7

我正在用 C 语言开发一个应用程序。父进程和子进程通过管道进行通信。在写入管道之前,父进程执行另一个语句。在示例代码中,我使用了 sleep(10) 来进行延迟。在子进程中,它应该从管道中读取数据。但是在子进程的管道读取端没有读取数据。

int main()
{
    int pid;
    FILE *fp;
    fp = fopen("test.txt","w");
    char *buff;
    int fd[2];
    int count = 0 ;
    pipe(fd);
    pid = fork();
    if(pid == 0)
    {
        close(fd[1]);
        ioctl(fd[0], FIONREAD, &count);
        fprintf(fp,"Value of count: %d ",count);
        buff = malloc(count);
        fprintf(fp,"\n TIME before read: %s",__TIME__);
        read(fd[0], buff, count);
        fprintf(fp,"\nbuffer: %s\n TIME after read %s", buff, __TIME__);
    }
    else{
        close(fd[0]);
        sleep(10);    //delay caused by application specific code replaced with sleep
        write(fd[1],"THIS is it",10);
    }
    fclose(fp);
    return 0;
}

如何让子进程等到另一端写入数据?

4

3 回答 3

6

您的管道以阻塞模式打开,您没有做任何更改,这可能是您想要的。

但是,由于您要做的第一件事是请求管道上等待的数据大小,然后盲目地跳入读取那么多字节(由于父级尚未写入任何内容,因此代码执行时该字节很可能为零) 你没有阻止,而是离开,因为你什么都没要求。

有很多方法可以做到这一点,包括选择循环。如果您希望在数据可用之前阻止读取,则在单个字节上执行此操作,然后填写剩余数据。

这绝不是如何正确执行此操作的示例,但它是一个简短示例,说明如何等待单个字节,请求管道的读取大小以获取其余数据,读取它,以及继续此操作,直到管道没有剩余数据并且父级关闭其末端:

我希望你觉得这对你有帮助。

#include <stdio.h>
#include <unistd.h>
#include <sys/ioctl.h>

int main()
{
    int pid = 0;

    // create pipe pair
    int fd[2];
    pipe(fd);

    pid = fork();
    if (pid == 0)
    {
        // child side
        char *buff = NULL;
        char byte = 0;
        int count = 0;

        // close write side. don't need it.
        close(fd[1]);

        // read at least one byte from the pipe.
        while (read(fd[0], &byte, 1) == 1)
        {
            if (ioctl(fd[0], FIONREAD, &count) != -1)
            {
                fprintf(stdout,"Child: count = %d\n",count);

                // allocate space for the byte we just read + the rest
                //  of whatever is on the pipe.
                buff = malloc(count+1);
                buff[0] = byte;
                if (read(fd[0], buff+1, count) == count)
                    fprintf(stdout,"Child: received \"%s\"\n", buff);
                free(buff);
            }
            else
            {   // could not read in-size
                perror("Failed to read input size.");
            }
        }

        // close our side
        close(fd[0]);
        fprintf(stdout,"Child: Shutting down.\n");
    }
    else
    {   // close read size. don't need it.
        const char msg1[] = "Message From Parent";
        const char msg2[] = "Another Message From Parent";
        close(fd[0]);
        sleep(5); // simulate process wait
        fprintf(stdout, "Parent: sending \"%s\"\n", msg1);
        write(fd[1], msg1, sizeof(msg1));
        sleep(5); // simulate process wait
        fprintf(stdout, "Parent: sending \"%s\"\n", msg2);
        write(fd[1], msg2, sizeof(msg2));
        close(fd[1]);
        fprintf(stdout,"Parent: Shutting down.\n");
    }
    return 0;
}

输出

Parent: sending "Message From Parent"
Child: count = 19
Child: received "Message From Parent"
Parent: sending "Another Message From Parent"
Parent: Shutting down.
Child: count = 27
Child: received "Another Message From Parent"
Child: Shutting down.
于 2013-03-04T08:27:26.900 回答
1

问题在于获取写入管道的字节数。你在 fork() 之后就得到了它。如果读取过程首先执行,它将不包含任何数据(并且计数为零)。如果写入过程先执行,它将包含一些数据。

如何让子进程等到另一端写入数据?

由于您以阻塞模式打开管道,因此您应该尽可能多地读取数据,而不是尝试获取写入数据的大小。


这是等待完整消息的修改示例:

#include <stdio.h>
#include <sys/ioctl.h>
int main()
{
int pid;
FILE *fp;
fp = fopen("test.txt","w");
char *buff = malloc(1024);
int fd[2];
int count = 0 ;
pipe(fd);
pid = fork();
        if(pid == 0)
        {
                close(fd[1]);

                int i = 0;

                while ( i < 10 )
                {
                    fprintf(fp,"\n TIME before read: %s \n",__TIME__);
                    read(fd[0], buff+i, 1);
                    ++ i;
                }

                fprintf(fp,"Full message received!\nbuffer: %s\n TIME after read %s\n", buff, __TIME__);
        }
        else{
                close(fd[0]);
                sleep(10);    //delay caused by application specific code replaced with sleep
                write(fd[1],"THIS is it",10);
        }
fclose(fp);
return 0;
}
于 2013-03-04T08:06:34.073 回答
1

我想之后

ioctl(fd[0], FIONREAD, &count);  
the count is 0.  

read(fd[0], buff, count) will get no data.  

try   
read(fd[0], buff, 10)
于 2013-03-04T08:07:58.000 回答