Using select instead of fork - c

Question

After looking at Beej's guide to network programming, I am trying to redo my server.c using select instead of fork. I am not too sure what is going wrong; my program compiles, but doesn't accept connections. I know that my loop containing i<=fdmax isn't functioning properly, but I can't figure out why. It seems like the if statements are not working appropriately.
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <sys/wait.h>
#include <errno.h>
#include <sys/select.h>
#define Connections 5
void SignalCatcher(int signum)
{
wait3(NULL,WNOHANG, NULL);
//wait(-1);
}
int main(int argc, char**argv)
{
int listenfd,connfd,n, i;
struct sockaddr_in servaddr,cliaddr;
socklen_t clilen;
pid_t childpid;
char mesg[1000];
FILE *inputFile;
inputFile = fopen("movie.txt", "r");
char returnMsg[1000];
int fdmax, newfd;
fd_set readfd;
fd_set mastersocket;
FD_ZERO(&mastersocket);
FD_ZERO(&readfd);
//Creating socket number
listenfd=socket(AF_INET,SOCK_STREAM,0);
//Setting up the internet address
bzero(&servaddr,sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
servaddr.sin_port=htons(32000);
bind(listenfd,(struct sockaddr *)&servaddr,sizeof(servaddr));
//Listening for clients
listen(listenfd,1024);
FD_SET(listenfd, &mastersocket);
fdmax=listenfd;
//signal(SIGCHLD, SIG_IGN);
signal(SIGCHLD, SignalCatcher);
//Infinite loop that waits for/accepts connections.
for(;;)
{
readfd = mastersocket;
clilen=sizeof(cliaddr);
if(select(fdmax+1, &readfd, NULL, NULL, NULL) == -1) {
perror("select");
exit(4);}
//connfd = accept(listenfd,(struct sockaddr *)&cliaddr,&clilen);
for(i=0; i<=fdmax;i++)
{
if (FD_ISSET(i, &readfd)){
if(i==listenfd){
printf("-SUCCESS\n");
clilen = sizeof cliaddr;
connfd = accept(listenfd,(struct sockaddr *)&cliaddr,&clilen); }
if (connfd!=-1)
{
// if ((childpid = fork()) == 0)
// {
close (listenfd);
for(;;)
{
n = recvfrom(connfd,mesg,1000,0,(struct sockaddr *)&cliaddr,&clilen);
if (n == -1 /*&& errno == EWOULDBLOCK*/) continue;
else if(n==0) break;
//sendto(connfd,mesg,n,0,(struct sockaddr *)&cliaddr,sizeof(cliaddr));
//write(connfd , mesg , strlen(mesg)); //both work
//write(connfd , "" , 1);
printf("-------------------------------------------------------\n");
printf("%d",listenfd);
mesg[n] = 0;
printf("Received the following:\n");
printf("%s",mesg);
printf("-------------------------------------------------------\n");
}
// }
close(connfd);
} //if connfd!=-1
}
} //for i<=fdmax
}
}

Answer 1

You can answer this question yourself by examining the functions' return values. In network programming, the usual idiom is something along the lines of:
if (foo() < 0) {
fprintf(stderr, "foo: %s\n", strerror(errno));
/* recover from error or... */
exit(EXIT_FAILURE);
}
...where foo() is one of bind(), listen(), accept(), send*/recv*() and so on.
Go ahead and try. errno will tell you what's wrong.
Besides, it's unclear why you are using select() at all. All you do is listen on a single socket, and you close it as soon as someone connects. Instead, you could just accept(listenfd).

Answer 2

firstly you should not close the listening fd it should be close while exiting the program. secondly we need to accept all incoming connections that are queued up on the listening socket.then if listening socket is not readable then an existing connection must be readable, so read the data for that connection. I just read once but u can read in loop for that connection until recv fails.
following code change should work:
//Infinite loop that waits for/accepts connections.
for(;;)
{
readfd = mastersocket;
clilen=sizeof(cliaddr);
if(select(fdmax+1, &readfd, NULL, NULL, NULL) == -1) {
perror("select");
exit(4);}
//connfd = accept(listenfd,(struct sockaddr *)&cliaddr,&clilen);
for(i=0; i<=fdmax;i++)
{
if (FD_ISSET(i, &readfd)){
if(i==listenfd){
printf("-SUCCESS\n");
clilen = sizeof cliaddr;
connfd = accept(listenfd,(struct sockaddr *)&cliaddr,&clilen);
if (connfd!=-1)
{
FD_SET(connfd, &mastersocket);
if (connfd > fdmax) {
fdmax = connfd;
}
}
}
else
{
n = recvfrom(i,mesg,1000,0,(struct sockaddr *)&cliaddr,&clilen);
if ((n == -1 /*&& errno == EWOULDBLOCK*/) || (n==0)) {
close (i);
FD_CLR (i, &mastersocket);
if (i == fdmax)
{
while (FD_ISSET(fdmax, &mastersocket) == 0)
fdmax -= 1;
}
}
//sendto(i,mesg,n,0,(struct sockaddr *)&cliaddr,sizeof(cliaddr));
//write(i , mesg , strlen(mesg)); //both work
//write(i , "" , 1);
printf("-------------------------------------------------------\n");
printf("%d",listenfd);
mesg[n] = 0;
printf("Received the following:\n");
printf("%s",mesg);
printf("-------------------------------------------------------\n");
}
}
} //for i<=fdmax
}

