Piping Data Between Tasks

Each end point of the pipe is a file descriptor, represented by an NSFileHandle object. You thus use NSFileHandle messages to read and write pipe data. A “parent” process creates the NSPipe object and holds one end of it. It creates an NSTask object for the other process and, before launching it, passes the other end of the pipe to that process; it does this by setting the standard input, standard output, or standard error device of the NSTask object to be the other NSFileHandle or the NSPipe itself (in the latter case, the type of NSFileHandle—reading or writing—is determined by the “set” method of NSTask).

The following example illustrates the above procedure:

- (void)readTaskData:(id)sender

{

    NSTask *pipeTask = [[NSTask alloc] init];

    NSPipe *newPipe = [NSPipe pipe];

    NSFileHandle *readHandle = [newPipe fileHandleForReading];

    NSData *inData = nil;

    // write handle is closed to this process

    [pipeTask setStandardOutput:newPipe];

    [pipeTask setLaunchPath:[NSHomeDirectory()

            stringByAppendingPathComponent:

                    @"PipeTask.app/Contents/MacOS/PipeTask"]];

    [pipeTask launch];

    while ((inData = [readHandle availableData]) && [inData length]) {

        [self processData:inData];

    }

    [pipeTask release];

}

The launched process in this example must get data and write that data (using the writeData: method of NSFileHandle), to its standard output device, which is obtained using the fileHandleWithStandardOutput method of NSFileHandle.

When the processes have no more data to communicate across the pipe, the writing process should simply send closeFile to its NSFileHandle end point. This causes the process with the “read” NSFileHandle to receive an empty NSData object, signalling the end of data. If the “parent” process created the NSPipe object with the init method, it should then release it.