Windows inter-process communication

AbstractAs people have higher and higher requirements on applications, single-process applications cannot meet people's requirements in many occasions. Writing multi-process/multi-thread programs has become an important feature of modern programming. In multi-process programming, inter-process communication is inevitable. Microsoft Win32 API provides a variety of inter-process communication methods, comprehensively describes the features of these methods, and compares and analyzes them, hoping to provide readers with a reference for choosing a communication method.
Keyword Process Communication IPC Win32 API

1Process and Process Communication 

A process is a program that loads memory and is ready to run. Each process has a private virtual address space, which consists of code, data, and system resources (such as files and pipelines) that it can use. Multi-process/multi-thread is a basic feature of the Windows operating system. Microsoft Win32 application programming interface (API) provides a large number of mechanisms to support data sharing and exchange between applications. These mechanisms are called interprocess communication, IPC), process communication refers to data sharing and data exchange between different processes.
Because there are many ways to use Win32 APIs for Process Communication, choosing an appropriate communication method becomes an important issue in application development, the following section analyzes and compares several methods of Process Communication in Win32.

2 Process Communication Method

2.1 file ing
Memory-mapped files allows the Process to Treat the file content as a piece of memory in the process address range. Therefore, the process does not need to use file I/O operations, and can read and modify the file content with simple pointer operations.
Win32 API allows multiple processes to access the same file ing object. Each process receives memory pointers in its own address space. By using these pointers, different processes can read or modify the file content to share data in the file.
There are three ways for an application to share a file ing object with multiple processes.
(1) Inheritance: the first process establishes a file ing object, and its child process inherits the handle of the object.
(2) named file ing: when creating a file ing object, the first process can specify a name for the object (which may be different from the file name ). The second process can use this name to open the file ing object. In addition, the first process can also pass the name to the second process through some other IPC Mechanisms (such as famous pipelines and email slots.
(3) handle replication: the first process establishes a file ing object, and then passes the object handle to the second process through other IPC Mechanisms (such as famous pipelines and email slots. The second process copies the handle to obtain the access permission for the ing object of the file.
File ing is a very effective way to share data among multiple processes and provides better security. However, file ing can only be used between processes on the local machine and not on the network. Developers must also control the synchronization between processes.
2.2 shared memory
In Win32 API, shared memory is a special case of file ing. When creating a file ing object, the process uses 0xffffffff to replace the handle, which means that the corresponding file ing object accesses the memory from the file on the operating system page, other processes can open the file ing object to access the memory block. Because shared memory is implemented by file ing, it also has good security and can only run between processes on the same computer.
2.3 Anonymous Pipeline
Pipe is a communication channel with two endpoints: a process with one end handle can communicate with a process with the other end handle. The pipeline can be one-way-one end is read-only, and the other end point is write-only. It can also be two-way. The two ends of a pipeline are both readable and writable.
An anonymous pipeline (anonymous pipe) is a one-way channel that transfers data between a parent process and a child process, or between two child processes of the same parent process. Generally, the parent process creates an MPS queue, and then the child process to communicate inherits the read endpoint handle or write endpoint handle of the channel, and then implements communication. The parent process can also create two or more sub-processes that inherit anonymous pipeline read and write handles. These sub-processes can communicate directly using pipelines without passing through the parent process.
An anonymous pipeline is an effective method for implementing sub-process standard I/O redirection on a single machine. It cannot be used online or between two unrelated processes.
2.4 Named Pipe
A named pipe (Named Pipe) is a one-way or two-way channel for communication between server processes and one or more client processes. Unlike an anonymous pipe, a named pipe can be used between unrelated processes and different computers. When the server creates a named pipe, a name is specified for it, any process can use this name to open the other end of the pipeline and communicate with the server process based on the given permissions.
The named pipeline provides a relatively simple programming interface, making it easier to transmit data over the network than to communicate with two processes on the same computer. However, it cannot communicate with multiple processes at the same time.
2.5 mail slot
Mailslots provides one-way communication between processes. Any process can establish a mail slot as a mail slot server. Other processes, called mail slot customers, can send messages to the mail slot server process by the name of the mail slot. The incoming message is kept in the mail slot until the server process reads it. A process can be either a mail slot server or a mail slot customer. Therefore, multiple mail slots can be established to implement bidirectional communication between processes.
You can use the mail slot to send messages to the mail slots on the local computer, the mail slots on other computers, or the mail slots with the same name on all computers in the specified network area. The length of a message for a broadcast communication cannot exceed 400 bytes. The length of a non-broadcast message is limited by the maximum message length specified by the mail slot server.
The mail slot is similar to the named pipe, but it transmits data through unreliable datagram (such as UDP packets in TCP/IP protocol, once a network error occurs, messages cannot be correctly received, while the named pipe transmits data based on reliable connections. However, the mail slot provides simplified programming interfaces and the ability to broadcast messages to all computers in the specified network area. Therefore, the mail slot is another option for applications to send and receive messages.
