select () C lib function always returns 0

Question

I'm currently trying to run a server to attend several "clients'"(local processes) requests at the same time using a Named Pipe as IPC. Clients are able to write on it but it seems the select() function on the server is not working properly, its returning 0 all the time.

This is the server's main code:

int main (int argc, char const *argv[]){

    fd_set set;         //fds to monitor
    Request * r;
    struct timeval tv;  //timeout
    Connection *c;      //where the fd will be saved
    fd_set foo;         //debugging purpose

    //opens the NamedPipe and saves de ReadOnly fd on c
    if( (c = openConnection()) == NULL) {
        return ERROR_OPEN_REQUEST_QUEUE;
    }

    //sets up the fds to monitor  FD_ZERO and FD_SET
    setConnection(c, &set); 

    FD_ZERO(&foo);

    tv.tv_sec = 2;
    tv.tv_usec = 0;

    while(1){

        int fdCount = select(2, &set, &foo, &foo, &tv);

            //it seems select directly modifies this value
            tv.tv_sec = 2;

            //saw it on another post        
            setConnection(c, &set); 
        if( fdCount > 0){

            r = getRequest(c);

            if( r != NULL ){
                TODO processRequest(r);
            }
        } else {
            printf("No requests to process\n");
        }
    }
    return 0;
}

Both server and client use openConnection to get the fd from the NamedPipe. openConnections calls this function and creates a Connection Object:

int * openNamedPipe(char * name) {
  char origin[] = "/tmp/";
  char myfifo[80];
  int * fd;
  fd = malloc(sizeof(int)*2);

  strcpy(myfifo,origin);
  strcat(myfifo,name);
  mkfifo(myfifo, 0777);

  fd[0] = open(myfifo, O_RDONLY|O_NONBLOCK);
  fcntl(fd[0], F_SETFL, fcntl(fd[0], F_GETFL) &~O_NONBLOCK);
  fd[1] = open(myfifo, O_WRONLY);

  return fd;
}

My questions are the following:

Is calling mkfifo() several times over the same pipe for each client a problem?
Same for opening/closing the pipe

I'm manually checking the fifo with cat, I'm able to read stuff from the shell. So if clients are able to write, the server should be able to read with the ReadOnly fd.

Adding setConnection function just in case:

void setConnection(Connection * connection, fd_set* set){
  FD_ZERO(set);
  FD_SET(connection -> np -> fd, set);
  return;
}

Have you read the [select man page](http://linux.die.net/man/2/select)? — hyde, May 12 '16 at 21:43
In particular, note that `select` modifies the file descriptor sets passed to it! — hyde, May 12 '16 at 21:45
Yeah, I read about that. That's why I'm Re-setting the fds after each select. Or should I do it before? — J. Nicastro, May 12 '16 at 21:47
Exactly, but the thing is that It shouldn't. As there are requests at the pipe ready to be read. — J. Nicastro, May 12 '16 at 21:50
I think you're running foul of the problem described in [Why `select()` always returns 0 after the first timeout?](https://stackoverflow.com/questions/3324078/why-select-always-return-0-after-the-first-timeout/3324123) or [Is it necessary to reset the `fd_set` between `select()` system calls?](https://stackoverflow.com/questions/4563577/is-it-necessary-to-reset-the-fd-set-between-select-system-call/4563608). You should know whether calling `mkfifo()` multiple times is a problem because your code should be checking whether `mkfifo()` succeeds or not. — Jonathan Leffler, May 12 '16 at 21:55
Also, you're saying it returns always `0`, however in the **code in the question you're not testing for 0! It can be negative for errors**. — Antti Haapala -- Слава Україні, May 12 '16 at 21:55
I tried to keep the code as clean as possible by eliminating mosts of the checks. mkfifo doesn't fail. Also I'm resetting the fd_set between the select() using the setConnection function. — J. Nicastro, May 12 '16 at 21:59
I'm actually seeing the pipe on the tmp folder on live action. Also open() is not failing. — J. Nicastro, May 12 '16 at 22:00
I'm now checking for select returning < 0 but it's just returning 0. — J. Nicastro, May 12 '16 at 22:03

score 4 · Accepted Answer · answered May 12 '16 at 22:01

4

First parameter to selectshould be highest numbered file descriptor +1. You are passing value 2, so your select is only concerned about fds 0 and 1 (if they are set in the passed sets). Does your pipe really use those? If not, you need to test in the code which is the highest and pass that to select.

Another thing, seems like you should just pass NULL instead of that foo, if you don't want to watch those events.

answered May 12 '16 at 22:01

hyde

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Now trying: int fdCount = select(c -> np -> fd, &set, NULL, NULL, &tv); But its not working either. Thanks for your answer though. – J. Nicastro May 12 '16 at 22:05
I forgot to add 1 to it. You are my savior. Thanks a lot! I spent 1 entire day working on this error. You saved me! – J. Nicastro May 12 '16 at 22:08
I thought it meant the max number of fds. It was 1 before, and change to 2 just in case. – J. Nicastro May 12 '16 at 22:10
@J.Nicastro Glad to be of help – hyde May 13 '16 at 03:59

select () C lib function always returns 0

1 Answers1