Internet Socket - Read/Write sync with Select()

[INFERNO2K]

If I have a function that's main purpose is to write(); to a client that expects a read(); in a function, do I want my select(); to use the read or write argument of the function call?

Example:

int displayMenu(int clientSock){

                  fd_set writefds;
          fd_zero(&writefds)
          fd_set(clientsock,&writefds);

        if(select(clientsock+1,NULL,&writeds,NULL,NULL)){
           for(int i =0;i<course_amount;i++)
         {
             write(clientsock,courses[i].coursecode);
fflush(stdout);
        }
    return 0;
}

What this will do is write to the client once and then hang (which is obviously something wrong in the setup I have with select).

What step/idea am I missing out on when I need to use select();

The hang is fixed if I cheat and use

sleep(1);

after the write();

[Salem]

You need to call select before each write.
And pay attention to the return result of write, in case it didn't send all the data in a single call.

[INFERNO2K]

int send_Courses_Menu(int clientSocket){

	fd_set writefds;
	int i, retval, bytes_written;
	
	FD_CLR(clientSocket,&writefds);
	FD_ZERO(&writefds);
	FD_SET(clientSocket,&writefds);
  	
	  	//if ((retval >= 1) && (FD_ISSET(clientSocket, &writefds))) {
	
	 while(bytes_written>0){
	  for(i=1;i<=__COURSE__AMOUNT__;i++){	
	  		select(clientSocket+1, NULL,&writefds, NULL, NULL);		
	  		bytes_written = write(clientSocket,courses[i].courseCode,strlen(courses[i].courseCode)+1);
	  }	
	}
	
	return 0;	
}

Function looks something like this and it still is'nt working properly

[vart]

why your array starts at 1?

[Salem]

I was thinking you should be calling select() with a timeout. As written, it does nothing useful at all.

If select() times out, you go away and analyse the input say, or redraw some part of the screen - whatever.
If select() returns an available fd, then you perform ONE I/O operation on that fd.

[INFERNO2K]

I'm still getting the read "hang" on the client. I am not sure what I am doing wrong.

Server function

int send_Courses_Menu(int clientSocket){

	int i, retval;
	fd_set writefds;
	
	FD_ZERO(&writefds);
	FD_SET(clientSocket,&writefds);

	for(i=1;i<=__COURSE__AMOUNT__;i++){		
		retval = select(clientSocket+1,NULL,&writefds,NULL,NULL);
		if(retval){
			write(clientSocket,courses[i].courseCode,strlen(courses[i].courseCode));
		}
	}
	
	return 0;	
}

client function

int fetch_Info(int sockfd){

	int i, bytesRead;
	char message[BUFSIZ + 1];

	system("clear");
	printf("Select a course to rate: \n\n");

	
	for(i=1;i<=__COURSE__AMOUNT__;i++){
		if((bytesRead = read(sockfd,message,BUFSIZ))>0){
		 	message[bytesRead] = '\0';
		 	sscanf(message,"&#37;s",courses[i].courseCode);	
		 	courses[i].courseNumber = i;
			printf("%d. %s\n",i, message);
		 }
	}	
	
	return 0;
}

Appreciate any help

[Salem]

> I'm still getting the read "hang" on the client. I am not sure what I am doing wrong.
You're not reading the manual page for select() to find out how to specify a timeout in the last parameter.

[INFERNO2K]

> I'm still getting the read "hang" on the client. I am not sure what I am doing wrong.
You're not reading the manual page for select() to find out how to specify a timeout in the last parameter.

Same result using a struct timeval

int send_Courses_Menu(int clientSocket){

	int i, retval;
	fd_set writefds;
	struct timeval to;
	
	to.tv_sec = 3;
	to.tv_usec = 0;
	
	FD_ZERO(&writefds);
	FD_SET(clientSocket,&writefds);

	for(i=1;i<=__COURSE__AMOUNT__;i++){		
		retval = select(clientSocket+1,NULL,&writefds,NULL,&to);
		if(retval){
			write(clientSocket,courses[1].courseCode,strlen(courses[1].courseCode));
		}
	}
	
	return 0;	
}

[Salem]

http://www.linuxmanpages.com/man2/select_tut.2.php