Wait() 导致程序在打开 FIFO 命名管道时暂停

Question

每当我使用等待函数时，我的代码都会在 open() 上暂停，没有它程序将无法运行。没有错误代码，但我想知道哪里出了问题。

代码最终会是controller.c会打开c1,c2,c3。它将等待 c1 获取输入并将其放入管道，然后管道将转到 c2 打印它，然后 c3 以不同的格式打印它。

int main(int ac, char**av)
{
    int fd;
    int test;
    char * myfifo = "/tmp/myfifo";
    unlink(myfifo);                     //makes sure no fifo already exists

    /* create the FIFO (named pipe) */
    if((mkfifo(myfifo, 0666)) <0)
        perror("MKFIFO: ");



    int statval;
    pid_t pid1 = fork();                    //creates a process for c1 to run in
    wait(&statval); 
    if ( pid1 == 0)                     //makes sure to only run the program in the child process
    {   
        printf("Opening c1\n");
        int x = execvp("./c1",av);          //Runs the other file based on the array
        if (x == -1)
            exit (1);
        perror("Error");                //used for error reporting
        printf("Closing c1\n");
        exit(0);                    //ends process when finished
    }
    if (WIFEXITED(statval))                 //if there is an exit code
    printf("\nChilds exit code %d\n", WEXITSTATUS(statval));//prints the exit status of the child process

    pid_t pid2 = fork();                    //creates a process for c2 to run in
    wait(&statval); 
    if ( pid2 == 0)                     //makes sure to only run the program in the child process
    {   
        printf("Opening c2\n");
        int x = execvp("./c2",av);          //Runs the other file based on the array
        if (x == -1)
            exit (1);
        perror("Error");                //used for error reporting
        exit(0);                    //ends process when finished
    }
    if (WIFEXITED(statval))                 //if there is an exit code
    printf("\nChilds exit code %d\n", WEXITSTATUS(statval));//prints the exit status of the child process

    unlink(myfifo);

C1:

printf("TEST1\n");
    char *line = NULL;
    size_t len = 0;
    ssize_t read;
    int fd;
        char * myfifo = "/tmp/myfifo";
        char buf[BUFSIZE];

    fd = open(myfifo, O_WRONLY);
    if (fd < 0)
        perror("OPEN: ");   

    //while (1) {
        printf("Enter a line of text\n"); 
        //scanf("%s", buf);
        if ((strcmp(line,"-999"))==0){
            printf("TEST");         
            //break;
        }
        //write(fd, buf, sizeof(buf));
            write(fd, "hi", sizeof("hi"));
    //}
    close(fd);
    unlink(myfifo);
        return 0;

C2:

int fd;
    char * myfifo = "/tmp/myfifo";
    char buf[BUFSIZE];

    /* open, read, and display the message from the FIFO */
    fd = open(myfifo, O_RDONLY);
    read(fd, buf, BUFSIZE);
    printf("Received: %s\n", buf);
    close(fd);

    return 0;

程序将打开 c1，打印 TEST 然后什么都不做，直到我手动退出。

Answer 1

在第一次分叉之后，parent 和 child wait() 立即出现。 child 的调用失败，因为它没有 un-waited-for children，但您忽略了结果。 child 然后执行程序 c1，它（假设执行成功）依次打开 FIFO 进行写入。这个阻塞直到 FIFO 的另一端打开，但是另一端不会被你的程序打开，因为 parent 进程正在等待 child在继续之前退出。您造成了死锁。

在另一个（或其他程序）打开 FIFO 的另一端之前，child 都无法继续打开 FIFO，因此 parent 必须分叉两个 child ren 在等待任何一个之前。此外，children 到 wait() 毫无意义，尽管他们对该函数的调用应该很快失败。

Wait() 导致程序在打开 FIFO 命名管道时暂停

Wait() causing program to pause when opening FIFO named pipe

c

pipe

mkfifo