Apple Darwin User Manual download pdf (Page 47)

Standard Input and Output

Another common API for interprocess communication is standard input and output. This API provides a pair

of unidirectional streams. Much like socket programming, the stream-based nature of standard input and

output requires you to keep additional state information if you need to associate responses to messages with

the original message. A good way to solve that problem is through the use of message queues, as described

in Message Queues (page 49).

One thing that makes standard input and output convenient is that they are largely set up for you. Every

process in a UNIX-based system has standard input and output automatically. You can take advantage of these

to communicate between a parent process (your main application) and its children (your helper host).

To communicate with child processes in Cocoa, you should use the NSTask API, described in NSTask Class

Reference. For more information on this method, read Creating and Launching an NSTask and Ending an

NSTask.

Alternatively, in BSD tools, you can accomplish the same thing at a file descriptor level using a few low-level

APIs as shown in this example:

#include <stdlib.h>

comm_channel *startchild(char *path)

{

comm_channel *channel = malloc(sizeof(*channel));

pid_t childpid;

int in_descriptors[2];

int out_descriptors[2];

/* Create a pair of file descriptors to use for communication. */

if (pipe(in_descriptors) == -1) {

fprintf(stderr, "pipe creation failed.\n");

goto error_exit;

}

if (pipe(out_descriptors) == -1) {

fprintf(stderr, "pipe creation failed.\n");

goto error_exit;

}

/* Create a new child process. */

if ((childpid = fork()) == -1) {

fprintf(stderr, "fork failed.\n");

Cross-Architecture Plug-in Support

Using Interprocess Communication

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Apple Darwin User Manual Page 47