NSPort in Objective-C

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NSPort is the base class that connects a data source to a run loop. Messages pushed onto the port are processed by the run loop. NSMachPort is a subclass that connects to a Mach kernel port for direct process-to-process communications. NSSocketPort can be connected to BSD pipes or sockets providing equivalent functionality between processes and systems. NSPorts are the foundation for distributed objects, several of which are demonstrated in the Distributed Objects section later in this chapter. NSMachPort is extremely efficient and the most common type of port used by run loops. User events, system events, deferred messages, timer events, and many other low-level messages are all processed by an application through its run loop. Most events are pushed onto the run loop s Mach port by the system or from within the application. Mach ports can also be used to send messages between processes. The one limitation is the security model of the operating system. All Mach kernel ports exist within a domain called a bootstrap namespace. A bootstrap namespace is created for each user that logs into a Mac OS X system. A process can only connect with the Mach ports in its namespace and its parent namespaces. Thus, an application could use Mach ports to establish communications with another process started by the same user, or a system daemon, but not with a process started by another user. NSSocketPorts are used when a run loop needs to communicate with a process outside its bootstrap namespace or possibly with another computer system. NSSocketPorts can be connected to a variety of different sources; the two most useful are BSD pipes and sockets. A pipe can be a named pipe in the file system. File system names are public to all processes, and provide a means for two processes in different bootstrap namespaces to communicate. Network sockets allow two processes to exchange data using a network transport protocol. The advantage is that the other process could be running on the same machine or one thousands of miles away. The disadvantage is that network ports are typically accessible from outside the computer system, which might not be appropriate for some communications and has security implications.
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NSPipe is little more than a wrapper for a pair of NSFileHandle objects. An NSPipe is used to interact with BSD pipes. Often, these are the traditional standard in, standard out, and standard error pipes that connect processes. Listing 13-1 shows how to launch an executable and capture the text output of the new process.
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Listin g 13-1. Capturing Standard Out
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Java ProcessBuilder pb = new ProcessBuilder("/bin/echo","Hello, Objective-C"); try { Process echo = pb.start(); InputStream stdOut = echo.getInputStream(); int c = (int)' '; System.out.print("echo says:");
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CHAPTER 13 COMMUNICATING NEAR AND FAR
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do { System.out.print((char)c); c = stdOut.read(); } while (c!=(-1)); } catch (IOException e) { e.printStackTrace(); } Objective-C NSTask *echo = [NSTask new]; NSPipe *stdOut = [NSPipe pipe]; [echo setLaunchPath:@"/bin/echo"]; [echo setArguments:[NSArray arrayWithObject:@"Hello, Objective-C"]]; [echo setStandardOutput:stdOut]; [echo launch]; NSFileHandle *outStream = [stdOut fileHandleForReading]; NSData *output = [outStream readDataToEndOfFile]; NSLog(@"echo says: %@",[NSString stringWithCString:[output bytes] length:[output length]]); In Java, the java.lang.Process object creates the required java.io.InputStream or java.io.OutputStream objects and provides them to you. In Objective-C, everything is backwards. You create the NSPipe or NSFileHandle object you want to communicate through, then pass it in a -setStandardInput:, -setStandardOutput:, or -setStandardError: message. This must be done before the process is launched. When launched, NSTask will connect the pipe or file handle you provided to the actual pipe attached to the process. Also note that the terminology is reversed. In Objective-C, the standardOut property is the standard out of the process (i.e., the process s output). In Java, java.lang.Process.getInputStream gets the InputStream connected to the process s standard out. In other words, pipe identities in Objective-C are from the perspective of the process. In Java, they are from the perspective of the parent process. To get the actual input data, the NSFileHandle for reading is obtained from the pipe. It s possible to bypass using NSPipe altogether, as the -setStandardInput:, -setStandardOutput:, and -setStandardError: messages all accept NSFileHandle objects too. So the code in Listing 13-1 could be easily rewritten to set an NSFileHandle as its connection to standard out. The effect would be the same.
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