How do I execute a command, feed data to its stdin, and read from its stdout in MacRuby?

Question

I'm attempting to execute a command, feed data to its stdin, and read from its stdout. I've tried using Ruby's Open3#popen3 as well as NSTask, exposed via MacRuby. The source for the program I'm writing is available here. I'm doing this in Xcode and MacRuby.

Here's some select code:

The entry point, just simply allowing me to easily switch between the two methods.

def do_gpg_cmd cmd
  do_gpg_cmd_nstask cmd
end

The ruby way, using Open3#popen3.

def do_gpg_cmd_ruby cmd
  gpg = "#{@gpg_path} --no-tty "
  cmd_output = ''
  logg "executing [#{cmd}]"
  Dispatch::Queue.concurrent.async do
    logg "new thread starting"
    Open3.popen3(gpg + cmd) do |stdin, stdout, stderr|
      stdin.write input_text
      stdin.close
      cmd_output = stdout.read
      output_text cmd_output
      stdout.close
      logg stderr.read
      stderr.close
    end
  end
  return cmd_output
end

In this approach, the application freezes (I'm testing by clicking the Sign button in the app, which runs gpg --clearsign --local-user $key).

When I kill the application, Xcode shows this in the thread diagnosic that automatically appears:

libsystem_kernel.dylib`__psynch_cvwait:
0x7fff84b390f0:  movl   $33554737, %eax
0x7fff84b390f5:  movq   %rcx, %r10
0x7fff84b390f8:  syscall
0x7fff84b390fa:  jae    0x7fff84b39101            ; __psynch_cvwait + 17 ; THIS LINE IS HIGHLIGHTED
0x7fff84b390fc:  jmpq   0x7fff84b3a4d4            ; cerror_nocancel
0x7fff84b39101:  ret    
0x7fff84b39102:  nop    
0x7fff84b39103:  nop

The Cocoa way, using NSTask.

def do_gpg_cmd_nstask cmd
  Dispatch::Queue.concurrent.async do
    fcmd = "--no-tty " + cmd
    task = NSTask.alloc.init
    task.setLaunchPath(@gpg_path)
    task.setArguments(fcmd.split(" ") << nil)

    task.arguments.each {|a| puts "ARG: [#{a}]" }

    inpipe = NSPipe.pipe
    outpipe = NSPipe.pipe
    errpipe = NSPipe.pipe

    task.setStandardOutput(outpipe)
    task.setStandardInput(inpipe)
    task.setStandardError(errpipe)

    output = outpipe.fileHandleForReading
    errput = errpipe.fileHandleForReading
    input = inpipe.fileHandleForWriting

    task.launch

    input.writeData input_text.dataUsingEncoding(NSUTF8StringEncoding)
    input.closeFile

    outdata = output.readDataToEndOfFile
    errdata = errput.readDataToEndOfFile
    output.closeFile
    errput.closeFile
    outstring = NSString.alloc.initWithData(outdata, encoding: NSUTF8StringEncoding)
    errstring = NSString.alloc.initWithData(errdata, encoding: NSUTF8StringEncoding)

    output_text outstring
    logg errstring
  end
end

When I run this, I receive this error in the Xcode debug output. I'm obviously outputting the ARG parts myself as ultra dumb logging. The subprocess is not executed.

ARG: [--no-tty]
ARG: [--clearsign]
ARG: [--local-user]
ARG: [0xC2808780]
ARG: []
2013-03-12 23:27:39.305 GPGBoard[84924:3503] -[NSNull fileSystemRepresentation]: unrecognized selector sent to instance 0x7fff75b05310
*** Dispatch block exited prematurely because of an uncaught exception:
/Users/colin/Library/Developer/Xcode/DerivedData/GPGBoard-bradukgmaegxvmbukhwehepzyxcv/Build/Products/Debug/GPGBoard.app/Contents/Resources/AppDelegate.rb:81:in `block': NSInvalidArgumentException: -[NSNull fileSystemRepresentation]: unrecognized selector sent to instance 0x7fff75b05310 (RuntimeError)

I suspect that problems of either approach are mutually exclusive: the Open3#popen3 problem may be related to blocking read, while the problem with NSTask is related to a pipe problem.

jtomschroeder jtomschroeder · Accepted Answer · 2013-03-13T17:50:51

This piece of code works for me and prints out the files in the current directory:

framework "Cocoa"
task = NSTask.new
task.launchPath = "/bin/ls"
task.arguments = ["-l", "-a"]
stdoutPipe = NSPipe.pipe
task.standardOutput = stdoutPipe
task.launch
data = stdoutPipe.fileHandleForReading.readDataToEndOfFile
puts NSString.alloc.initWithData data, :encoding => NSASCIIStringEncoding

Now if I replace task.arguments = ["-l", "-a"] with task.arguments = "-l -a".split(" ") << nil I get the following error:

macruby[86209:707] -[NSNull fileSystemRepresentation]: unrecognized selector sent to instance 0x7fff77d6f310

So, I think your issue is task.setArguments(fcmd.split(" ") << nil). Change it to task.setArguments(fcmd.split(" ")) and you should no longer get the NSNull problem.

How do I execute a command, feed data to its stdin, and read from its stdout in MacRuby?

2 Answers