[Linux process communication] Anonymous pipe

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Table of Contents

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Preface

Prerequisite knowledge supplement

Principles of pipe communication

Characteristics of pipelines

Pipeline creation

Create interface pipe()

Example demonstration and analysis

Pipeline closed

Expand knowledge and supplement

Foreword

The content of this chapter is anonymous pipes. If you want to learn named pipes step by step link named pipes detailed explanation click here! ! !

Let’s start learning about anonymous pipe communication:

An anonymous pipe is an IPC mechanism used for communication between related processes. It is usually created by a parent process and can only be used by related processes.
An anonymous pipe is nameless, it has no name in the file system and can only be used between the process that created it and its child processes.
Reading and writing anonymous pipes requires the use of file descriptors (file descriptor 0 represents standard input, file descriptor 1 represents standard output, etc.), where 0 represents the reading end and 1 represents the writing end. For the convenience of reading, the following anonymous pipes are referred to as pipes

Prerequisite knowledge supplement

Before learning about process communication, let’s talk about some basic knowledge:

Purpose of inter-process communication:

Data transfer: One process needs to send its data to another process

Resource sharing: sharing the same resources between multiple processes.

Notification event: A process needs to send a message to another process or a group of processes to notify it that a certain event has occurred (such as notifying the parent process when the process terminates).

Process control: Some processes hope to completely control the execution of another process (such as the Debug process). At this time, the control process hopes to intercept all traps and exceptions of another process and be able to know its status changes in time.

Processes are independent:

Processes are the basic execution units in operating systems. They run in independent memory spaces and have their own code, data and system resources. This independence is to ensure that an error or crash in one process will not affect other processes, thus improving the stability and security of the system. Each process has its own address space, stack, registers, etc., which makes them isolated from each other.

Inter-process interaction cost:

Due to the characteristics of process independence, the cost of data interaction between processes is relatively high. This is because different processes cannot share memory directly. They must use some communication mechanisms to share and interact with data. This communication mechanism introduces a certain amount of overhead, including data copying, process switching, and synchronization, making inter-process communication relatively slow.

Information exchange required:

Although processes are isolated from each other, in practical applications, information exchange between different processes is sometimes required. This can be because they need to share data, work together, or require some form of coordination. For example, a server process needs to receive client requests and provide responses, which requires information exchange between processes. Or in parallel computing, different processes may need to coordinate tasks and share results to speed up calculations.

In order to realize information interaction between processes, the operating system provides various process communication mechanisms, such as pipes, message queues, shared memory, signals, sockets, etc. Each mechanism has its advantages and applicable scenarios, and developers choose the appropriate communication method based on specific needs. Although process communication is costly, it is a key element in fields such as multitasking, distributed systems, and parallel computing, helping different processes work together, share data, and implement complex applications.

Principles of Pipeline Communication

The principle of pipe communication is through a buffer maintained by the operating system kernel. When one process writes data, the data is stored in the buffer, and then another process can read the data from the buffer. Typically, the parent process creates the pipe and owns its writing end, while the child process inherits the reading end of the pipe. This allows the parent and child processes to work together. Pipes are a communication method based on file descriptors, in which the file descriptors on the write side and the read side serve as the interface of the communication channel, allowing one-way transmission of data flow.

Characteristics of pipelines

Pipe (Pipe) is a special mechanism used for inter-process communication (IPC) in computer science and operating systems. It has the following characteristics:

One-way data flow: Pipes are unidirectional, and data can only flow from one process to another. This means that one process can write data to the pipe, while another process can only read this data from the pipe.
Based on file descriptors: Pipes use file descriptors (File Descriptors) to identify and access, one of which is used for reading (reading side) and the other for writing (writing side). Processes can transfer data through file descriptors.
Kernel buffering: The data transfer of the pipe is carried out through the kernel’s buffer, which transfers data between the writing end and the reading end. This buffer is usually of limited size, so if writes occur faster than reads, or reads occur faster than writes, blocking conditions or data loss can occur.
Blocking and non-blocking operations: Reading and writing from a pipe can be blocking or non-blocking. In blocking mode, if the operation cannot be completed immediately, the process will be blocked waiting for the condition to be met. In non-blocking mode, if the operation cannot be completed immediately, it will return immediately without waiting.
Process Relationship: Typically, a pipe is created by a parent process and can be shared with one or more child processes. A parent process can create a pipe and then pass the pipe’s file descriptor to the child process, allowing them to communicate using the pipe.
Named Pipe: Named pipes allow multiple processes to communicate between unrelated processes. They have a name in the file system, and multiple processes can communicate by opening the same named pipe. This is different from anonymous pipes, which are typically used between processes that are related.
Lifecycle: A pipe is usually associated with the process that created it. Once the parent process that created it or the process that created the named pipe terminates, the pipe will be closed and no longer available.
Lightweight and fast: Pipes are a lightweight communication mechanism because they do not involve heavy protocol or network communication. This makes them very fast and efficient for local inter-process communication.

Pipeline Creation

Create interface pipe()

Let us first learn about the system interface pipe() that needs to be called to create a pipeline.

pipe() is a system call usually used to create pipes in Unix/Linux operating systems. This call is very simple, its purpose is to create a channel for inter-process communication, where one process can write data to the channel and another process can read data from the channel.

