Unable to display real time Process() output to NSTextView in release mode

Question

I wanted to display output from Process() to NSTextView in real time. Below are my code snippets:

Code Block func displayOutput (_ task:Process) {
		let outputPipe = Pipe()
		setvbuf(stdout, nil, _IONBF, 0)
		dup2(outputPipe.fileHandleForWriting.fileDescriptor, STDOUT_FILENO)
		task.standardOutput = outputPipe
		outputPipe.fileHandleForReading.readabilityHandler = { pipe in
			if let outputString = String(data: pipe.availableData, encoding: String.Encoding.utf8) {
				 if outputString.count > 0 {
						DispatchQueue.main.async(execute: {
							print(outputString, separator: "\n", to: &self.outputTextView.string)
							let range = NSRange(location:self.outputTextView.string.count,length:0)
					  self.outputTextView.scrollRangeToVisible(range)
						}
				}
			}
		}
}

The above code snippets only works in debug mode. Nothing displays on the NSTextView besides the initial text when it's in release mode.

Foundation

374

Posted by

Sylvia971

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Answer 1

What is this code supposed to be doing?

Code Block  dup2(outputPipe.fileHandleForWriting.fileDescriptor, STDOUT_FILENO)

AFAICT it’s changing STDOUT_FILENO of the parent process, which seems like a mistake.

Also, this code is broken:

Code Block  if let outputString = String(data: pipe.availableData, encoding: String.Encoding.utf8) {

The problem is that pipes are byte streams, and thus might split a UTF-8 sequence. For example, if the task prints naïve, the byte sequence you get back is 6E 61 C3 AF 76 65. If the stream splits that byte sequence between C3 and AF, both the before and after byte sequences are invalid UTF-8 and thus String(data:encoding:) will return nil.

As to what’s causing your main problem, it’s most likely because no one is holding on to outputPipe which is causing it to be deallocated which then closes everything down. However, it’s hard to be sure without more context.

Pasted in below is my test code for this. Note that this exercises two different cases:

The child process terminating on its own.
You clicking the Stop button to terminate the child process early.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@apple.com"

Code Block  class ProcessState {
    var process: Process
    var fileHandleForReading: FileHandle
    init(process: Process, fileHandleForReading: FileHandle) {
        self.process = process
        self.fileHandleForReading = fileHandleForReading
    }
}
var stateQ: ProcessState?
func start() -> ProcessState? {
    print("will start")
    let pipe = Pipe()
    let state = ProcessState(process: Process(), fileHandleForReading: pipe.fileHandleForReading)
    state.process.executableURL = URL(fileURLWithPath: "/bin/sh")
    state.process.arguments = [
        "-c", "for i in 1 2 3 4 5; do date; sleep 1; done"
    ]
    state.process.terminationHandler = { _ in
        OperationQueue.main.addOperation {
            guard state === self.stateQ else { return }
            self.stateQ = nil
            self.stop(state: state, needsTerminate: false)
        }
    }
    state.process.standardOutput = pipe.fileHandleForWriting
    state.fileHandleForReading.readabilityHandler = { _ in self.readData(state: state) }
    do {
        try state.process.run()
    } catch {
        print("did not start")
        return nil
    }
    return state
}
func readData(state: ProcessState) {
    print(state.fileHandleForReading.availableData as NSData)
}
func stop(state: ProcessState, needsTerminate: Bool) {
    state.fileHandleForReading.readabilityHandler = nil
    state.fileHandleForReading.closeFile()
    state.process.terminationHandler = nil
    if needsTerminate {
        state.process.terminate()
    }
    print("did stop")
}

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eskimo

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Unable to display real time Process() output to NSTextView in release mode

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