification.userInfo
objectForKey:NSFileHandleNotificationDataItem];
if (data.length > 0)
{
[errorData_ appendData:data];
[errHandle_ readInBackgroundAndNotify];
}
else
errHandle_ = nil;
}
}
- (BOOL) waitTillFinished
{
[task_ waitUntilExit];
[[NSNotificationCenter defaultCenter] removeObserver:self
name:NSFileHandleReadCompletionNotification object:nil];
[task_ terminate];
result_ = [task_ terminationStatus];
outHandle_ = nil;
errHandle_ = nil;
return (result_ == 0);
}
- (id) initWithCommand:(NSString*)cmd andArgs:(NSArray*)args
{
generatingCmd_ = cmd;
generatingArgs_ = args;
task_ = [[NSTask alloc] init];
NSPipe* outPipe = [[NSPipe alloc] init]; // Create the pipe to
write standard out to
NSPipe* errPipe = [[NSPipe alloc] init]; // Create the pipe to
write standard error to
outHandle_ = [outPipe fileHandleForReading];
errHandle_ = [errPipe fileHandleForReading];
outputData_ = [[NSMutableData alloc] init];
errorData_ = [[NSMutableData alloc] init];
[task_ setLaunchPath:cmd];
[task_ setArguments:args];
[task_ setStandardInput:[NSPipe pipe]];
[task_ setStandardOutput:outPipe];
[task_ setStandardError:errPipe];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(gotOutput:) name:NSFileHandleReadCompletionNotification
object:nil];
[[NSNotificationCenter defaultCenter] addObserver:self
selector:@selector(gotError:) name:NSFileHandleReadCompletionNotification
object:nil];
[outHandle_ readInBackgroundAndNotify];
[errHandle_ readInBackgroundAndNotify];
return self;
}
+ (void) execute:(NSString*)cmd withArgs:(NSArray*)args
{
ShellTask* shellTask = [[ShellTask alloc] initWithCommand:cmd
andArgs:args];
[shellTask->task_ launch]; // Start the process
[shellTask waitTillFinished];
NSString* outStr = [[NSString alloc]
initWithData:shellTask->outputData_ encoding:NSUTF8StringEncoding];
NSString* errStr = [[NSString alloc] initWithData:shellTask->errorData_
encoding:NSUTF8StringEncoding];
fprintf(stderr, "output:\n%s\n", [outStr UTF8String]);
if ([errStr length] > 0)
fprintf(stderr, "error:\n%s\n", [errStr UTF8String]);
}
@end
-------------
Interestingly now the output now just looks like:
output:
total 0
drwxr-xr-x 3 jason staff 102 Mar 27 23:57 gcdGroups.app
drwxr-xr-x 3 jason staff 102 Mar 27 23:02 .
drwxr-xr-x@ 4 jason staff 136 Mar 27 23:02 ..
output:
total 0
drwxr-xr-x 3 jason staff 102 Mar 27 23:57 gcdGroups.app
drwxr-xr-x 3 jason staff 102 Mar 27 23:02 .
drwxr-xr-x@ 4 jason staff 136 Mar 27 23:02 ..
output:
output:
output:
total 0
drwxr-xr-x 3 jason staff 102 Mar 27 23:57 gcdGroups.app
drwxr-xr-x 3 jason staff 102 Mar 27 23:02 .
drwxr-xr-x@ 4 jason staff 136 Mar 27 23:02 ..
output:
output:
output:
output:
output:
total 0
drwxr-xr-x 3 jason staff 102 Mar 27 23:57 gcdGroups.app
drwxr-xr-x 3 jason staff 102 Mar 27 23:02 .
drwxr-xr-x@ 4 jason staff 136 Mar 27 23:02 ..
output:
output:
total 0
drwxr-xr-x 3 jason staff 102 Mar 27 23:57 gcdGroups.app
drwxr-xr-x 3 jason staff 102 Mar 27 23:02 .
drwxr-xr-x@ 4 jason staff 136 Mar 27 23:02 ..
So you can see various tasks where just "skipped". I think this corresponds to
the cases where maybe the DispatchWorker is not quitting...
Complete ready to run project and code is at:
http://jasonfharris.com/files/misc_snippets/gcdGroupsNotifications.zip_______________________________________________
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