CODEWITHSUNDEEP
clinuxposix-select
user720694
user720694

Reputation: 2075

select keeps waiting indefinitely

I'm getting a strange result while experimenting with select().

fd_set tempset,set; //set of descriptors

FD_ZERO(&set); //initialize descriptor set to NULL

FD_SET(serversock,&set); //add server's socket to descriptor set 'set'

timeout.tv_sec=2;
timeout.tv_usec=0;

while(1){

tempset=set;
timeout.tv_sec=2;
timeout.tv_usec=0;

printf("%s","Waiting on select\n");

int ret=select(FD_SETSIZE,&tempset,NULL,NULL,&timeout);

if(ret==0) printf("timeout ");

else if(ret==-1){
    printf("Error occured\n");
    exit(0);
}

else if(ret>0){ //socket ready for reading
    
    for(i=0;i<FD_SETSIZE;i++){
        
    if(FD_ISSET(i,&tempset)){ //check if it's serversock
        
        if(i==serversock){
              //accept connection and read/write
              printf("Client connected\n");
       
    }//i==serversock close
    
    }
}//for ends 
}

When I remove the the line printf("%s","Waiting on select\n"); select keeps waiting indefinitely. However, when I reinsert the line, everything works as expected (tested with a client connection)

Am I missing something?

Upvotes: 1

Views: 909

Answers (1)

Remy Lebeau
Remy Lebeau

Reputation: 595349

You are misususing FD_SETSIZE, which you should not be using directly at all. You are only putting one socket into the fd_set, so there is no need to even loop through it at all (but if you did need to, use the fd_set.fd_count member instead).

Try this:

fd_set set;
timeval timeout;

while(1){ 
    FD_ZERO(&set);
    FD_SET(serversock, &set);

    timeout.tv_sec = 2; 
    timeout.tv_usec = 0; 

    printf("%s","Waiting on select\n"); 

    int ret = select(serversock+1, &set, NULL, NULL, &timeout); 

    if (ret==0){
        printf("timeout "); 
    }

    else if (ret==-1){ 
        printf("Error occured\n"); 
        exit(0); 
    } 

    else if (ret>0){ //socket ready for reading 
        ... = accept(serversock, ...);
        printf("Client connected\n"); 
    }
}

Or:

fd_set set;
timeval timeout;
int max_fd = 0;

while(1){ 
    FD_ZERO(&set);

    FD_SET(serversock, &set);
    max_fd = serversock;

    // FD_SET() other sockets and updating max_fd as needed...

    timeout.tv_sec = 2; 
    timeout.tv_usec = 0; 

    printf("%s","Waiting on select\n"); 

    int ret = select(max_fd+1, &set, NULL, NULL, &timeout); 

    if (ret==0){
        printf("timeout "); 
    }

    else if (ret==-1){ 
        printf("Error occured\n"); 
        exit(0); 
    } 

    else if (ret>0){ //socket(s) ready for reading 
        for (u_int i = 0; i < set.fd_count; ++i){
            ... = accept(set.fd_array[i], ...);
            printf("Client connected\n"); 
        }
    }
}

Upvotes: 4

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