TCP and UDP server using select

Last Updated : 05 Nov, 2021

Prerequisites: TCP UDP

In previous articles, we have seen a TCP server and a UDP server. But now we can combine our concurrent TCP echo server and iterative UDP server into a single server that uses select to multiplex TCP and UDP socket.

The Select function is used to select between TCP and UDP sockets. This function gives instructions to the kernel to wait for any of the multiple events to occur and awakens the process only after one or more events occur or a specified time passes.

Example - kernel will return only when one of these conditions occurs

Any Descriptor from {1, 2, 3} is ready for reading
Any Descriptor from {4, 5, 6} is ready for writing
Time 5sec has passed

The entire process can be broken down into the following steps :

Server:

Create TCP i.e Listening socket
Create a UDP socket
Bind both sockets to the server address.
Initialize a descriptor set for select and calculate a maximum of 2 descriptors for which we will wait
Call select and get the ready descriptor(TCP or UDP)
Handle new connection if the ready descriptor is of TCP OR receive datagram if the ready descriptor is of UDP

UDP Client:

Create a UDP socket.
Send a message to the server.
Wait until a response from the server is received.
Close socket descriptor and exit.

TCP Client:

Create a TCP socket.
Call connect to establish a connection with the server.
When the connection is accepted write a message to a server.
Read the response of the Server.
Close socket descriptor and exit.

Necessary functions:

int select(int maxfd, fd_set *readsset, fd_set *writeset, 
fd_set *exceptset, const struct timeval *timeout);
Returns: positive count of descriptors ready, 0 on timeout, -1 error

Arguments:

maxfd: maximum number of descriptors ready.
timeout: How long to wait for select to return.

struct timeval{
long tv_sec;
long tv_usec;
};
if timeout==NULL then wait forever
if timeout == fixed_amount_time then wait until specified time
if timeout == 0 return immediately.

readset: Descriptor set that we want the kernel to test for reading.
writeset: Descriptor set that we want the kernel to test for writing.
exceptset: Descriptor set that we want the kernel to test for exception conditions.

int read(int sockfd, void * buff, size_t nbytes);
Returns:  number of bytes read from the descriptor. -1 on error

Arguments:

sockfd: Descriptor which receives data.
buff: Application buffer socket descriptor data is copied to this buffer.
nbytes: Number of bytes to be copied to application buffer.

Server.c

// Server program 
#include <arpa/inet.h> 
#include <errno.h> 
#include <netinet/in.h> 
#include <signal.h> 
#include <stdio.h> 
#include <stdlib.h> 
#include <strings.h> 
#include <sys/socket.h> 
#include <sys/types.h> 
#include <unistd.h> 
#define PORT 5000 
#define MAXLINE 1024 
int max(int x, int y) 
{ 
    if (x > y) 
        return x; 
    else
        return y; 
} 
int main() 
{ 
    int listenfd, connfd, udpfd, nready, maxfdp1; 
    char buffer[MAXLINE]; 
    pid_t childpid; 
    fd_set rset; 
    ssize_t n; 
    socklen_t len; 
    const int on = 1; 
    struct sockaddr_in cliaddr, servaddr; 
    char* message = "Hello Client"; 
    void sig_chld(int); 

    /* create listening TCP socket */
    listenfd = socket(AF_INET, SOCK_STREAM, 0); 
    bzero(&servaddr, sizeof(servaddr)); 
    servaddr.sin_family = AF_INET; 
    servaddr.sin_addr.s_addr = htonl(INADDR_ANY); 
    servaddr.sin_port = htons(PORT); 

    // binding server addr structure to listenfd 
    bind(listenfd, (struct sockaddr*)&servaddr, sizeof(servaddr)); 
    listen(listenfd, 10); 

    /* create UDP socket */
    udpfd = socket(AF_INET, SOCK_DGRAM, 0); 
    // binding server addr structure to udp sockfd 
    bind(udpfd, (struct sockaddr*)&servaddr, sizeof(servaddr)); 

    // clear the descriptor set 
    FD_ZERO(&rset); 

