Why doesn't select() react on received messages? - c

Question

I'm trying to abort sending UDP data to specific hosts when I receive data back from any of them.
So far what I've tried is creating a thread inside other function that sends data.
Thread is supposed to be checking for incoming data on socket descriptors for maximum of 100 secs. However select() function only waits for timeout and doesn't respond when data is received. Sockets are already created in calling function, here I'm only passing file descriptors. Also this is modified code snippet from Beej's Guide to Network Programming.
void *bot_listen(void *arg){
printf("Thread starting\n");
size_socks *sock_size=(size_socks*)arg;
int n, rv;
fd_set readfds;
struct timeval tv;
thr_running=1;
FD_ZERO(&readfds);
for(int i=0;i<sock_size->size;i++) {
FD_SET(sock_size->sockets[i], &readfds);
}
n = sock_size->size + 1;
tv.tv_usec = 0;
tv.tv_sec = 100;
rv = select(n, &readfds, NULL, NULL, &tv);
if (rv == -1) {
perror("select"); // error occurred in select()
} else if (rv == 0) {
printf("Timeout occurred! No data after 100 seconds.\n");
} else {
bot_recv_message=1;
}
thr_running=0;
}
void send_dgrams(int num_ip, struct message ip_port_pairs, char *message){
struct addrinfo hints, *res;
struct sockaddr_in *dest;
char udp_ip[INET_ADDRSTRLEN];
char str[INET6_ADDRSTRLEN];
char udp_port[22];
char *p;
int *sockfd, time_elapsed=0;
size_socks ss;
printf("Message, beginning of send_dgrams: %s\n", message);
pthread_t listen_thread;
sockfd = (int*)malloc(sizeof(int)*num_ip);
dest = malloc(sizeof(struct sockaddr_in)*num_ip);
p=&(ip_port_pairs.command);
p++;
for(int i =0; i<num_ip;i++) {
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_DGRAM;
strcpy(udp_ip, p);
p+=INET_ADDRSTRLEN;
strcpy(udp_port,p);
p+=22;
getaddrinfo(udp_ip, udp_port, &hints, &res);
sockfd[i] = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
dest[i].sin_family = res->ai_family;
dest[i].sin_port = ((struct sockaddr_in *) res->ai_addr)->sin_port;
dest[i].sin_addr = ((struct sockaddr_in *) res->ai_addr)->sin_addr;
memset(dest[i].sin_zero, '\0', sizeof(dest[i].sin_zero));
}
ss.size=num_ip;
ss.sockets=sockfd;
pthread_create(&listen_thread, NULL, bot_listen, (void*)&ss);
while(bot_recv_message==0 && time_elapsed<100) {
for (int i = 0; i < num_ip; i++) {
printf("Sending this message: %s to socket %d\n", message, sockfd[i]);
inet_ntop(AF_INET, &(dest[i].sin_addr), str, INET_ADDRSTRLEN);
printf("Sending to this address: %s\n", str);
printf("Sending to this port: %d\n", ntohs(dest[i].sin_port));
printf("Did bot receive message? :%d\n", bot_recv_message);
sendto(sockfd[i], message, strlen(message) + 1, 0, (struct sockaddr *) &dest[i], sizeof(dest[i]));
}
sleep(1);
time_elapsed+=1;
}
for(int i=0;i<num_ip;i++){
close(sockfd[i]);
}
bot_recv_message=0;
}

Answer 1

This could be the problem:
n = sock_size->size + 1;
...
rv = select(n, &readfds, NULL, NULL, &tv);
Value of the 1st parameter of select() won't be big enough.
Man page of the select() says the following about the 1st parameter:
nfds should be set to the highest-numbered file descriptor in any of
the three sets, plus 1. The indicated file descriptors in each set
are checked, up to this limit (but see BUGS).
So try something like this:
int max_fd = -1;
for(int i=0;i<sock_size->size;i++) {
FD_SET(sock_size->sockets[i], &readfds);
if (sock_size->sockets[i] > max_fd)
max_fd = sock_size->sockets[i];
}
...
rv = select(max_fd + 1, &readfds, NULL, NULL, &tv);