子进程和父进程之间的通信

Question

我正在尝试在两个进程之间进行通信，一个发送指令的父进程和一个返回响应的子进程。

    void create_player(Game *g, Player *p, int player, char* command) {

       int send[2];
       int receive[2];
       int file;

        if(pipe(send)!=0) {
            _exit(99);
        }
        if(pipe(receive)!=0) {
            _exit(99);
        }

        g->player[player].id = fork();

        if(g->player[player].id < 0) {
            fprintf(stderr, "Unable to start subprocess\n");
            fflush(stderr);
            exit(5);
        }

        //Parent process
        else if(g->player[player].id>0) {
            g->player[player].send = fdopen(send[1], "w");
            g->player[player].receive = fdopen(receive[0], "r");

            close(send[0]);
            close(receive[1]);

        //Child process
        }  else {

           close(send[1]);
           close(receive[0]);
           dup2(send[0], 0);
           dup2(receive[1], 1);
           close(send[0]);
           close(receive[1]);

           file = open("/dev/null", O_WRONLY);

           for(i =0; i < player; i++) {
                fclose(g->player[i].send);
                fclose(g->player[i].receive);
            }

            char width[1024];
            char playerId[26];
            char numOfPlayers[26];
            char seed[1024];
            char numOfCarriages[1024];

            sprintf(seed, "%d", g->seed);
            sprintf(width, "%d", g->numOfCarriages);
            sprintf(playerId, "%d", player);
            sprintf(numOfPlayers, "%d", g->numOfPlayers);

            char* args[] = {command, numOfPlayers, playerId, width, seed, NULL};

            execlp(command, command, numOfPlayers, playerId, width, seed, (char*) 0);

            _exit(99);

}

当我运行代码块来创建子进程时，没有任何消息通过(父标准输出 -> 子标准输入)发送。我添加了一些示例来展示我如何处理消息传递。任何帮助将不胜感激。

在父进程中:父进程向子进程发送消息

//Send message to start the game
void send_startGame(Game *g) {
    fprintf(g->player[g->currentPlayer].send, "startGame\n");
    fflush(g->player[g->currentPlayer].send);
}

在子进程中:子消息接收来自父进程的消息

void read_message(Game *g) {
    char message[2048];

    if(fgets(message, 2048, stdin) == NULL) {
        fprintf(stderr, "Error in message\n");
    }

    //Receive start game message
    if(strncmp("startGame\n", message, 9)==0){
        start_game(g);
    }

}

Answer 1

尝试减少你的代码，子代码中有错误的关闭:

       close(send[1]);
       dup2(send[0], 0);
       close(send[0]);

在 dup2 之后，文件描述符 send[0] 不再与发送管道输入相关，并且它可能会意外关闭子进程中的其他文件描述符。

您的代码不会从父级的标准输出到子级的标准输入进行通信。下面是一个小示例，将父级的标准输出重定向到管道的输入，并将管道的输出重定向到子级的标准输入。

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

int main(int argc, char **argv) {
    int send[2];
    if(pipe(send)!=0) {
        perror("pipe");
    } else {
        int pid = fork();
        if(pid < 0) {
            perror("fork");
        }
        else if(pid>0) {
            //Parent process
            close(send[0]);  // close pipe input 
            dup2(send[1], 1); // replace stdout with pipe output

            // send message to child
            fprintf(stdout, "send to child\n");
            fflush(stdout);
        }  else {
            //Child process                     
            close(send[1]);      // close pipe output                       
            dup2(send[0], 0); // replace stdin with pipe input

            char message[2048];
            if(fgets(message, 2048, stdin) != NULL) {
                fprintf(stderr, "message from parent:%s", message);
            }
        }
    }
}

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