First time using select(), maybe a basic question? - c

Question

i've been working for a few days with this server using select(). What it does, is that, i have two arrays of clients (one is "suppliers", and the other is "consumers"), and the mission of the server is to check whether the suppliers have something to send to the consumers, and in case affirmative, send it.
The second part of the server is that, when the consumers have received the suppliers' info, they send a confirmation message to the same suppliers that sent the info.
When a client connects, it gets recognized as "undefined", until it sends a message with the word "supplier" or "consumer" (in Spanish, as i'm from there), when the server puts it in the correct clients array.
Well, what the server does is not very important here. What's important is that, i'm doing both parts with two different "for" loops, and that's where i'm getting the problems. When the first user connects to the server (be it a supplier or a consumer), the server gets stuck in the first or second loop, instead of just continuing its execution. As it's the first time i'm using select(), i may be missing something. Could you guys give me any sort of help?
Thanks a lot in advance.
for(;;)
{
rset=allset;
nready=select(maxfd+1,&rset,NULL,NULL,NULL);
if (FD_ISSET(sockfd, &rset))
{
clilen=sizeof(cliente);
if((connfd=accept(sockfd,(struct sockaddr *)&cliente,&clilen))<0)
{
printf("Error");
}
IP=inet_ntoa(cliente.sin_addr);
for(i=0;i<COLA;i++)
{
if(indef[i]<0)
{
indef[i]=connfd;
IPind[i]=IP;
break;
}
}
FD_SET(connfd,&allset);
if(connfd > maxfd)
{
maxfd=connfd;
}
if(i>maxii)
{
maxii=i;
}
if(--nready<=0)
{ continue; }
}// Fin ISSET(sockfd)
for(i=0;i<=maxii;i++)
{
if((sockfd1=indef[i])<0)
{ continue; } //!
if(FD_ISSET(sockfd1,&rset))
{
if((n=read(sockfd1,comp,MAXLINE))==0)
{
close(sockfd1);
FD_CLR(sockfd1,&allset);
indef[i]=-1;
printf("Cliente indefinido desconectado \n");
}
else
{
comp[n]='\0';
if(strcmp(comp,"suministrador")==0)
{
for(j=0;j<=limite;j++)
{
if(sumi[j]<0)
{
IPsum[j]=IPind[i];
sumi[j]=indef[i];
indef[i]=-1;
if(j>maxis)
{
maxis=j;
}
break;
}
}
}
else if(strcmp(comp,"consumidor")==0)
{
for(o=0;j<=limite;j++)
{
if(consum[o]<0)
{
IPcons[o]=IPind[i];
consum[o]=indef[i];
indef[o]=-1;
if(o>maxic)
{
maxic=o;
}
break;
}
}
}
if(--nready <=0)
{
break;
}
}
}
}//fin bucle for maxii
for(i=0;i<=maxis;i++)
{
if((sockfd2=sumi[i])<0)
{ continue; }
if(FD_ISSET(sockfd2,&rset))
{
if((n=read(sockfd2,buffer2,MAXLINE))==0)
{
close(sockfd2);
FD_CLR(sockfd2,&allset);
sumi[i]=-1;
printf("Suministrador desconectado \n");
}
else
{
buffer2[n]='\0';
for(j=0;j<=maxic;j++)
{
if((sockfd3=consum[j])<0)
{ continue; }
else
{
strcpy(final,IPsum[i]);
strcat(final,":");
strcat(final,buffer2);
write(sockfd3,final,sizeof(final));
respuesta[i]=1;
}
}
break; // ?
}
}
}//fin for maxis
for(i=miniic;i<=maxic;i++)
{
if((sockfd4=consum[i])<0)
{ continue; }
if(FD_ISSET(sockfd4,&rset))
{
if((n=read(sockfd4,buffer3,MAXLINE))==0)
{
close(sockfd4);
FD_CLR(sockfd4,&allset);
consum[i]=-1;
printf("Consumidor desconectado \n");
}
else
{
buffer3[n]='\0';
IP2=strtok(buffer3,":");
obj=strtok(NULL,":");
for(j=0;j<100;j++)
{
if((strcmp(IPsum[j],IP2)==0) && (respuesta[j]==1))
{
write(sumi[j],obj,sizeof(obj));
miniic=i+1;
respuesta[j]=0;
break;
}
}
}
}
}

Answer 1

Hmm, I think your logic is all wrong. It should look something more like this (warning, untested pseudo-code):
for (;;)
{
// First, set up the fd_sets to specify the sockets we want to be notified about
fd_set readSet; FD_CLR(&readSet);
fd_set writeSet; FD_CLR(&writeSet);
int maxFD = -1;
for (int i=0; i<num_consumers; i++)
{
if (consumer_sockets[i] > maxFD) maxFD = consumer_sockets[i];
FD_SET(consumer_sockets[i], &readSet);
if (consumer_has_data_he_wants_to_send[i]) FD_SET(consumer_sockets[i], &writeSet);
}
for (int i=0; i<num_producers; i++)
{
if (producer_sockets[i] > maxFD) maxFD = producer_sockets[i];
FD_SET(producer_sockets[i], &readSet);
if (producer_has_data_he_wants_to_send[i]) FD_SET(producer_sockets[i], &writeSet);
}
// Now we block in select() until something is ready to be handled on a socket
int selResult = select(maxFD+1, &readSet, &writeSet, NULL, NULL);
if (selResult < 0) {perror("select"); exit(10);}
for (int i=0; i<num_consumers; i++)
{
if (FD_ISSET(consumer_sockets[i], &readSet)
{
// There is some incoming data ready to be read from consumer_socket[i], so recv() it now
[...]
}
if (FD_ISSET(consumer_sockets[i], &writeSet)
{
// There is buffer space in consumer_socket[i] to hold more outgoing
// data for consumer_socket[i], so send() it now
[...]
}
}
for (int i=0; i<num_producers; i++)
{
if (FD_ISSET(&producer_sockets[i], &readSet)
{
// There is some data ready to be read from producer_socket[i], so recv() it now
[...]
}
if (FD_ISSET(producer_sockets[i], &writeSet)
{
// There is buffer space in producer_socket[i] to hold more outgoing
// data for producer_socket[i], so send() it now
[...]
}
}
}
Note that to really do this properly, you'd need to set all of your sockets to non-blocking I/O and be able to handle partial reads and writes (by storing the partial data into a local memory-buffer associated with that consumer/producer, until you have enough data to act on), otherwise you risk having a call to recv() or send() block, which would prevent the event loop from being able to service any of the other consumers or producers. Ideally the only place you should ever block in is select()... every other call should be non-blocking. But if you want to keep things simple to start out with, you may be able to get away with using blocking I/O for a while.

Answer 2

You might read the introductory tutorial
Look at blocking vs non-blocking connections