管道:写在管道的前面

Question

我在“TIMM_OSAL_WriteToFrontOfPipe”和“TIMM_OSAL_WriteToPipe”中遇到了链接

在TIMM_OSAL_WriteToPipe中，数据只是为了编写管道的末端。在TIMM_OSAL_WriteToFrontOfPipe中，在写入端写入数据，然后再读又写。

我试着用下面的示例程序复制相同的内容。但又不明白读写的意义。有人能给它加点光吗？

int main(void)
{
int fd[2];

int status = pipe (fd);
if (status) {
    fprintf(stderr, "** pipe create failed");
    return 0;
}

status = fork();
if (status < 0)
    fprintf (stderr, "** fork failure!");

if (status == 0){
    //close (fd[1]);
    char buf[5] = {0,};
    int ret_size;
    fprintf(stderr, "child going to sleep...\n");
    sleep (10);
    fprintf(stderr, "\nchild woken up...\n");
    ret_size = read (fd[0], buf, 5);
    fprintf (stderr, "child printing ...\n");
    fprintf (stderr, "%s\n", buf);
} else {
    char buf[5] = {0};
    int read_sz;
    int write_sz;
    //close (fd[0]);
    strcpy(buf,"1");
    write_sz = write (fd[1], buf, 1);
    strcpy(buf,"2");
    write_sz = write (fd[1], buf, 1);
    strcpy(buf,"3");
            write_sz = write (fd[1], buf, 1);
    strcpy(buf,"4");
    write_sz = write (fd[1], buf, 1);
    if (write_sz < 0)
        fprintf (stderr, "write failed");
    else
        fprintf(stderr, "written 1234\n");

    memset(buf, 0, sizeof(buf));

    strcpy(buf,"5");
    write(fd[1], buf, 1);

    memset(buf, 0, sizeof(buf));

    read_sz =  read (fd[0], buf, 5);
    if (read_sz > 0)
        fprintf(stderr, "read data :\n%s\n", buf);

    //write back to pipe
    write (fd[1], buf, 5);

}
return 0;
}

输出：

written 1234
read data :
12345
child going to sleep...
child woken up...
child printing ...
12345

我也尝试过不使用fork，在相同的过程中读和写。

Answer 1

WriteToFrontOfPipe的意思是：

|----------------CURRENT_DATA| -> |------CURRENT_DATA YOUR_DATA|

因为管道的作用类似于队列，所以需要三个操作：

|------YOUR_DATA CURRENT_DATA| //写入数据

|-------------------YOUR_DATA| //读取尾将数据放在前面

|------CURRENT_DATA YOUR_DATA| //保存以前的数据