Thread: Help with non-blocking IO

Hi everyone,

New user on this forum but also new to C and C++.

I was hoping someone could enlight a really beginner on something I've been struggling with in the last days.

Here is my task for an R&D Project. I've build a client app and a server app using sockets with Adobe AIR. Works well so far.

The server app is including a C compiled executable to act as a wrapper for another C library.

I've made the front (server app) talking with the C process that I launch from it. The app is basically able to send and receive data with the C process (just string for now). Pretty much like you would do from with command line.

I had 2 approaches to exchange data (just string for now):

- std::cin and std::cout
- read and write

They both work well.

The problem:

The C library I've included is using a thread to get information from a server, I made it working with some examples I had, but the example is using a usleep and it makes the while loop stopping which is a problem with the I/O I need (if I got the code right). Also the std::cin and read method stops the while loop which is a problem as I need to loop for the thread to process the events I receive from the server.

With a bit of search, I found out that what I probably need is called "non-blocking IO".

Great, but my knowledge of C is so light that I haven't been able to make any non-blocking IO examples working (I have like 6 google pages of visited links I tried to find a solution I would be able to able to understand/implement at my level)...

The closest I got is the following code (also attached as a file), which stops looping as soon as I receive an input (basically not working).

I was hoping someone could explain me a bit how I can do that (simply if possible, as I'm really a beginner), the best would be a simple command line program that shows the loop and IO working? Or tell me what is wrong in that code, in which I understand only half of it, as I tried bits and pieces from everywhere.

I understand I need to put my nose in some C, C++ book, which I will do, but right now, having this non-blocking IO working would make me able to continue.

Thanks in advance for any help.

Romuald

Code:

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <signal.h>
#include <pthread.h>

#include <string>
#include <sstream>
#include <iostream>

#include "appkey.h"
#include "libspotify/api.h"
#include "spotify.h"

#include <sys/socket.h>
#include <termios.h>

#define STDIN 0  // file descriptor for standard input
#define BUFFER_SIZE 8192 
#define NB_ENABLE 0
#define NB_DISABLE 1

bool exitloop = false;
pthread_t mainthread = 0;
char buf[BUFFER_SIZE];

static void sigIgn(int signo) { }

int kbhit() {
    struct timeval tv;
    fd_set fds;
    tv.tv_sec = 0;
    tv.tv_usec = 0;
    FD_ZERO(&fds);
    FD_SET(STDIN_FILENO, &fds); //STDIN_FILENO is 0
    select(STDIN_FILENO+1, &fds, NULL, NULL, &tv);
    return FD_ISSET(STDIN_FILENO, &fds);
}

void nonblock(int state) {
    struct termios ttystate;
    //get the terminal state
    tcgetattr(STDIN_FILENO, &ttystate);
    if (state==NB_ENABLE) {
        //turn off canonical mode
        ttystate.c_lflag &= ~ICANON;
        //minimum of number input read.
        ttystate.c_cc[VMIN] = 1;
    }
    else if (state==NB_DISABLE) {
        //turn on canonical mode
        ttystate.c_lflag |= ICANON;
    }
    //set the terminal attributes.
    tcsetattr(STDIN_FILENO, TCSANOW, &ttystate);
}

int main(int argc, char* argv[]) {
    int i = 0;
	int cnt; 
	mainthread = pthread_self();
	signal(SIGIO, &sigIgn);
	if(spotify->InitSession())
		printf("valid session.\n");
	else
		printf("invalid session.\n");
	spotify->SetCredentials("*************", "**************");
	spotify->Login();
	sigset_t sigset;
	sigemptyset(&sigset);
	sigaddset(&sigset, SIGIO);
    nonblock(NB_ENABLE);
    while(!i) {
        i=kbhit();
        if (i!=0) {
			while (!feof(stdin)) { 
				cnt = read(STDIN_FILENO, buf, sizeof buf); 
				write(STDOUT_FILENO, buf, cnt );
			} 
        }
		int timeout = -1;
		pthread_sigmask(SIG_BLOCK, &sigset, NULL);
		spotify->ProcessEvents(&timeout);
		pthread_sigmask(SIG_UNBLOCK, &sigset, NULL);
    }
    nonblock(NB_DISABLE);
    return 0;
}

Right, that sounds harder than I thought.

The server where we will be using this server app will a Mac. We don't do anything cross-platform right now.

Could you tell where should I start then?

Thanks for your answer.

Romu

On Mac OS X, check out kqueue(): works with sockets but not tty's, apparently. The FreeBSD forum should have more info on this, since OSX is based on that, and they have Asynchronous IO in FreeBSD.

Source:
Asynchronous I/O - Wikipedia, the free encyclopedia

Certainly above my pay grade, as a programmer.

Hi again,

Thanks for that information, I'll dig into that.

If I can ask again, as I had a go with another piece of code.

In the following code, I used the select() and I can now loop to get the input and perform the thread every second.

But I get an error in the console "select(): Interrupted system call" while I call a "processEvent" method on the library (code not visible).

Code:

spotify->ProcessEvents(&timeout);

This wasn't hapening in the first piece of code I posted.

Any idea what that that is, what could interupt a this select call or how I could solve it?

Thanks again for your time.

Romu

Code:

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <signal.h>
#include <pthread.h>

#include <string>
#include <sstream>
#include <iostream>

#include "appkey.h"
#include "libspotify/api.h"
#include "spotify.h"

#include <sys/socket.h>
#include <termios.h>

#define BUFFER_SIZE 8195
#define FD_NUM 0

fd_set rfds;
struct timeval tv;
int retval;
int cnt;
int readResult;
char buffer[BUFFER_SIZE];
pthread_t mainthread = 0;

static void sigIgn(int signo) { }

int timed() {
	
	/* Watch stdin (fd 0) to see when it has input. */
	FD_ZERO(&rfds);
	FD_SET(FD_NUM, &rfds);
	
	/* Wait up to five seconds. */
	tv.tv_sec = 1;
	tv.tv_usec = 0;
	
	retval = select(1, &rfds, NULL, NULL, &tv);
	/* Don't rely on the value of tv now! */
	
	if (retval == -1) {
		perror("select()");
		return 0;
	}
	else if (retval) {
		/* FD_ISSET(0, &rfds) will be true. */
		readResult = read(FD_NUM, buffer, BUFFER_SIZE);
		printf("Data is available now: %s\n", buffer);
		
		//write(0, buffer, cnt);
		
		return 1;
	}
	else {
		printf("No data within five seconds.\n");
		return 2;
	}
}

int main(void) {
	
	mainthread = pthread_self();
	signal(SIGIO, &sigIgn);
	
	if(spotify->InitSession())
		printf("valid session.\n");
	else
		printf("invalid session.\n");
	
	spotify->SetCredentials("breellondon", "spotifybreel1234");
	spotify->Login();
	
	sigset_t sigset;
	sigemptyset(&sigset);
	sigaddset(&sigset, SIGIO);
	
	while (timed() != 0) {
		
		std::cout << "looping true\n";
		
		int timeout = -1;
		pthread_sigmask(SIG_BLOCK, &sigset, NULL);
		spotify->ProcessEvents(&timeout);
		pthread_sigmask(SIG_UNBLOCK, &sigset, NULL);
		
	}
	
	return 0;
}