Named pipes and C

jobless · 10-20-2008, 08:27 PM

Hello guys,

I created a named pipe on my Ubuntu machine using the command

Code:

mkfifo mypipe

Now I am trying to read from that pipe in a C program doing the following.

Code:

void listen() {
    int fp;
    char buf[255];

    fp = open("mypipe", O_RDONLY);
    if(fp == -1) {
        printf("Could not open the pipe\n");
    }

    while(1) {
        read(fp, buf, sizeof(buf));
        printf("%s", buf);
    }

    close(fp);
}

When I write to the pipe using the following command

Code:

echo "Hello" > mypipe

The C program just prints the text continuously without any break. Is it something wrong with my program or is it how the pipes work? And I can't check the returned string and see if it matched the previous read.

Thanks for your help.

SciYro · 10-20-2008, 09:36 PM

I think the problem is your code. You assume read() alters the buffer when it hits the end-of-file, a mistake I believe:

Code:

void listen() {
    int fp;
    char buf[255];
    int l; // read length

    buf[0] = '\0'; // in case there is nothing to read
    buf[255] = '\0'; // garentee a end-of-line

    fp = open("mypipe", O_RDONLY);
    if(fp == -1) {
        printf("Could not open the pipe\n");
    }

    do {
        l = read(fp, buf, 254);
        printf("%s", buf);
    } while (l > 0);

    close(fp);
}

jobless · 10-21-2008, 01:31 PM

The solution you provided worked. But it printed duplicates. So I changed it to print only when the read length is greater than 0. Also, reinitialize the buffer so that you guarantee that only the current information will be printed instead of the one from the previous read.

Thanks for all your help.

Code:

void listen() {
    int fp;
    char buf[255];
    int l; // read length
    int i;

    buf[0] = '\0'; // in case there is nothing to read
    buf[255] = '\0'; // garentee a end-of-line

    fp = open("mypipe", O_RDONLY);
    if(fp == -1) {
        printf("Could not open the pipe\n");
    }

    while(1) {
        l = read(fp, buf, 254);
        if(l) // print only if the read length is greater than 0
            printf("%s", buf);

        for(i=0; i<255; i++) // initialize the whole buffer again
            buf[i] = '\0';
    }

    close(fp);
}