IPV4 UDP server client program with Select system call

IPv4 AF_INET UDP

• In this program, you are going to learn

• How to create a Socket ?

• How to bind a socket ?

• How to send a data ?

• How to recv a data ?

• How to use socket APIs ?

  • socket

  • bind

  • sendto

  • recvfrom

Topics in this section,

• IPV4 AF_INET UDP SOCKET FAQs

• Step 1: Sequence Diagram for SERVER.c

• Step 2: Program for Server.c

• Step 3: Compile and Execute Server.c

• Step 4: Sequence Diagram for CLIENT.c

• Step 5: Program for Client.c

• Step 6: Compile and Execute Client.c

• Summary

IPV4 AF_INET UDP SOCKET : FAQs

Let us answer few basic questions in this socket

What does socket(AF_INET SOCK_DGRAM,IPPROTO_UDP) create?

See Answer

A UDP socket

What does AF_INET signify in socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)?

See Answer

IPv4 address family

Why might you choose SOCK_DGRAM when creating a socket?

See Answer

For connectionless communication

What is the purpose of IPPROTO_UDP in socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)?

See Answer

It is optional and can be omitted.

How does a UDP socket differ from a TCP socket created using socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)?

See Answer

UDP is connectionless, while TCP is connection-oriented,

What is the typical use case for a UDP socket created with socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)?

See Answer

DNS resolution

How would you bind a specific IP address and port to a UDP socket created using socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)?

See Answer

Use the bind() function after creating the socket.

What is the purpose of the select system call in network programming?

See Answer

To block and wait for activity on one or more file descriptors.

How does select help in handling multiple sockets efficiently?

See Answer

It provides a way to wait for readiness on multiple sockets without blocking the entire program.

What types of file descriptors can be monitored using select?

See Answer

sockets, files, timerfd, socketpair, message_queue, Namedpipes and shared_memory.

What is the significance of the timeout parameter in the select function?

See Answer

It specifies the maximum duration to wait for any file descriptor to become ready.

How do you handle errors when using the select system call?

See Answer

Check the return value for -1 to detect errors, Use perror to print error messages.

How does select handle a set of file descriptors with different states (e.g., reading, writing, exception)?

See Answer

Preparing File Descriptor Sets:

select(readfds, writefds, exceptfds);

Setting Up Readiness Conditions:

If you are interested in monitoring file descriptors for readability, you add them to the readfds set.

FD_ZERO(&readfds);

FD_SET(fd1, &readfds);

Setting Up Writability Conditions:

If you are interested in monitoring file descriptors for writability, you add them to the writefds set.

FD_ZERO(&writefds);

FD_SET(fd2, &writefds);

Setting Up Exceptional Conditions:

If you are interested in monitoring file descriptors for exceptional conditions, you add them to the exceptfds set.

FD_ZERO(&exceptfds);

FD_SET(fd3, &exceptfds);

How does select Checking Ready File Descriptors?

See Answer

After select returns, you can check the sets to determine which file descriptors are ready for the specified conditions.

if (FD_ISSET(fd1, &readfds)) {
        // fd1 is ready for reading
}
if (FD_ISSET(fd3, &writefds)) {
        // fd2 is ready for writing
}
if (FD_ISSET(fd4, &exceptfds)) {
        // fd3 has an exceptional condition
}

What does it mean if select returns 0?

See Answer

No file descriptors are ready within the specified timeout.

Step 1: Sequence Diagram for SERVER.c

Step 2 : program for server.c

• There are many functions used in socket. We can classify those functions based on functionalities.

  • Create Socket

  • Bind Socket

  • Select

  • Recvfrom data_packet

  • Sendto data_packet

  • Close socket

• socket() is used to create a new socket. For example,

server_socket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

• bind() is used to associate the socket with a specific address and port. For example,

ret = bind(server_socket, (struct sockaddr*)&server_addr, sizeof(server_addr));

• select() is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,

ret = select(server_socket + 1, &read_fds, NULL, NULL, NULL);

• recvfrom is commonly used with UDP sockets, where communication is connectionless. it provides information about the source (sender) of the data, including the sender’s IP address and port number. For example,

len = recvfrom(server_socket, buffer, BUFFER_SIZE, 0, (struct sockaddr*)&client_addr, &client_addr_len);

• sendto is used to send the encoded message to the specified server address and port using a UDP socket. For example,

ret = sendto(server_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, client_addr_len);

• close is used to close the socket To free up system resources associated with the socket. For example,

(void)close(server_socket);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <signal.h>

