使用重定向 I/O 与子进程通信时出现死锁

Posted 2023-02-22

【中文标题】使用重定向 I/O 与子进程通信时出现死锁

【英文标题】：Deadlock when communicating with child process with redirected I/O

【发布时间】：2018-06-24 12:38:48

【问题描述】：

我正在编写一个程序以编程方式与 SPIN 模型检查器的交互式模块进行通信。为此，我需要重定向 SPIN 的 I/O，从我的程序中启动它，然后反复读取和写入它。为了进行测试，我使用了以下只有一个输入和一个输出的短程序，而不是 spin：

#include <string> 
#include <iostream> 

using std::cin;
using std::cout;
using std::string;

void sprint(string s);
int main() 
   
    std::string s = "empty";
    cin >> s;
    cout << "\n\tthe text is: " << s;
    return 0;    
 

我的程序，主要来自this answer，是：

#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <cstring>
#include <unistd.h>
#include <errno.h>
#include <iostream>

#define PIPE_READ 0
#define PIPE_WRITE 1
#define maxReadSize 2048
using std::cout;
using std::cin;
using std::string;

int createChild(const char* szCommand, const char* aArguments, const char* szMessage);

int main()

    createChild("./inout" , "inout", "hello");
    return 0;



int createChild(const char* szCommand, const char* aArguments, const char* szMessage) 

    int cStdinPipe[2];
    int cStdoutPipe[2];
    int nChild;
    char nChar;
    int nResult;

    if (pipe(cStdinPipe) < 0) 
     
        return -1;
    
    if (pipe(cStdoutPipe) < 0) 
    
        close(cStdinPipe[PIPE_READ]);
        close(cStdinPipe[PIPE_WRITE]);
        return -1;
    

    nChild = fork();
    if (0 == nChild) 
    
        // child continues here
        cout.flush();
        close(cStdinPipe[PIPE_WRITE]);

        // redirect stdin
        if (dup2(cStdinPipe[PIPE_READ], STDIN_FILENO) == -1) 
        
            exit(errno);
        

        // redirect stdout
        if (dup2(cStdoutPipe[PIPE_WRITE], STDOUT_FILENO) == -1) 
        
            exit(errno);
        

        // all these are for use by parent only
        close(cStdinPipe[PIPE_READ]);
        close(cStdoutPipe[PIPE_READ]);
        close(cStdoutPipe[PIPE_WRITE]); 

        // run child process image
        nResult = execlp(szCommand, aArguments, NULL);

        // if we get here at all, an error occurred, but we are in the child
        // process, so just exit
        exit(nResult);
     
    else if (nChild > 0) 
    
        // parent continues here
        string messageFromChild = "";
        string messageFromParent = "";
        char readBuffer[maxReadSize];
        int bytesWritten = 0;
        int bytesRead = 0;

        // close unused file descriptors, these are for child only
        close(cStdinPipe[PIPE_READ]);
        close(cStdoutPipe[PIPE_WRITE]); 

        // write to child
        if (NULL != szMessage) 
        
            bytesWritten = write(cStdinPipe[PIPE_WRITE], szMessage, strlen(szMessage));
        

        // read from child
        bytesRead = read(cStdoutPipe[PIPE_READ], readBuffer, maxReadSize);

        cout << "\nChild says: " << readBuffer << "\n";

        // done with these in this example program, you would normally keep these
        // open of course as long as you want to talk to the child
        close(cStdinPipe[PIPE_WRITE]);
        close(cStdoutPipe[PIPE_READ]);

        std::cout << "\n\nParent ending";
     
    else 
    
        // failed to create child
        close(cStdinPipe[PIPE_READ]);
        close(cStdinPipe[PIPE_WRITE]);
        close(cStdoutPipe[PIPE_READ]);
        close(cStdoutPipe[PIPE_WRITE]);
    
    return nChild;

运行此程序会导致死锁，其中子进程卡在cin 上，而父进程卡在read() 上。删除其中任何一个调用都会导致两个程序运行到终止并正常退出。 write() 和 cout 都可以正常工作。

【问题讨论】：

现在卡在select()

【参考方案1】：

问题出在read() 和write() 如何处理各种流/文件/管道。我的write() 无法与已完成写入的read() 通信。 Read() 依赖于 EOF，它仅在管道的写入端关闭后发送。当然，一旦关闭，它就永远消失了，我无法重新打开它来发送另一条消息。

因此，我切换到命名管道 (fifo)，以便在每次写入后重新打开管道。