使用命名管道将命令发送到正在运行的进程

Question

I have a program that runs as a service, which takes the standard input from the console, and prints the feedback to the screen. It can be thought as the following simple example, while the printf() function is actually some more complicated function.

//keyin.c
int main(int argc, char *argv[])
{
    char buf[80];
    int i = 1;

    while(i++){ 
    printf("enter %d string: \n", i);
    scanf("%s", buf);
    printf("print %d string: %s \n\n", i, buf);
    sleep(2);
    }

    return 0;
}

Now I want to use a named pipe as the input so that I can use another program to pass desired commands to the service by writing to the pipe, instead of typing in the console.

I created a named pipe by:

mknod MYFIFO p

and then I redirect the input by:

root@ubuntu: ./keyin < MYFIFO

and then I write the command into pipe by:

root@ubuntu: echo "test" > MYFIFO

I expect the service take the "test", print it out, and wait for the next input, however, after the first run, it keeps printing out "test" infinitely, and never responds to the pipe file again, even when I delete it.

What did I do wrong?

Answer 1

You should check the return value of scanf() . Note that for every time you get the same repeated buffer, scanf() would return a negative value.

int main(int argc, char *argv[]){
   char buf[80];
   int i = 1;
       while(i++){
       if(scanf("%s", buf) > 0){
           printf("print %d string: %s \n\n", i, buf);
       }
       sleep(2);
    }
    return 0;
}

Also, note that a named pipe is not like a normal pipe or the console and will behave differently.