c - 使用管道在父子之间传递整数值

Question

我对如何正确使用 pipe() 在两个进程之间传递整数值感到有点困惑。

在我的程序中，我首先创建了一个管道，然后我创建了它。我假设我有“两个”管道？

据我了解，这是我的任务。我的 parent 通过 for 循环检查某个操作的整数值“i”，增加计数变量，并将值保存到数组中。每次检查后，我的 parent 应该通过管道将一个整数值“i”传递给我的 child 。我的 child 然后使用该整数值，对该值进行一些检查，并且应该增加一个计数变量，并将结果保存在一个 [shared?] 数组中。最终; child 应该将它的最终计数返回给 parent ，然后 parent 打印出这两个计数和“共享”数组。

-> 我不确定是否需要共享数组或是否需要保存结果。我可能只需要计数 - 家庭作业模棱两可，我正在等待教授的回复。还;我什至可以在进程之间做一个共享数组吗？对我来说，这听起来像是某个问题的开始。

-> 以下是我的问题:一;如何使用管道表示整数？我只看到它们用于字符数组，以前的答案似乎不认为这是可能的或合法的..？我不确定。我找不到解决办法。

-> 如何使用单向管道将整数传递给 child ？让 child 还东西？我不确定我如何能够……区分这两个管道。我确实“知道”[或认为我知道]我必须关闭每个管道的一个未使用部分以避免“一些模糊的问题”。

抱歉提出愚蠢的问题；我还没有在这门课上学过进程(除了 fork)或管道(根本)——所以我真的不确定从哪里开始!

这是我的部分代码 - 它不漂亮而且不起作用，我不希望它起作用。它更像是一个 shell 占位符。一旦我弄清楚如何使用管道 - 我可能会使代码有意义。

int main(void)
{
int fd[2];
pid_t childpid;
pid_t parentpid;
int i;
int threecount = 0;
int fivecount = 0;;
int results [MAXSIZE];

parentpid = getpid(); //Get current process ID number

pipe(fd);
childpid = fork(); 

if(childpid == 0){
close(fd[0]); //Closing this for some other reason

}
int j = 0;

if(childpid > 0)
close(fd[1]); //Closing this for some reason


if( childpid == -1 )
{
    perror("Failed to fork\n");
    return 1;
}
if (childpid > 0)
{
    for(i = 1; i < MAXSIZE;i++)
    {
        if(i % 5 == 0)
        {
            fivecount++;
            i = results[j];
            j++; 
            wait(NULL);
        }
    }
}
else if (childpid == 0)
    {
        if(i % 3 == 0) //This i here should probably be the i value above, piped to the child
        {
            threecount++;
            i = results[j];  //This should be part of th pipe
            j++;    //Trying to keep count of that shared array, not really the right way to do it though.
        }

    }
    printf("%d  %d  \n", fivecount,threecount);
    return 0;

Answer 1

这是关于使用管道将 int 从父进程发送到子进程的示例(并且没有错误检查，顺便说一句)，因为子进程是从 fork() 启动的。发送和接收数据变得更加复杂(显然)，但我不能为您做一切。这只是 fork 并等待来自 child 的 int(实际上，是 int 使用的字节数)。

更新:在此之后添加了send+response双向通信示例。有关详细信息，请参阅第二个代码 list 。

希望对您有所帮助。

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

int main(int argc, char *argv[])
{
    int fd[2];
    int val = 0;

    // create pipe descriptors
    pipe(fd);

    // fork() returns 0 for child process, child-pid for parent process.
    if (fork() != 0)
    {
        // parent: writing only, so close read-descriptor.
        close(fd[0]);

        // send the value on the write-descriptor.
        val = 100;
        write(fd[1], &val, sizeof(val));
        printf("Parent(%d) send value: %d\n", getpid(), val);

        // close the write descriptor
        close(fd[1]);
    }
    else
    {   // child: reading only, so close the write-descriptor
        close(fd[1]);

