Handle multiple socket connections with fd_set and select on Linux

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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.


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.


	Handle multiple socket connections with select and fd_set on Linux
	Silver Moon ( m00n.silv3r@gmail.com)
#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");
    //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 )
    //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");
	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)
    //accept the incoming connection
    addrlen = sizeof(address);
    puts("Waiting for connections ...");
        //clear the socket set
        //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)
            //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) ) 
            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);
        //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
                    //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

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 : , port : 57831 
Welcome message sent successfully
Adding to list of sockets as 0
New connection , socket fd is 5 , ip is : , port : 57832 
Welcome message sent successfully
Adding to list of sockets as 1
New connection , socket fd is 6 , ip is : , port : 57833 
Welcome message sent successfully
Adding to list of sockets as 2
New connection , socket fd is 7 , ip is : , port : 57834 
Welcome message sent successfully

The client terminal can be like this

$ telnet localhost 8888
Connected to localhost.
Escape character is '^]'.
ECHO Daemon v1.0 

There are other functions that can perform tasks similar to select. pselect , poll , ppoll


1. http://pubs.opengroup.org/onlinepubs/7908799/xsh/select.html
2. http://linux.die.net/man/2/select

Last Updated On : 18th May 2013

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About Silver Moon

Php developer, blogger and Linux enthusiast. He can be reached at m00n.silv3r@gmail.com. Or find him on

  • MhenryDpai

    I get the success on connect message multiple times from different telnet sessions – however I do not get anything echoed back to telnet client

  • Filip Jarno

    The best explanation of sockets and select that I found, thanks!

  • mohamad

    i implemented such as this code, but when server send to client , client not recieved.why?!

    • http://www.binarytides.com/ Silver Moon

      is the client able to connect to server in the first place ?
      does the server indicate that it received a new connection ?

      if yes, then whatever message the client sends to server, it should get back the same and print it.

  • Jyx

    Nice article, but isn’t it a problem in the part “//add new socket to array of sockets” where you assign max_clients? I.e. you set “i = max_clients;” what if a client with socket number less than max_clients has disconnected earlier. Then in the loop you will find it in the client_socket[i] which is equal to zero. Then you will add the new socket in that location and assign max_clients to that socket. However there could be sockets with higher id still available.

    • http://www.binarytides.com/ Silver Moon

      i is just a temporary loop variable.
      i = max_clients is used just to quit the loop, putting break instead of that would do the same thing.

      and empty position in client_sockets array is searched everytime a new socket connection comes in and the loop starts i from 0.

  • Florian

    You saved me MANY headaches.

  • http://www.smarkwardt.de Sascha

    wow, the article is perfect for me…


  • Nishant

    I go to this link while googling for “handling multiple socket connections”. This is exactly what I was looking for. Thanks!

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