2.6 clipboard
Clipped board is essentially a group of functions and messages used to transmit data in Win32 APIs. It provides an intermediary for data sharing between Windows applications) -The pasting Mechanism provides a shortcut for different applications to share data of different formats. When a user performs a cut or copy operation in an application, the application puts the selected data in one or more formats on the clipboard. Then, any other application can pick up data from the clipboard and select a suitable format from the given format.
The clipboard is a very loose exchange medium that supports any data format. Each format is identified by an unsigned integer in a standard (predefined) clipboard format, this value is a constant defined by Win32 API. For non-standard formats, you can use the register clipboard format function to register it as the new clipboard format. You only need to use the clipboard to exchange data in the same format or convert the data to a certain format. However, the clipboard can only be used in Windows-based programs and cannot be used on the network.
2.7 Dynamic Data Exchange
Dynamic Data Exchange (DDE) is a form of inter-process communication in which data is exchanged between applications in the shared content. Applications can use DDE for one-time data transmission, or dynamically exchange data between applications by sending update values when new data appears.
Like clipboard, DDE supports both standard data formats (such as text and bitmap) and custom data formats. However, their data transmission mechanisms are different. One obvious difference is that clipboard operations are almost always used as a one-time response to user-specified operations-for example, selecting the paste command from the menu. Although DDE can also be started by the user, it continues to play a role without further user intervention. DDE has three data exchange methods:
(1) Cold Chain: data exchange is a one-time data transmission, the same as the clipboard.
(2) temperature chain: when data is exchanged, the server notifies the customer, and then the customer must request new data.
(3) Hot chain: when data is exchanged, the server automatically sends data to the customer.
DDE exchange can occur between applications of different computers in a single machine or network. Developers can also define custom DDE data formats for specific IPC between applications, which have more closely coupled communication requirements. Most Windows-based applications support DDE.
2.8 object connection and embedding
Applications Use object connection and Embedding (OLE) technology to manage composite documents (documents composed of multiple data formats ), OLE provides services that make it easier for an application to call other applications for data editing. For example, the word processor supported by Ole can nest workbooks. When you want to edit a workbook, the OLE library can automatically start the workbook editor. When you exit the workbook editor, the table is updated in the original word processor document. Here, the workbook editor becomes an extension of the word processor. If you use DDE, you need to start the workbook editor explicitly.
Similar to DDE technology, most Windows-based applications support OLE technology.
2.9 dynamic Connection Library
Global data in the Win32 dynamic connection library (DLL) can be shared by all the processes that call the DLL, which opens up a new way for inter-process communication, of course, pay attention to synchronization during access.
Although data can be shared between processes through DLL, we do not advocate this method from the perspective of data security. It is better to use shared memory with access control.
2.10 Remote Procedure Call
Remote Procedure Call (RPC) provided by Win32 API enables applications to call functions remotely, which makes Process Communication over RPC over the network as simple as function call. RPC can be used between different processes on a single machine or in the network.
Since the RPC provided by Win32 API complies with the OSF-DCE (Open Software Foundation distributed computing environment) standard. Therefore, RPC applications written using Win32 APIs can communicate with RPC applications that support Dec on other operating systems. RPC developers can build high-performance, tightly coupled distributed applications.
2.11 NetBIOS Functions
Win32 API provides NetBIOS functions for low-level network control, which is mainly used to compile interfaces with windows for the IBM NetBIOS system. Unless there are applications with special low-level network functional requirements, it is recommended that other applications do not use the NetBIOS function for inter-process communication.
2.12 sockets
The Windows Sockets specification is a set of network programming interfaces in Windows defined in the example of socket interfaces popular in U. C. Berkeley University bsd unix. In addition to the original library functions of Berkeley socket, a group of functions for windows are also extended, allowing programmers to make full use of the Windows message mechanism for programming.
Currently, more and more network applications are using sockets to implement process communication. The main reason is that sockets is more cross-platform than other IPC Mechanisms. In addition, Winsock 2.0 not only supports TCP/IP protocol, other protocols (such as IPX) are also supported ). The only drawback of sockets is that it supports underlying communication operations, which makes it inconvenient to transmit simple data between standalone processes. In this case, the wm_copydata message described below will be more suitable.
2.13 wm_copydata message
Wm_copydata is a very powerful but little-known message. When an application transmits data to another application, the sender only needs to call the sendmessage function. The parameters are the handle of the destination window, the start address of the transmitted data, and the wm_copydata message. The receiver only needs to process the wm_copy data message as it does other messages, so that the receiving and receiving sides can share data.
Wm_copydata is a very simple method, which is actually implemented through file ing at the underlying layer. Its disadvantage is that it is not flexible and can only be used in a single-host Windows platform.

3 conclusion

Win32 API provides many options for applications to implement inter-process communication. How can Developers choose? The following questions should be considered before deciding which IPC method to use:
(1) Whether the application works in a network environment or a standalone environment.