Creation of process in QT

---

Preface

This article will teach you how to create processes in QT.

1. Introduction to QProcess class

The QProcess class is a class in Qt used to start and control external processes. It provides a series of methods to execute external commands, interact with the process, and obtain the output information of the process. The following are some common functions of the QProcess class:

1. Start the process: Use the start() method to start the external process. You can set the program path and command line parameters to be executed, as well as the working directory and environment variables.

2. Process status: You can use the state() method to obtain the current status of the process, including running, stopped, completed, etc. You can also use the error() method to get the error code when any error occurs.

3. Process interaction: You can use the write() method to write data to the standard input of the process for interaction with the process. You can also use the closeWriteChannel() method to close the standard input, indicating that no more data will be written. You can use the readAllStandardOutput() and readAllStandardError() methods to obtain the standard output and standard error output of the process.

4. Wait for the process to complete: Use the waitForFinished() method to wait for the process to complete execution. You can optionally set a timeout and terminate the process after timeout.

5. Signal and slot mechanism: The QProcess class provides some useful signals, such as started() indicates that the process has started, finished() indicates that the process execution is completed, errorOccurred() indicates that an error occurred during execution, etc., you can use these signals to Handles the status and errors of the process.

6. Process exit code: Use the exitCode() method to obtain the exit code of the process, which is used to determine the result of the process execution.

2. Create process code

To create a process in Qt, you can use Qt's QProcess class. The QProcess class provides an interface for starting and controlling external processes. Here is a simple example showing how to create a process in Qt:

#include <QCoreApplication>
#include <QProcess>

int main(int argc, char *argv[])
{
    
    
    QCoreApplication a(argc, argv);
    
    // 创建一个QProcess对象
    QProcess process;
    
    // 设置要执行的程序
    process.setProgram("path/to/your/executable");
    
    // 设置要传递给程序的参数
    QStringList arguments;
    arguments << "arg1" << "arg2";
    process.setArguments(arguments);
    
    // 启动进程
    process.start();
    
    // 等待进程完成
    process.waitForFinished();
    
    // 获取进程的输出
    QByteArray output = process.readAllStandardOutput();
    
    // 打印输出结果
    qDebug() << "Process output:" << output;
    
    return a.exec();
}

3. Comparison of process creation in QT and thread creation in Linux

There are some flaws and differences in creating a process in Qt compared to directly creating a process in Linux. Here are some common pitfalls in creating processes in QT:

1. Large overhead: In Qt, creating a process usually involves operating system calls and resource allocation, which adds additional overhead. Compared with using the fork() function to create a child process in Linux, QProcess in Qt needs to start an independent external process, which involves the overhead of creating a process and establishing an inter-process communication pipeline.

2. Cross-platform compatibility: Qt is a cross-platform framework that can run on a variety of operating systems. Therefore, Qt's QProcess class provides a universal way to start and control external processes, no longer relying on operating system-specific functions for creating processes. This generic design may cause some platform-related details to be difficult to maintain and obtain.

比较而言，在Linux中使用fork()函数创建进程存在以下优势：

1. High efficiency: When using the fork() function to create a child process, the code and data of the parent process are copied and resources are shared. In this way, the overhead required to create a process is relatively low and more efficient.

2. Flexibility and directness: Using the fork() function, you can directly control the execution path of the child process and execute different codes according to different roles of the process. By judging the return value of the fork() function, the parent process and the child process can be clearly distinguished.

3. Lower-level control: In Linux, you can directly access the POSIX multi-thread library to create, manage and synchronize threads in a lower-level way, with higher flexibility. You can more precisely control thread execution, shared data, and more.

To sum up, compared with directly creating a process in Linux, creating a process in Qt has some disadvantages such as higher overhead and cross-platform compatibility. However, Qt provides the convenience and cross-platform advantages of wrapping external processes. In contrast, using the fork() function to create threads in Linux is relatively more efficient and more flexible. It can directly control different processes to execute different code paths, and provides lower-level thread control. You need to choose the appropriate way to create a process or create a thread based on specific project requirements and platform features.

Summarize

This article will explain it here.