        // now read the data (will block)
        read(fd[0], &val, sizeof(val));
        printf("Child(%d) received value: %d\n", getpid(), val);

        // close the read-descriptor
        close(fd[0]);
    }
    return 0;
}

输出:

Parent(5943) send value: 100
Child(5945) received value: 100

---

更新:扩展为包括使用两个管道集的发送+响应

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>

// some macros to make the code more understandable
//  regarding which pipe to use to a read/write operation
//
//  Parent: reads from P1_READ, writes on P1_WRITE
//  Child:  reads from P2_READ, writes on P2_WRITE
#define P1_READ     0
#define P2_WRITE    1
#define P2_READ     2
#define P1_WRITE    3

// the total number of pipe *pairs* we need
#define NUM_PIPES   2

int main(int argc, char *argv[])
{
    int fd[2*NUM_PIPES];
    int val = 0, len, i;
    pid_t pid;

    // create all the descriptor pairs we need
    for (i=0; i<NUM_PIPES; ++i)
    {
        if (pipe(fd+(i*2)) < 0)
        {
            perror("Failed to allocate pipes");
            exit(EXIT_FAILURE);
        }
    }

    // fork() returns 0 for child process, child-pid for parent process.
    if ((pid = fork()) < 0)
    {
        perror("Failed to fork process");
        return EXIT_FAILURE;
    }

    // if the pid is zero, this is the child process
    if (pid == 0)
    {
        // Child. Start by closing descriptors we
        //  don't need in this process
        close(fd[P1_READ]);
        close(fd[P1_WRITE]);

        // used for output
        pid = getpid();

        // wait for parent to send us a value
        len = read(fd[P2_READ], &val, sizeof(val));
        if (len < 0)
        {
            perror("Child: Failed to read data from pipe");
            exit(EXIT_FAILURE);
        }
        else if (len == 0)
        {
            // not an error, but certainly unexpected
            fprintf(stderr, "Child: Read EOF from pipe");
        }
        else
        {
            // report what we received
            printf("Child(%d): Received %d\n", pid, val);

            // now double it and send it back
            val *= 2;

            printf("Child(%d): Sending %d back\n", pid, val);
            if (write(fd[P2_WRITE], &val, sizeof(val)) < 0)
            {
                perror("Child: Failed to write response value");
                exit(EXIT_FAILURE);
            }
        }

        // finished. close remaining descriptors.
        close(fd[P2_READ]);
        close(fd[P2_WRITE]);

        return EXIT_SUCCESS;
    }

    // Parent. close unneeded descriptors
    close(fd[P2_READ]);
    close(fd[P2_WRITE]);

    // used for output
    pid = getpid();

    // send a value to the child
    val = 42;
    printf("Parent(%d): Sending %d to child\n", pid, val);
    if (write(fd[P1_WRITE], &val, sizeof(val)) != sizeof(val))
    {
        perror("Parent: Failed to send value to child ");
        exit(EXIT_FAILURE);
    }

    // now wait for a response
    len = read(fd[P1_READ], &val, sizeof(val));
    if (len < 0)
    {
        perror("Parent: failed to read value from pipe");
        exit(EXIT_FAILURE);
    }
    else if (len == 0)
    {
        // not an error, but certainly unexpected
        fprintf(stderr, "Parent(%d): Read EOF from pipe", pid);
    }
    else
    {
        // report what we received
        printf("Parent(%d): Received %d\n", pid, val);
    }

    // close down remaining descriptors
    close(fd[P1_READ]);
    close(fd[P1_WRITE]);

    // wait for child termination
    wait(NULL);

    return EXIT_SUCCESS;
}

(编译，例如，gcc thisfile.c -o test)

输出

Parent(2794): Sending 42 to child
Child(2797): Received 42
Child(2797): Sending 84 back
Parent(2794): Received 84