IMPORTANT Rather than use the code below, I recommend that you adopt Swift’s shiny-new Subprocess package. That’s what I’m doing! (-:
Running a child process using Process (or NSTask in Objective-C) is easy, but piping data to and from the child’s stdin and stdout is surprisingly tricky. I regularly see folks confused by this. Moreover, it’s easy to come up with a solution that works most of the time, but suffers from weird problems that only show up in the field [1].
I recently had a couple of DTS incidents from folks struggling with this, so I sat down and worked through the details. Pasted below is the results of that effort, namely, a single function that will start a child process, pass it some data on stdin, read the data from the child’s stdout, and call a completion handler when everything is done.
There are some things to note here, some obvious, some not so much:
I’ve included Swift and Objective-C versions of the code. Both versions work the same way. The Swift version has all the comments. If you decide to base your code on the Objective-C version, copy the comments from there.
I didn’t bother collecting stderr. That’s not necessary in many cases and, if you need it, it’s not hard to extend the code to handle that case.
I use Dispatch I/O rather than FileHandle to manage the I/O channels. Dispatch I/O is well suited to this task. In contrast, FileHandle has numerous problems working with pipes. For the details, see Whither FileHandle?.
This single function is way longer than I’d normally tolerate. This is partly due to the extensive comments and party due to my desire to maintain focus. When wrapping Process it’s very easy to run afoul of architecture astronaut-ism. Indeed, I have a much more full-featured Process wrapper sitting on my hard disk, but that’s going to stay there in favour of this approach (-:
Handling a child process correctly involves some gnarly race conditions. The code has extensive comments explaining how I deal with those.
If you have any questions or comments about this, put them in a new thread. Make sure to tag that thread with Foundation and Inter-process communication so that I see it.
Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"
[1] Indeed, this post shows that I’ve made this sort of mistake myself )-:
