Client-server restaurant simulation - c

Question

I am making a client-server with select() in C that simulates a fast-food or whatever.
I have clients that order a random "food" between 1-5. The server decides every 30 sec. what is the most preferred food from all the clients. He serves those clients and they close connection, and for the rest, sends them a message to wait. The clients that are not served try 2 more times, otherwise they leave.
My question is, how can I make the server check every 30 sec. what their orders are?
I tried making an array, but I can't figure out how to make the server 'check' continuously every 30 sec. and set the array to 0, then.
Here is the pseudocode:
**client**
patience=0;served=0;
do
{send random 1-5
receieve message. if 1->served=1; if 0, patience++;
}while patience !=3 and served!=1;
if served==1 send -1
else send -2
close connection
**Server**
while(1)
{
serves clients in a concurent manner
select
adds client in socket list
serves client
waits message
if 1-5 adds in vector
//here I don't know how to make it wait for 30 sec.
//If I put sleep(30), it's going to sleep for each client every time. I want it to permanently check every 30 sec and send messages to clients.
send respones(0 or 1, depending on if the max order is the same as the client's)
if -1, thanks for the food
if -2, going somewhere else
close client connection
}

Answer 1

You may want to try sleep() function in a loop, what it will do is pause your program for that much duration and then execute the statements that follow.
sleep(30);
Is what you want i suppose. Have a look here for more about sleep.
What your code should be like is:
for(i=0; i<=100; i++)//Checks 100 times
{
sleep(30);
checkserverforupdate();
}
Update
Code:
while(1){//runs infinite times
sleep(30);//pauses here for 30 seconds everytime before/after running the following code
*client**
patience=0;served=0;
do
{send random 1-5
receieve message. if 1->served=1; if 0, patience++;
}while patience !=3 and served!=1;
if served==1 send -1
else send -2
close connection
**Server**
while(1)
{
serves clients in a concurent manner
select
adds client in socket list
serves client
waits message
if 1-5 adds in vector
//here I don't know how to make it wait for 30 sec.
//If I put sleep(30), it's going to sleep for each client every time. I want it to permanently check every 30 sec and send messages to clients.
send respones(0 or 1, depending on if the max order is the same as the client's)
if -1, thanks for the food
if -2, going somewhere else
close client connection
}
}
See what happens with sleep(30) is that the program pauses for the time of 30 seconds and then executes the code written after it and if you put it in a while loop, it waits every time.
Update 2:
Code for how to get the current time in C:
here
/* localtime example */
#include <stdio.h>
#include <time.h>
int main ()
{
time_t rawtime;
struct tm * timeinfo;
time ( &rawtime );
timeinfo = localtime ( &rawtime );
printf ( "Current local time and date: %s", asctime (timeinfo) );
return 0;
}
Update 3:
So your code should be like this:
time_t rawtime;
struct tm * timeinfo;
time ( &rawtime );
timeinfo = localtime ( &rawtime );
printf ( "Current local time and date: %s", asctime (timeinfo) );
All of your code here
and then get the time again,
time_t rawtime1;
.....
and then you may calculate the difference between them and put a sleep statement as you wish. If the time you want it to pause is in x then,
sleep(x);
return 0;

Answer 2

You can use a thread to wait for 30 secs and when the thread returns do a check to find out the preferred food from all the clients. This link for threading will help.

Answer 3

As you are into pseudocode here goes
t = time now
start loop
... do stuff
t2 = time now
delta = t2 - t
sleep (30 - delta % 30)
end of loop

Answer 4

The select() function in C has a timeout parameter. The correct logic would be something like:
nextTime = now() + 30 seconds;
while(running) {
currentTime = now();
if( nextTime <= currentTime() ) {
makeFood();
nextTime += 30 seconds;
} else {
timeout = nextTime - currentTime;
if(select(..., timeout) > 0) {
// Check FD sets here
}
}
}
EDIT (added explanation below).
You have to have some sort of data structure to track that state (e.g. which foods have been requested by which clients). This data structure isn't shown above.
The // Check FD sets part modifies the current state. For example, when a new client orders some food you might add the details to a linked list of clients waiting for orders. When a client gets sick of waiting you'd remove their order from the list of clients waiting for orders.
The makeFood() function would look at the (previously stored) state and determine which type of food to make; then inform clients to either keep waiting or that their food was made (and remove clients that did receive their food).
EDIT 2 - The problem with using sleep().
If you sleep until it's the right time to send replies and then receive packets, and then send replies; then the time it takes to receive packets varies (depending on the number of packets received) which means that the time that you send replies varies (depending on the number of packets received).
A time-line might look like this:
12:00:59 - stop sleeping
12:00:59 - check for received packets and find there's none
12:00:59 - do 1 second of processing
12:01:00 - start sending reply packets
12:01:02 - finish sending reply packets
12:01:02 - sleep for the remaining 27 seconds
12:01:29 - stop sleeping
12:01:29 - check for received packets and find there's lots of them
12:01:29 - process received packets for 3 seconds
12:00:33 - do 1 second of processing (3 seconds too late)
12:01:34 - start sending reply packets (3 seconds too late)
12:01:02 - finish sending reply packets (3 seconds too late)
12:01:02 - sleep for the remaining 24 seconds
In this case, the replies are sent between 0 and 3 seconds too late because it takes between 0 and N seconds to handle received packets. This timing variation is called "jitter".
To fix the "jitter", you handle received packets while you're waiting so that the time spent handling those received packets doesn't influence the time that replies are sent.
In this case, the time-line looks like this:
12:00:59 - stop waiting and receiving packets
12:00:59 - do 1 second of processing
12:01:00 - start sending reply packets
12:01:02 - finish sending reply packets
12:01:02 - receive packets while waiting for the remaining 27 seconds
12:01:29 - stop waiting and receiving packets
12:00:29 - do 1 second of processing
12:01:00 - start sending reply packets
12:01:02 - finish sending reply packets
12:01:02 - receive packets while waiting for the remaining 27 seconds
In this case, the replies are always sent at the correct time.
EDIT 3 - The other problem with using sleep().
The first problem with sleep exists because it's impossible to predict the ideal amount of time to spent sleeping in advance, which causes you to wake up too late (which causes "jitter"). The second problem is the effect this has only the server's ability to handle periods of "almost maximum load".
Basically, you sleep too long, and as load increases you reach a point where you've slept too long and left yourself with insufficient time to process the load you should've received while you were waiting.
This causes oscillations when there's constant "almost maximum load". For example, if there's enough load that it takes 29 seconds to do everything (receive packets, process them and send reply packets); then for one 30 second period you sleep too long, fail to handle the load in time and finish too late; then you realise that you need to sleep for "negative seconds" (but you can't turn back time - the best you can do is skip the sleep()). This pushes the excess load onto the next 30 second period of time. After several (many) of these "sleep skipped but still starting too late" 30 seconds periods you eventually catch up; and as soon as that happens you sleep too long again and start the entire thing all over again.
Of course this assumes there's constant "almost maximum load". Real life is almost never like that. In practice you tend to get fluctuations - e.g. one 30 second period with "slightly higher than maximum average load", followed by a 30 second period with "slightly less than maximum average load". What we're really talking about here is the ability of the server to recover quickly from the occasional "slightly higher than maximum average load" periods.
For my method, the server recovers as quickly as possible. For the "sleep()" method it takes much longer to recover, resulting in prolonged "lag surges".