    // get maxfd 
    maxfdp1 = max(listenfd, udpfd) + 1; 
    for (;;) { 

        // set listenfd and udpfd in readset 
        FD_SET(listenfd, &rset); 
        FD_SET(udpfd, &rset); 

        // select the ready descriptor 
        nready = select(maxfdp1, &rset, NULL, NULL, NULL); 

        // if tcp socket is readable then handle 
        // it by accepting the connection 
        if (FD_ISSET(listenfd, &rset)) { 
            len = sizeof(cliaddr); 
            connfd = accept(listenfd, (struct sockaddr*)&cliaddr, &len); 
            if ((childpid = fork()) == 0) { 
                close(listenfd); 
                bzero(buffer, sizeof(buffer)); 
                printf("Message From TCP client: "); 
                read(connfd, buffer, sizeof(buffer)); 
                puts(buffer); 
                write(connfd, (const char*)message, sizeof(buffer)); 
                close(connfd); 
                exit(0); 
            } 
            close(connfd); 
        } 
        // if udp socket is readable receive the message. 
        if (FD_ISSET(udpfd, &rset)) { 
            len = sizeof(cliaddr); 
            bzero(buffer, sizeof(buffer)); 
            printf("\nMessage from UDP client: "); 
            n = recvfrom(udpfd, buffer, sizeof(buffer), 0, 
                        (struct sockaddr*)&cliaddr, &len); 
            puts(buffer); 
            sendto(udpfd, (const char*)message, sizeof(buffer), 0, 
                (struct sockaddr*)&cliaddr, sizeof(cliaddr)); 
        } 
    } 
}

TCP_Client.c

// TCP Client program 
#include <netinet/in.h> 
#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <sys/socket.h> 
#include <sys/types.h> 
#define PORT 5000 
#define MAXLINE 1024 
int main() 
{ 
    int sockfd; 
    char buffer[MAXLINE]; 
    char* message = "Hello Server"; 
    struct sockaddr_in servaddr; 

    int n, len; 
    // Creating socket file descriptor 
    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) < 0) { 
        printf("socket creation failed"); 
        exit(0); 
    } 

    memset(&servaddr, 0, sizeof(servaddr)); 

    // Filling server information 
    servaddr.sin_family = AF_INET; 
    servaddr.sin_port = htons(PORT); 
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1"); 

    if (connect(sockfd, (struct sockaddr*)&servaddr, 
                            sizeof(servaddr)) < 0) { 
        printf("\n Error : Connect Failed \n"); 
    } 

    memset(buffer, 0, sizeof(buffer)); 
    strcpy(buffer, "Hello Server"); 
    write(sockfd, buffer, sizeof(buffer)); 
    printf("Message from server: "); 
    read(sockfd, buffer, sizeof(buffer)); 
    puts(buffer); 
    close(sockfd); 
}

UDP_client.c

// UDP client program 
#include <arpa/inet.h> 
#include <netinet/in.h> 
#include <stdio.h> 
#include <stdlib.h> 
#include <strings.h> 
#include <sys/socket.h> 
#include <sys/types.h> 
#define PORT 5000 
#define MAXLINE 1024 
int main() 
{ 
    int sockfd; 
    char buffer[MAXLINE]; 
    char* message = "Hello Server"; 
    struct sockaddr_in servaddr; 

    int n, len; 
    // Creating socket file descriptor 
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) { 
        printf("socket creation failed"); 
        exit(0); 
    } 

    memset(&servaddr, 0, sizeof(servaddr)); 

    // Filling server information 
    servaddr.sin_family = AF_INET; 
    servaddr.sin_port = htons(PORT); 
    servaddr.sin_addr.s_addr = inet_addr("127.0.0.1"); 
    // send hello message to server 
    sendto(sockfd, (const char*)message, strlen(message), 
        0, (const struct sockaddr*)&servaddr, 
        sizeof(servaddr)); 

    // receive server's response 
    printf("Message from server: "); 
    n = recvfrom(sockfd, (char*)buffer, MAXLINE, 
                0, (struct sockaddr*)&servaddr, 
                &len); 
    puts(buffer); 
    close(sockfd); 
    return 0; 
}