Getting data from NSTask in real-time using notifications doesn't work

Question

I got a Cocoa command-line program in which I try to run NSTask program (tshark to monitor network) and get data from it in real-time. So I make a NSFileHandle , call waitForDataInBackgroundAndNotify to send notifications and then register my help class to Notification center to process the data, but not a single notification is sent to my help class.

Does anybody have an idea of what could be wrong?

Thanks in advance

Here is my code:

#import <Foundation/Foundation.h>
#import <string>
#import <iostream>

@interface toff : NSObject {}
-(void) process:(NSNotification*)notification;
@end

@implementation toff
-(void) process:(NSNotification*)notification{
    printf("Packet caught!\n");
}
@end

int main (int argc, const char * argv[]){
    @autoreleasepool {
        NSTask* tshark = [[NSTask alloc] init];
        NSPipe* p = [NSPipe pipe];
        NSFileHandle* read = [p fileHandleForReading];
        toff* t1 = [[toff alloc] init];
        NSNotificationCenter* nc = [NSNotificationCenter defaultCenter];

        [read waitForDataInBackgroundAndNotify];
        [nc addObserver:t1 selector:@selector(process:) name:nil object:nil];

        printf("Type 'stop' to stop monitoring network traffic.\n");
        [tshark setLaunchPath:@"/usr/local/bin/tshark"];
        [tshark setStandardOutput:p];
        [tshark launch];

        while(1){
            std::string buffer;
            getline(std::cin, buffer);
            if(buffer.empty()) continue;
            else if(buffer.compare("stop") == 0){
                [tshark interrupt];
                break;
            }
        }

        //NSData* dataRead = [read readDataToEndOfFile];
        //NSLog(@"Data: %@", dataRead);
        //NSString* stringRead = [[NSString alloc] initWithData:dataRead encoding:NSUTF8StringEncoding];
        //NSLog(@"Output: %@", stringRead);

    }
    return 0;
}

EDIT: When I uncomment commented section of code and delete all that notification stuff, all desired data are extracted from file handle after task finish.

I was also wondering, if problem can't be in fact, that my program is 'Command line tool' so I am not sure if it has run loop - as Apple documentation says is needed (in waitForDataInBackgroundAndNotify message of NSFileHandle):

You must call this method from a thread that has an active run loop.

Answer 1

Your program finishes immediately after launching the task. You need to tell the task to wait until exit. That will run the run loop, waiting for the child process to exit and coincidentally enabling the file handle to monitor the pipe. When the task does exit, the method will return and you can proceed to the end of main.

Answer 2

Have a look at asynctask.m, sample code that shows how to implement asynchronous stdin, stdout & stderr streams for processing data with NSTask (there may be room for improvement though).

Answer 3

There's a new API since 10.7, so you can avoid using NSNotifications.

task.standardOutput = [NSPipe pipe];
[[task.standardOutput fileHandleForReading] setReadabilityHandler:^(NSFileHandle *file) {
    NSData *data = [file availableData]; // this will read to EOF, so call only once
    NSLog(@"Task output! %@", [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]);

    // if you're collecting the whole output of a task, you may store it on a property
    [self.taskOutput appendData:data];
}];

Probably you want to repeat the same above for task.standardError.

IMPORTANT:

When your task terminates, you have to set readabilityHandler block to nil; otherwise, you'll encounter high CPU usage, as the reading will never stop.

[task setTerminationHandler:^(NSTask *task) {

    // do your stuff on completion

    [task.standardOutput fileHandleForReading].readabilityHandler = nil;
    [task.standardError fileHandleForReading].readabilityHandler = nil;
}];

This is all asynchronous (and you should do it async), so your method should have a ^completion block.