#define BUFFER_SIZE 1024

int server_socket = -1;

static void sigint_handler(int signo)
{
  (void)close(server_socket);
  sleep(2);
  (void)printf("Caught sigINT!\n");
  exit(EXIT_SUCCESS);
}

void register_signal_handler(
int signum,
void (*handler)(int))
{
  if (signal(signum, handler) == 
  SIG_ERR) {
     printf("Cannot handle signal\n");
     exit(EXIT_FAILURE);
  }
}

void validate_convert_port(
char *port_str,
struct sockaddr_in *sock_addr)
{
 int port;

 if (port_str == NULL) {
   perror("Invalid port_str\n");
   exit(EXIT_FAILURE);
 }

 if (sock_addr == NULL) {
   perror("Invalid sock_addr\n");
   exit(EXIT_FAILURE);
 }

 port = atoi(port_str);

 if (port == 0) {
     perror("Invalid port\n");
     exit(EXIT_FAILURE);
 }

 sock_addr->sin_port = htons(
 (uint16_t)port);
 printf("Port: %d\n",
 ntohs(sock_addr->sin_port));
}

void recv_send(
char *buffer,
struct sockaddr_in *client_addr)
{
  int len, ret;

  socklen_t client_addr_len = sizeof(
  client_addr);

  len = recvfrom(server_socket, 
  buffer, BUFFER_SIZE, 0, 
  (struct sockaddr*)&client_addr, 
  &client_addr_len);
      
  if (len > 0) {
    buffer[len] = '\0';
    printf("Received: %s\n", 
    buffer);

    memset(buffer, 0, 
    sizeof(buffer));
    strncpy(buffer, "HELLO", 
    strlen("HELLO") + 1);
    buffer[strlen(buffer) + 1] = '\0';
        
    ret = sendto(server_socket, 
    buffer, 
    strlen(buffer), 0, 
    (struct sockaddr*)&client_addr, 
    client_addr_len);
        
    if (ret < 0) {
       perror("sendto");
       exit(EXIT_FAILURE);
      }
    } else if (len < 0) {
        perror("recvfrom");
        exit(EXIT_FAILURE);
    }
    printf("Sent buffer = %s\n", 
    buffer);
}

int main(int argc, char *argv[]) 
{
  int ret;
  struct sockaddr_in 
  server_addr, 
  client_addr;
  fd_set read_fds;
  char buffer[BUFFER_SIZE];

  register_signal_handler(SIGINT,
      sigint_handler);

  if (argc != 2) {
    printf("%s <port-number>",
        argv[0]);
    exit(EXIT_FAILURE);
  }

  memset(&server_addr, 0, 
  sizeof(server_addr));
  server_addr.sin_family = AF_INET;
  server_addr.sin_addr.s_addr = 
  INADDR_ANY;
  validate_convert_port(argv[1],
  &server_addr);

  server_socket = socket(AF_INET, 
                  SOCK_DGRAM, 
                  IPPROTO_UDP);

  if (server_socket < 0) {
    perror("socket");
    return -3;
  }

  ret = bind(server_socket, 
  (struct sockaddr*)&server_addr, 
  sizeof(server_addr));
  
  if (ret < 0) {
    perror("bind");
    (void)close(server_socket);
    return -4;
  }

  printf("UDP listining\n");

  while (1) {
    FD_ZERO(&read_fds);
    FD_SET(server_socket, &read_fds);

    ret = select(server_socket + 1, 
    &read_fds, NULL, NULL, NULL);

    if (ret < 0) {
      perror("select");
      break;
    }

    if (FD_ISSET(server_socket, 
    &read_fds)) {
      recv_send(buffer, &client_addr);
      sleep(2);     
    }
  }

  (void)close(server_socket);
  
  return 0;
}

Step 3: Compile and Execute server.c

$ gcc -o server server.c

$ sudo ./server 8080

Port: 8080
UDP listining
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
^CCaught sigINT!

Step 4 : Sequence Diagram for CLIENT.c

Step 5: program for client.c

• Create Socket

• Select

• Sendto data_packet

• Recvfrom data_packet

• Close socket

• socket is used to create a new socket. For example,

client_socket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);

• select is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,

ret = select(client_socket + 1, &read_fds, NULL, NULL, NULL);

• recvfrom is commonly used with UDP sockets, where communication is connectionless. it provides information about the source (sender) of the data, including the sender’s IP address and port number. For example,

len = recvfrom(client_socket, buffer, BUFFER_SIZE, 0, NULL, NULL);

• sendto is used to send the encoded message to the specified server address and port using a UDP socket. For example,

ret = sendto(client_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&server_addr, sizeof(server_addr));

• close is used to close the socket To free up system resources associated with the socket. For example,

(void)close(client_socket);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <signal.h>

#define BUFFER_SIZE 1024

int client_socket = -1;

static void sigint_handler(int signo)
{
  (void)close(client_socket);
  sleep(2);
  (void)printf("Caught sigINT!\n");
  exit(EXIT_SUCCESS);
}

void validate_convert_port(
char *port_str,
struct sockaddr_in *sock_addr)
{
 int port;

 if (port_str == NULL) {
   perror("Invalid port_str\n");
   exit(EXIT_FAILURE);
 }

 if (sock_addr == NULL) {
   perror("Invalid sock_addr\n");
   exit(EXIT_FAILURE);
 }

 port = atoi(port_str);

 if (port == 0) {
   perror("Invalid port\n");
   exit(EXIT_FAILURE);
 }

 sock_addr->sin_port = htons(
 (uint16_t)port);
 printf("Port: %d\n",
 ntohs(sock_addr->sin_port));
}

