Handle multiple socket connections with fd_set and select on Linux
Handle multiple socket connections
When writing server programs using sockets , it becomes necessary to handle multiple connections at a time , since a server needs to serve multiple clients.
There are many ways to do so. On linux this can be done in various ways like forking , threading , select method etc.
In this tutorial we shall use the select method approach. The select function allows the program to monitor multiple sockets for a certain "activity" to occur. For example if there is some data to be read on one of the sockets select will provide that information.
fd_set
An fd_set is a set of sockets to "monitor" for some activity. There are four useful macros : FD_CLR, FD_ISSET, FD_SET, FD_ZERO for dealing with an fd_set.
FD_ZERO - Clear an fd_set FD_ISSET - Check if a descriptor is in an fd_set FD_SET - Add a descriptor to an fd_set FD_CLR - Remove a descriptor from an fd_set
//set of socket descriptors fd_set readfds; //socket to set FD_SET( s , &readfds);
select function
The select method takes a list of socket for monitoring them. Here is how :
activity = select( max_fd + 1 , &readfds , NULL , NULL , NULL);
The select function blocks , till an activity occurs. For example when a socket is ready to be read , select will return and readfs will have those sockets which are ready to be read.
Code
/**
Handle multiple socket connections with select and fd_set on Linux
*/
#include <stdio.h>
#include <string.h> //strlen
#include <stdlib.h>
#include <errno.h>
#include <unistd.h> //close
#include <arpa/inet.h> //close
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/time.h> //FD_SET, FD_ISSET, FD_ZERO macros
#define TRUE 1
#define FALSE 0
#define PORT 8888
int main(int argc , char *argv[])
{
int opt = TRUE;
int master_socket , addrlen , new_socket , client_socket[30] , max_clients = 30 , activity, i , valread , sd;
int max_sd;
struct sockaddr_in address;
char buffer[1025]; //data buffer of 1K
//set of socket descriptors
fd_set readfds;
//a message
char *message = "ECHO Daemon v1.0 \r\n";
//initialise all client_socket[] to 0 so not checked
for (i = 0; i < max_clients; i++)
{
client_socket[i] = 0;
}
//create a master socket
if( (master_socket = socket(AF_INET , SOCK_STREAM , 0)) == 0)
{
perror("socket failed");
exit(EXIT_FAILURE);
}
//set master socket to allow multiple connections , this is just a good habit, it will work without this
if( setsockopt(master_socket, SOL_SOCKET, SO_REUSEADDR, (char *)&opt, sizeof(opt)) < 0 )
{
perror("setsockopt");
exit(EXIT_FAILURE);
}
//type of socket created
address.sin_family = AF_INET;
address.sin_addr.s_addr = INADDR_ANY;
address.sin_port = htons( PORT );
//bind the socket to localhost port 8888
if (bind(master_socket, (struct sockaddr *)&address, sizeof(address))<0)
{
perror("bind failed");
exit(EXIT_FAILURE);
}
printf("Listener on port %d \n", PORT);
//try to specify maximum of 3 pending connections for the master socket
if (listen(master_socket, 3) < 0)
{
perror("listen");
exit(EXIT_FAILURE);
}
//accept the incoming connection
addrlen = sizeof(address);
puts("Waiting for connections ...");
while(TRUE)
{
//clear the socket set
FD_ZERO(&readfds);
//add master socket to set
FD_SET(master_socket, &readfds);
max_sd = master_socket;
//add child sockets to set
for ( i = 0 ; i < max_clients ; i++)
{
//socket descriptor
sd = client_socket[i];
//if valid socket descriptor then add to read list
if(sd > 0)
FD_SET( sd , &readfds);
//highest file descriptor number, need it for the select function
if(sd > max_sd)
max_sd = sd;
}
//wait for an activity on one of the sockets , timeout is NULL , so wait indefinitely
activity = select( max_sd + 1 , &readfds , NULL , NULL , NULL);
if ((activity < 0) && (errno!=EINTR))
{
printf("select error");
}
//If something happened on the master socket , then its an incoming connection
if (FD_ISSET(master_socket, &readfds))
{
if ((new_socket = accept(master_socket, (struct sockaddr *)&address, (socklen_t*)&addrlen))<0)
{
perror("accept");
exit(EXIT_FAILURE);
}
//inform user of socket number - used in send and receive commands
printf("New connection , socket fd is %d , ip is : %s , port : %d \n" , new_socket , inet_ntoa(address.sin_addr) , ntohs(address.sin_port));
//send new connection greeting message
if( send(new_socket, message, strlen(message), 0) != strlen(message) )
{
perror("send");
}
puts("Welcome message sent successfully");
//add new socket to array of sockets
for (i = 0; i < max_clients; i++)
{
//if position is empty
if( client_socket[i] == 0 )
{
client_socket[i] = new_socket;
printf("Adding to list of sockets as %d\n" , i);
break;
}
}
}
//else its some IO operation on some other socket :)
for (i = 0; i < max_clients; i++)
{
sd = client_socket[i];
if (FD_ISSET( sd , &readfds))
{
//Check if it was for closing , and also read the incoming message
if ((valread = read( sd , buffer, 1024)) == 0)
{
//Somebody disconnected , get his details and print
getpeername(sd , (struct sockaddr*)&address , (socklen_t*)&addrlen);
printf("Host disconnected , ip %s , port %d \n" , inet_ntoa(address.sin_addr) , ntohs(address.sin_port));
//Close the socket and mark as 0 in list for reuse
close( sd );
client_socket[i] = 0;
}
//Echo back the message that came in
else
{
//set the string terminating NULL byte on the end of the data read
buffer[valread] = '\0';
send(sd , buffer , strlen(buffer) , 0 );
}
}
}
}
return 0;
}
The source code has been put up on the following url
https://gist.github.com/silv3rm00n/5604330
Compile and run the above program. Then connect to it using telnet from 3 different terminals.
$ telnet localhost 8888
Now whatever you type and send to server will be send back as it is, or echoed.
The server terminal would show details of connections like this :
Waiting for connections... New connection , socket fd is 4 , ip is : 127.0.0.1 , port : 57831 Welcome message sent successfully Adding to list of sockets as 0 New connection , socket fd is 5 , ip is : 127.0.0.1 , port : 57832 Welcome message sent successfully Adding to list of sockets as 1 New connection , socket fd is 6 , ip is : 127.0.0.1 , port : 57833 Welcome message sent successfully Adding to list of sockets as 2 New connection , socket fd is 7 , ip is : 127.0.0.1 , port : 57834 Welcome message sent successfully
The client terminal can be like this
$ telnet localhost 8888 Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. ECHO Daemon v1.0 ccc ccc ddd ddd fff fff
There are other functions that can perform tasks similar to select. pselect , poll , ppoll
Resources
1. http://pubs.opengroup.org/onlinepubs/7908799/xsh/select.html
2. http://linux.die.net/man/2/select
