c socket prog - select() problems - c

Question

I'm new to network programming. I have to write a simple client/server program in C. The server will listen for a connection and the client will connect to the server, send a message, and receive an echo back from the client. We have to update this using select() to handle connections to multiple clients at the same time from the server process. I tried to implement select() on the client side like instructed,but I think I'm having an infinite loop on the client side in if(FD_ISSET(clientSockfd, &readfds)) part.
//client1.c
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <resolv.h>
#include <arpa/inet.h>
const int BUF_SIZE = 512;
int main(int argc, char *argv[]) {
char buf[BUF_SIZE], buf2[BUF_SIZE];
char *msg;
struct sockaddr_in serverInfo;
int clientSockfd, errorCheck, readVal, numfd;
struct hostent *hostName;
fd_set readfds;
//make sure user entered correct arguments when starting client
if(argc != 3)
{
printf("error: must enter 'programName portNumber hostname'\n");
exit(errno);
}
//create socket and error check socket() call
clientSockfd=socket(AF_INET, SOCK_STREAM, 0);
if (clientSockfd == -1)
{
perror("error creating socket");
exit(errno);
}
//assign sockaddr_in info for RemoteAddr
bzero(&serverInfo, sizeof(serverInfo));
serverInfo.sin_family=AF_INET;
hostName=gethostbyname(argv[2]);
if(hostName == NULL)
{
herror("Error when calling gethostbyname()");
exit(errno);
}
memcpy((unsigned char *) &serverInfo.sin_addr, (unsigned char *)hostName->h_addr, hostName->h_length); //copy IP address to be used
serverInfo.sin_port=htons(atoi(argv[1])); //port number to be used, given in command line, must be converted to network byte order
//connect to server side
if(connect(clientSockfd, (struct sockaddr *)&serverInfo, sizeof(serverInfo)) == -1)
{
perror("error when connecting to server");
exit(errno);
}
while(1)
{
FD_ZERO(&readfds); //zero out set
FD_SET(fileno(stdin), &readfds);
FD_SET(clientSockfd, &readfds);
int maxfd = fileno(stdin);
if(maxfd < clientSockfd) maxfd = clientSockfd;
numfd = select(maxfd, &readfds, 0, 0, 0); //call select()
if(numfd > 0)
{
if(FD_ISSET(clientSockfd, &readfds))
{
//make sure buf is empty so it doesnt print extra chars
bzero(buf2, BUF_SIZE);
read(clientSockfd, buf2, BUF_SIZE);
}
if(FD_ISSET(fileno(stdin), &readfds))
{
bzero(buf, BUF_SIZE);
fgets(buf, BUF_SIZE-1, stdin);
printf("echo from server: %s\n", buf);
errorCheck = write(clientSockfd, buf, strlen(buf)+1);
if(errorCheck == -1)
{
perror("error writing");
}
}
}
else if(numfd == 0)
{
perror("Error using select()\n");
exit(0);
}
else
printf("no data\n");
}
//close connection to server
errorCheck = close(clientSockfd);
if(errorCheck == -1)
{
perror("Error closing connection.");
exit(errno);
}
return 0;
}
here is the server..
//server.c
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <sys/socket.h>
#include <resolv.h>
#include <arpa/inet.h>
#include <errno.h>
const int ASSIGNED_PORT = 17000;
const int BUF_SIZE = 512;
int main() {
int serverfd, clientfd;
struct sockaddr_in serverSock; //NOTE: a pointer to sockaddr_in can be cast to a pointer to
// a struct sockaddr - useful for connect()
char buf[BUF_SIZE];
int errorCheck, msgLength;
//create socket with error checking (-1 ret on error)
serverfd = socket(AF_INET, SOCK_STREAM, 0);
if(serverfd < 0 )
{
perror("socket failed.");
exit(errno);
}
//assign sockaddr_in info for server
bzero(&serverSock, sizeof(serverSock)); //set to all 0's
serverSock.sin_family = AF_INET;
serverSock.sin_addr.s_addr = htonl(INADDR_ANY);
serverSock.sin_port = htons(ASSIGNED_PORT);
//bind a name to the socket with error checking (0 ret on success)
errorCheck = bind(serverfd, (struct sockaddr*)&serverSock, sizeof(serverSock));
if(errorCheck < 0 )
{
perror("bind failed.");
exit(errno);
}
//listen for connections with error checking (0 ret on success)
errorCheck = listen(serverfd, 10);
if(errorCheck < 0 )
{
perror("listen failed.");
exit(errno);
}
printf("Listening for connections. Enter CNTRL-c to kill server.\n");
//create infinite loop to accept, read, write, and close connections with error hecking
while(1)
{
//accept the connection from the client
clientfd = accept(serverfd, 0, 0);
if(clientfd == -1)
{
perror("error accepting connection.");
exit(errno);
}
//read data from the client
bzero(buf, BUF_SIZE);
msgLength = read(clientfd, buf, BUF_SIZE-1);
if(msgLength == -1)
{
perror("error reading from client");
close(clientfd);
close(serverfd);
exit(errno);
}
if(buf[0] '\0')
{
printf("connection closing");
exit(0);
}
//print what the client sent
printf("Message from client: %s\n", buf);
//echo back what the client sent
errorCheck = write(clientfd, buf, strlen(buf)+1);
if(errorCheck == -1 )
{
perror("error writing to client");
exit(errno);
}
//close the connection
errorCheck = close(clientfd);
if(errorCheck == -1)
{
perror("error closing connection");
exit(errno);
}
}
errorCheck = close(serverfd);
if(errorCheck==-1)
{
perror("error closing server, exiting program now");
sleep(6);
exit(errno);
}
return 0;
}

Answer 1

I think the problem (or at least, a problem) in your client-side code is that you are passing the magic number 32 to select(). That's incorrect. What you should be passing is the maximum of all of the socket numbers, plus one. For example, something like this:
int maxfd = fileno(stdin);
if (maxfd < clientSockFD) maxfd = clientSockFD;
// further maximizations for other sockets would go here, if you had any other sockets...
numfd = select(maxfd+1, &readfds, 0, 0, 0);

Answer 2

You need to use select() on the server side to handle multiple clients, not on the client side.