void validate_convert_addr(
char *ip_str,
struct sockaddr_in *sock_addr)
{
  if (ip_str == NULL) {
   perror("Invalid ip_str\n");
   exit(EXIT_FAILURE);
 }

 if (sock_addr == NULL) {
   perror("Invalid sock_addr\n");
   exit(EXIT_FAILURE);
 }

 printf("IP Address: %s\n", ip_str);

 if (inet_pton(AF_INET, ip_str,
 &(sock_addr->sin_addr)) <= 0) {
    perror("Invalid address\n");
    exit(EXIT_FAILURE);
  }
}

void recv_data(char *buffer)
{
  int ret, len;

  len = recvfrom(client_socket, 
  buffer, 
  BUFFER_SIZE, 0, NULL, NULL);
      
  if (len > 0) {
    buffer[len] = '\0';
    (void)printf("Received: %s\n",
    buffer);

  } else if (len == 0) {
     printf("Connection closed\n");
     exit(EXIT_FAILURE);
   }
}

void register_signal_handler(
int signum,
void (*handler)(int))
{
  if (signal(signum, handler) == SIG_ERR)
  {
     printf("Cannot handle signal\n");
     exit(EXIT_FAILURE);
  }
}

int main(int argc, char *argv[])
{
  int ret, len;
  struct sockaddr_in 
  server_addr;
  fd_set read_fds;
  char buffer[BUFFER_SIZE];
  char *str = "HI!";

  socklen_t tcp_addr_len = sizeof(
  server_addr);

  register_signal_handler(SIGINT,
  sigint_handler);

  if (argc != 3) {
    printf("%s<port-number><ip-addr>\n",
    argv[0]);
    exit(EXIT_FAILURE);
  }

  memset(&server_addr, 0, 
  sizeof(server_addr));
  server_addr.sin_family = AF_INET;
  validate_convert_port(argv[1],
  &server_addr);
  validate_convert_addr(argv[2],
  &server_addr);

  client_socket = socket(AF_INET, 
                  SOCK_DGRAM, 
                  IPPROTO_UDP);

  if (client_socket < 0) {
    perror("socket");
    return -1;
  }

  while (1) {
    ret = sendto(client_socket, str, 
    strlen(str), 0, 
    (struct sockaddr*)&server_addr, 
    sizeof(server_addr));
    printf("sendbuffer = %s\n", str);

    if (ret < 0) {
       perror("send error\n");
       (void)close(client_socket);
       break;
    }
    
    FD_ZERO(&read_fds);
    FD_SET(client_socket, &read_fds);

    ret = select(client_socket + 1, 
    &read_fds, NULL, NULL, NULL);

    if (ret < 0) {
      perror("select");
      break;
    }

    if (FD_ISSET(client_socket, 
    &read_fds)) {
      recv_data(buffer);
    }
  }

  (void)close(client_socket);
  
  return 0;
}

Step 6: Compile and Execute client.c

$ gcc -o client client.c

$ sudo ./client 8080 127.0.0.1

Port: 8080
IP Address: 127.0.0.1
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
^CCaught sigINT!

$ sudo ./server 8080

$ sudo ./client 8080 127.0.0.1

program to run with elevated privileges, listen on port 8080, and bind to the loopback address 127.0.0.1.

<port_number> <ip_address> decided by the user based on the connection.

Enhanced Socket Flexibility with AF_INET and PF_INET Domains

Default Domain:

By default, the socket is configured to work in the AF_INET domain, handling all types of network data.

Additional Domain Support:

We expand the socket’s capabilities to also function in the PF_INET domain, allowing it to operate similarly to AF_INET.

Socket Creation:

We set up a network connection point known as a socket using socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP).

Working Scenario:

Despite the change in domain to PF_INET, the socket continues to operate the same way, handling general network data.

Summary

Socket API	Learning
socket	Create a new socket
bind	Associate the socket with a specific address and port
select	Monitor multiple file descriptors (usually sockets) for read, write, or error conditions.
recvfrom	It provides information about the source (sender) of the data, including the sender’s IP address and port number.
sendto	Send the encoded message to the specified server address and port using a UDP socket.

See Also

• Previous topic

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• Other sockets

  • af_unix_tcp

  • af_unix_udp

  • ipv4_raw_af_inet_icmp

  • ipv4_raw_af_inet_raw

  • ipv4_raw_af_inet_tcp

  • ipv4_raw_af_inet_udp

  • ipv6_af_inet6_tcp

  • ipv6_af_inet6_udp

  • //TODO ipv6_raw_af_inet6_icmp

  • //TODO ipv6_raw_af_inet6_tcp

  • //TODO ipv6_raw_af_inet6_udp

  • // TODO ipv6_raw_af_inet6_raw

  • raw_af_packet_raw_htons_ETH_P_ALL

  • raw_af_packet_tcp_htons_ETH_P_ALL

  • raw_af_packet_udp_htons_ETH_P_ALL

• Other IPCs

  • Message_queues

  • NamedPipes

  • Netlink

  • Shared_Memory

  • Shared_Memory_2_FDS

  • SocketPair

  • Timerfd