Write to NSTasks standard input after launch

Question

I am currently trying to wrap my head around the hole NSTask, NSPipe, NSFileHandle business. So I thought I write a little tool, which can compile and run C code. I also wanted to be able to redirect my stdout and stdin to a text view.

Here is what I got so far. I used code from this post to redirect my stdio: What is the best way to redirect stdout to NSTextView in Cocoa?

NSPipe *inputPipe = [NSPipe pipe];
// redirect stdin to input pipe file handle
dup2([[inputPipe fileHandleForReading] fileDescriptor], STDIN_FILENO);
// curInputHandle is an instance variable of type NSFileHandle
curInputHandle = [inputPipe fileHandleForWriting];

NSPipe *outputPipe = [NSPipe pipe];
NSFileHandle *readHandle = [outputPipe fileHandleForReading];
[readHandle waitForDataInBackgroundAndNotify];
// redirect stdout to output pipe file handle
dup2([[outputPipe fileHandleForWriting] fileDescriptor], STDOUT_FILENO);

// Instead of writing to curInputHandle here I would like to do it later
// when my C program hits a scanf
[curInputHandle writeData:[@"123" dataUsingEncoding:NSUTF8StringEncoding]];

NSTask *runTask = [[[NSTask alloc] init] autorelease];
[runTask setLaunchPath:target]; // target was declared earlier
[runTask setArguments:[NSArray array]];
[runTask launch];

NSNotificationCenter *center = [NSNotificationCenter defaultCenter];
[center addObserver:self selector:@selector(stdoutDataAvailable:) name:NSFileHandleReadCompletionNotification object:readHandle];

And here the stdoutDataAvailable method

- (void)stdoutDataAvailable:(NSNotification *)notification
{
    NSFileHandle *handle = (NSFileHandle *)[notification object];
    NSString *str = [[NSString alloc] initWithData:[handle availableData] encoding:NSUTF8StringEncoding];
    [handle waitForDataInBackgroundAndNotify];
    // consoleView is an NSTextView
    [self.consoleView setString:[[self.consoleView string] stringByAppendingFormat:@"Output:\n%@", str]];
}

This Program is working just fine. It is running the C program printing the stdout to my text view and reading "123" from my inputPipe. Like indicated in my comment above I would like to provide the input while the task is running once it is needed.

So there are two questions now.

1. Is there a way to get a notification as soon as somebody tries to read data from my inputPipe?
2. If the answer to 1 is no, is there a different approach I can try? Maybe using a class other than NSTask?

Any help, sample code, links to other resources are highly appreciated!

Answer 1

I'm not sure whether you can detect a "pull" on an NSPipe. I do have a vague sense that polling for write-availability with select() or using kqueue to look for I/O availability events on the underlying file descriptor of your NSFileHandle might do the trick, but I'm not very familiar with using those facilities in this way.

Do you have to support arbitrary C programs, or is it a special daemon or something you've developed?

If it's your own program, you could watch for requests for feedback on outputPipe, or just blast input onto the inputPipe as you find out what it is you want to send, and let the C program consume it when it's ready; if it's somebody else's code, you may be able to hook scanf and friends using a link-time method (since it's code you're compiling) like the one described in Appendix A-4 of:

http://www.cs.umd.edu/Library/TRs/CS-TR-4585/CS-TR-4585.pdf

The gist of it is to make a .dylib with your custom I/O functions (which may send some sigil to your app indicating that they need input), link that into the built program, set an environment variable (DYLD_BIND_AT_LAUNCH=YES) for the launched task, and run it. Once you've got those hooks in, you can provide whatever conveniences you want for your host program.