Here is some important information about the pipe() call,

Function prototype:
```
int pipe(int pipefd[2]);
```
- pipefd is an integer array containing two elements. pipefd[0] is the file descriptor for reading, pipefd[1] is the file descriptor for writing.
Return value:
- If pipe() is called successfully, it will return 0.
- If it fails, it returns -1 and sets the errno variable to indicate the type of error.
Error handling:
- If pipe() fails, you can use the perror() function to print an error message for debugging and error handling.
Purpose after pipeline creation:
- After creating the pipe, you can use the file descriptor pipefd[0] to read the data in the pipe and the file descriptor pipefd[1] to write the data. These two file descriptors are the interfaces for pipe communication.
Pipes are one-way:
- Pipes are one-way communication, and data can only flow from the writing end to the reading end. If you need bidirectional communication, you’ll typically create two pipes, one for each direction.
Inter-process communication:
- Pipes are often used to implement inter-process communication, such as data transfer between parent and child processes.
Note:
- After creating the pipe, make sure to close unnecessary file descriptors when they are no longer needed to free up resources.

Example demonstration and analysis

The following is an example of creating a pipeline using C language, and the corresponding text description:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main() {
    int pipe_fd[2]; // File descriptor used to store the pipe, pipe_fd[0] is the read end, pipe_fd[1] is the write end

    if (pipe(pipe_fd) == -1) {
        perror("Pipe creation failed");
        exit(1);
    }

    printf("Pipe created successfully.\
");
    printf("Read end file descriptor: %d\
", pipe_fd[0]);
    printf("Write end file descriptor: %d\
", pipe_fd[1]);

    close(pipe_fd[0]); // Close the reading end
    close(pipe_fd[1]); // Close the write end

    printf("Pipe closed.\
");

    return 0;
}

The above code creates a pipeline and performs the following operations:

The pipe_fd[2] array is used to store the file descriptor of the pipe. pipe_fd[0] is the read end of the pipe. pipe_fd[1] is The write end of the pipe.
A pipe is created by calling the pipe() function. If creation fails, it prints an error message and exits.
After successfully creating the pipe, the program prints a success message and displays the file descriptors for the read and write sides.
Next, the program closes the read and write ends of the pipe, which is good practice to ensure resources are released.

This example shows how to use the pipe function to create a pipe and print the associated file descriptor. After the pipeline is successfully created, data transmission and inter-process communication can be performed through these file descriptors.

Pipeline closed

The operation of closing a pipe involves the management of file descriptors. You can use the close() system call to close the read or write end of the pipe. In C, a pipe can be closed as follows:

Close the reader:

close(pipe_fd[0]); // where pipe_fd[0] is the read-side file descriptor of the pipe

Close writing end:

close(pipe_fd[1]); // where pipe_fd[1] is the write-side file descriptor of the pipe

In actual programming, when you no longer need to read or write to the pipe, you should close the corresponding file descriptor. This helps free up resources and ensures that unexpected data reads and writes don’t occur. Additionally, if you don’t close a file descriptor when it’s no longer needed, you can cause a resource leak.

Expand knowledge supplement

Why does the parent process open the read and write ends separately?

When creating a pipe, the parent process usually needs to open both the read and write sides of the pipe. This is because pipes are a two-way communication mechanism that allows data to flow in both directions, and the parent process may need to both write data to and read data from the pipe. Therefore, the parent process opens file descriptors for reading and writing respectively.

Why does the parent process close the corresponding reading and writing?

Once the parent process has completed the pipe operations it needs, it usually closes the file descriptors it no longer needs to release resources and avoid resource leaks. For example, if the parent process only needs to write data to the pipe, it will close the reading side to ensure that it does not accidentally try to read data from it, and vice versa.

Who decides what reading and writing is closed between father and son?

Deciding which process closes which file descriptor depends on the needs of the process. Typically, the parent process creates the pipe and determines how it is used. A child process can inherit the file descriptors of the parent process, but it can choose to close unnecessary file descriptors according to its own needs. For example, if the child process only needs to read data, it can close the file descriptor on the write side.

Pipes are sequential: Pipes ensure that data is read in the order in which they were written. This means that the data written to the pipe will

They are read in the order they were written, maintaining the orderliness of the data.

Blocking waiting: Inside the pipeline, both read operations (read) and write operations (write) can cause the process to block and wait. if

There is no data to read, and the read operation will wait for the data to arrive. Likewise, if the pipe is full, write operations will wait for space to become available. this

Data consistency and integrity are ensured.

Pipeline has access control mechanism: The pipe has its own access control mechanism to ensure that only one process can write data and another process can read data. This maintains synchronization and mutual exclusion, ensuring ordering and consistency of data between processes.

Waiting queue: In the implementation of pipelines, the operating system usually uses waiting queues to manage the blocking state of the process. When a process has no

When the method continues execution, it will be placed in the waiting queue and awakened at the appropriate time to continue execution.

The above is my summary of process communication and pipeline knowledge. I hope it will be helpful to you! ! !