Inter-process communication (IPC) can be defined as set of techniques used for exchanging data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods can divided into methods for message passing, synchronization, shared memory, and remote procedure calls (RPC).
Below is a screenshot of Activity Monitor on mac os. I wonder what command line I can use to get the memory usage. I tried below command: $ top -l 1 -n 0 Processes: 399 total, 3 running, 396 sleeping, 2460 threads 2018/03/07 19:54:01 Load Avg: 7.41, 6.55, 6.23 CPU usage: 58.38% user, 12.97% sys, 28.63% idle SharedLibs: 191M resident, 54M data, 24M linkedit.
Reasons for allowing two processes to communicate with each other may be different :
- Information sharing
- Computational speedup
- Modularity
- Convenience
- Privilege separation
In this article we will discuss various techniques available on Mac to accomplish IPC. In the subsequent articles we will see the implementation part for various techniques.
Let's look at the techniques available for IPC one by one.
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1. Shared File : Most naive solution where multiple processes will share a common file. It could be a simple .txt file or a .sqlite database. The obvious problems in this method are
- Clients need to continuously poll to see if server has written something in the file.
- Write problems if multiple processes are trying to write to the shared file at the same time.
2. Shared Memory : This is another implementation for IPC where a memory section is shared between different processes.In other words process A writes to this memory and B can read from this memory, or vice verse. This is fast and data doesn't have to be copied around. The downside is that it's really difficult to coordinate changes to the shared memory area.
I hate my job! mac os. 3. Mach Ports :The fundamental services and primitives of the OS X kernel are based on Mach 3.0. Mach 3.0 was originally conceived as a simple, extensible, communications microkernel. It is capable of running as a stand–alone kernel, with other traditional operating-system services such as I/O, file systems, and networking stacks running as user-mode servers.
However, in OS X, Mach is linked with other kernel components into a single kernel address space. This is primarily for performance. It is much faster to make a direct call between linked components than it is to send messages or do remote procedure calls (RPC) between separate tasks. This modular structure results in a more robust and extensible system than a monolithic kernel would allow, without the performance penalty of a pure microkernel.
The only disadvantage is complexity of implementation and less documentation.
4. Sockets : While most TCP/IP connections are established over a network between two different machines, it is also possible to connect two processes running on the same machine without ever touching a network using TCP/IP.
Using TCP/IP sockets for interprocess communication (IPC) is not very different from using them for network communications. In fact, they can be used in exactly the same way.But if the intent is only for local IPC create a socket in the AF_UNIX family to get a socket that's only for local communication and uses more flexible addressing than TCP/IP allows.
5. Apple Events : These are the only IPC mechanism which is universally supported by GUI applications on Mac OS X for remote control. Operation like opening a telling a application to open a file or to quit etc. can be done using these.
AppleScript is a scripting language built on top of Apple Events which can be used using scripting bridge in a Mac application.
6. Distributed Notifications : A notification center manages the sending and receiving of notifications. It notifies all observers of notifications meeting specific criteria. The notification information is encapsulated in NSNotification objects. Client objects register themselves with the notification center as observers of specific notifications posted by other objects. When an event occurs, an object posts an appropriate notification to the notification center.
Posting a distributed notification is an expensive operation. The notification gets sent to a system wide server that then distributes it to all the processes that have objects registered for distributed notifications. The latency between posting the notification and the notification's arrival in another process is unbounded. In fact, if too many notifications are being posted and the server's queue fills up, notifications can be dropped.
7. Pasteboard : Every time a copy-paste happens between applications, that's IPC happening using pasteboard. Inter-application drag and drop also uses the pasteboard. It is possible to create custom pasteboards which only desired applications can access for passing data back and forth between applications.
Like distributed notifications, pasteboard work by talking to a central pasteboard server using mach ports.
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8. Distributed Objects : This mechanism enables a Cocoa application to call an object in a different Cocoa application (or a different thread in the same application). The applications can even be running on different computers on a network.
Distributed objects operates by having the server make public, an object to which other client processes can connect. Once a connection is made, the client process invokes one of the public object's methods as if the object existed in the client process.
Distributed Objects normally runs over mach ports but can also be used with sockets, allowing it to work between computers as well.
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Written By: HEM DUTT, Sr. Engineer/Tech Lead (Mac OSX development), Mindfire Solutions
