IPV4 UDP server client program with Poll 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 poll system call?

See Answer

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

How does poll differ from poll in terms of usability?

See Answer

poll is more efficient than poll for monitoring multiple file descriptors.

What types of file descriptors can be monitored using poll?

See Answer

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

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

See Answer

It uses different structures for each state in the pollfd array.

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

See Answer

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

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

See Answer

Array of pollfd Structures:

Before calling poll, you need to create an array of pollfd structures, where each structure represents a file descriptor and its associated events.

struct pollfd fds[NUM_FDS];

NUM_FDS is the number of file descriptors you want to monitor.

Initialize pollfd Structures:

For each file descriptor you want to monitor, initialize the corresponding pollfd structure with the following information:

fd: The file descriptor to monitor. events: The events of interest (e.g., POLLIN for readability, POLLOUT for writability). revents: Initially set to zero. After the poll call, this field is updated to indicate the events that occurred.

fds[0].fd = fd1;

fds[0].events = POLLIN;

fds[0].revents = 0;

fds[1].fd = fd2;

fds[1].events = POLLIN;

fds[1].revents = 0;

Call poll:

After initializing the pollfd array, call the poll function, providing the array, the number of file descriptors, and a timeout

int ready_fds = poll(fds, NUM_FDS, timeout_ms);

ready_fds will contain the number of file descriptors that are ready.

How does poll Checking Ready File Descriptors?

See Answer

After the poll call, loop through the pollfd array and check the revents field for each file descriptor to determine which events occurred.

for (int i = 0; i < NUM_FDS; ++i) {
        if (fds[i].revents & POLLIN) {
                // File descriptor i is ready for reading
        }

        if (fds[i].revents & POLLOUT) {
                // File descriptor i is ready for writing
        }
        // Check other events if needed (e.g., POLLERR, POLLHUP)
}

What does it mean if poll 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

  • Poll

  • 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));

• poll() is used for monitoring multiple file descriptors to see if I/O is possible on any of them.

ret = poll(fds, 1, 1000);

• 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>
#include <poll.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("Sentbuffer = %s\n",
    buffer);
}

int main(int argc, char *argv[])
{
  int ret;
  struct sockaddr_in
  server_addr,
  client_addr;
  char buffer[BUFFER_SIZE];
  struct pollfd fds[1];

  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 -1;
  }

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

  if (ret < 0) {
    perror("bind");
    (void)close(server_socket);
    return -2;
  }

  printf("UDP listining\n");

  memset(fds, 0, sizeof(fds));
  fds[0].fd = server_socket;
  fds[0].events = POLLIN;

  while (1) {

    ret = poll(fds, 1, 1000);

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

    if (fds[0].revents & POLLIN) {
      recv_send(buffer, 
      &client_addr);
    }
  }

  (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
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
Sentbuffer = HELLO
Received: HI
^CCaught sigINT!

Step 4 : Sequence Diagram for CLIENT.c

Step 5: program for client.c

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

  • Create Socket

  • Poll

  • 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);

• poll() is used for monitoring multiple file descriptors to see if I/O is possible on any of them.

ret = poll(fds, 2, 1000);

• 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));

• 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);

• 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>
#include <poll.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;
  char buffer[BUFFER_SIZE];
  struct pollfd fds[1];
  char *str = "HI";

  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;
    }

    fds[0].fd = client_socket;  
    fds[0].events = POLLIN;

    ret = poll(fds, 2, 1000);

    if (ret < 0) {
       perror("poll");
       (void)close(client_socket);
       break;
    }

    if (fds[0].revents & POLLIN) {
       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 127.0.0.1

$ 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
poll	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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  • IPV4 UDP server client program with Poll system call

• Next topic

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  • //TODO ipv6_raw_af_inet6_icmp

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