我如何 PIPE 文件输入或 output 缓冲区刷新？

Question

today my code is had problem to input & output buffer flush problem.

this problem is

first Process(#1) 's code....

int sign_sender(char* command)
{
    int f , len;

    f=open(PIPE_NAME,O_RDWR);

    if(f<0)
    {
        printf("open fail : %s\n",strerror(errno));
        exit(1);
    }

    while(1)
    {
        len=strlen(command);
        if(len<=0)
        {
            continue;
        }
        write(f,command,len);
        printf("write...\n");
        sleep(1);
    }
    return 1;
}

this is write string non blocking...

and.

Second process(#2) is...

int sign_receiver()
{
    int f , len;
    char temp_buffer[128]={0};

    f=open(PIPE_NAME,O_RDWR|O_NONBLOCK);
    if(f<0)
    {
        printf("open fail \n");
        exit(1);
    }

    while(1)
    {
        if((len=read(f,temp_buffer,sizeof(temp_buffer)))<0)
        {
            printf("read error. len = %d\n",len);
        }
        printf("read...\n");
        if(len>0)
        {
            printf("len : %d , sign process recv : [%s]\n",len,temp_buffer);
        }
        sleep(1);
    }

    close(f);
    unlink(PIPE_NAME);

    return 1;
} 

like this. process(#1) is first run. and 5 sec after process(#2) is run..

process(#2) 's printf....

read...
len : 15 , sign process recv : [hellohellohello]
read...
len : 5 , sign process recv : [hellohellohello]
read...
len : 5 , sign process recv : [hellohellohello]
read...
len : 5 , sign process recv : [hellohellohello]
read...
len : 5 , sign process recv : [hellohellohello]
read...
len : 5 , sign process recv : [hellohellohello]

i want this printf.

read...
len : 5 , sign process recv : [hello]
read...
len : 5 , sign process recv : [hello]
read...
len : 5 , sign process recv : [hello]
read...
len : 5 , sign process recv : [hello]

i dont know why.... i'm so noob of C...

Answer 1

The reason why [hellohellohello] keeps getting written after the first read, is because you do not clear your buffer in-between each read operation. So what you actually see on the next lines where it reads:

len: 5, sign process recv: [hellohellohello]

Is the message hello that was just read, and that overrided the 5 first letters contained in your buffer's previous state, followed by what is remaining in the buffer, hence the repetition of the message.

Here is an example of how you can clear the buffer:

int sign_receiver()
{
    int f , len;
    char temp_buffer[128]={0};

    f=open(PIPE_NAME,O_RDWR|O_NONBLOCK);
    if(f<0)
    {
        printf("open fail \n");
        exit(1);
    }

    while(1)
    {
        if((len=read(f,temp_buffer,sizeof(temp_buffer)))<0)
        {
            printf("read error. len = %d\n",len);
        }
        printf("read...\n");
        if(len>0)
        {
            printf("len : %d , sign process recv : [%s]\n",len,temp_buffer);
            memset(temp_buffer, 0, 128); <- Adding this line
        }
        sleep(1);
    }

    close(f);
    unlink(PIPE_NAME);

    return 1;
}

But that won't get you rid of the first [hellohellohello] , and that is because you are writing on the named pipe several times before reading it (3 times in your case). These messages are buffered in a system buffer, until it is retrieved by the read calls.

Your Non-Blocking read doesn't mean that it will discard the piled up data accumulated in the system buffer, it just means that its call will return immediately, even if no data has been read.

I'd suggest inverting the order in which you call the two processes, running executable 2 before executable 1, if you don't want messages to add up on the first read.

You could also add an extra definition in your messages, with a header/footer, or a delimiter that would let you parse messages inside the payload.