Answer 3

I tried your code with changes and its working fine to me.
Also if you want only one client communicate with server at a time then enable the while loop for reading client data.
server.c
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <sys/wait.h>
#include <errno.h>
#include <sys/select.h>
#define Connections 5
void SignalCatcher(int signum)
{
wait3(NULL,WNOHANG, NULL);
//wait(-1);
}
int main(int argc, char**argv)
{
int listenfd,connfd,n, i;
struct sockaddr_in servaddr,cliaddr;
socklen_t clilen;
pid_t childpid;
char mesg[1000];
FILE *inputFile;
inputFile = fopen("movie.txt", "r");
char returnMsg[1000];
int fdmax, newfd;
fd_set readfd;
fd_set mastersocket;
FD_ZERO(&mastersocket);
FD_ZERO(&readfd);
//Creating socket number
listenfd=socket(AF_INET,SOCK_STREAM,0);
//Setting up the internet address
bzero(&servaddr,sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
servaddr.sin_port=htons(32000);
bind(listenfd,(struct sockaddr *)&servaddr,sizeof(servaddr));
//Listening for clients
listen(listenfd,1024);
FD_SET(listenfd, &mastersocket);
fdmax=listenfd;
//signal(SIGCHLD, SIG_IGN);
signal(SIGCHLD, SignalCatcher);
//Infinite loop that waits for accepts connections.
for(;;)
{
readfd = mastersocket;
clilen=sizeof(cliaddr);
if(select(fdmax+1, &readfd, NULL, NULL, NULL) == -1) {
perror("select");
exit(4);}
//connfd = accept(listenfd,(struct sockaddr *)&cliaddr,&clilen);
for(i=0; i<=fdmax;i++)
{
if (FD_ISSET(i, &readfd)){
if(i==listenfd){
printf("-SUCCESS\n");
clilen = sizeof cliaddr;
connfd = accept(listenfd,(struct sockaddr *)&cliaddr,&clilen);
if (connfd!=-1)
{
FD_SET(connfd, &mastersocket);
if (connfd > fdmax) {
fdmax = connfd;
}
}
}
else
{
//while(1) {
n = recvfrom(i,mesg,1000,0,(struct sockaddr *)&cliaddr,&clilen);
if ((n == -1 /*&& errno == EWOULDBLOCK*/) || (n==0)) {
close (i);
FD_CLR (i, &mastersocket);
if (i == fdmax)
{
while (FD_ISSET(fdmax, &mastersocket) == 0)
fdmax -= 1;
}
// break;
}
//sendto(i,mesg,n,0,(struct sockaddr *)&cliaddr,sizeof(cliaddr));
//write(i , mesg , strlen(mesg)); //both work
//write(i , "" , 1);
printf("-------------------------------------------------------\n");
printf("%d",listenfd);
mesg[n] = 0;
printf("Received the following:\n");
printf("%s",mesg);
printf("-------------------------------------------------------\n");
}
//} /* end of while */
}
} //for i<=fdmax
}
}
client.c
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#define SERVER_PORT 32000
int main (int argc, char *argv[]) {
int sd, rc, i;
struct sockaddr_in localAddr, servAddr;
struct hostent *h;
char message[1000] ;
servAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
servAddr.sin_family = AF_INET;
servAddr.sin_port = htons( SERVER_PORT );
/* create socket */
sd = socket(AF_INET, SOCK_STREAM, 0);
if(sd<0) {
perror("cannot open socket ");
exit(1);
}
/* bind any port number */
localAddr.sin_family = AF_INET;
localAddr.sin_addr.s_addr = htonl(INADDR_ANY);
localAddr.sin_port = htons(0);
rc = bind(sd, (struct sockaddr *) &localAddr, sizeof(localAddr));
if(rc<0) {
printf("%s: cannot bind port TCP %u\n",argv[0],SERVER_PORT);
perror("error ");
exit(1);
}
/* connect to server */
rc = connect(sd, (struct sockaddr *) &servAddr, sizeof(servAddr));
if(rc<0) {
perror("cannot connect ");
exit(1);
}
while(1)
{
printf("Enter message : ");
scanf("%s" , message);
rc = send(sd, message, strlen(message) + 1, 0);
if(rc<0) {
perror("cannot send data ");
close(sd);
exit(1);
}
printf("To_server: data%u sent (%s)\n",strlen(message),message);
}
return 0;
}