IPV4 UDP server client program with Epoll 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

  • epoll_create1

  • epoll_ctl

  • epoll_wait

  • 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 primary purpose of the epoll system call?

See Answer

To efficiently monitor multiple file descriptors for I/O events

What types of file descriptors can be monitored using epoll?

See Answer

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

What data structure is used by epoll to store events?

See Answer

Hash table

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

See Answer

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

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

See Answer

Create the epoll Instance:

Before monitoring file descriptors, the application creates an epoll instance using the epoll_create system call.

int epoll_fd = epoll_create1(0);

Register File Discriptors:

The application registers file descriptors with the epoll instance using the epoll_ctl system call. It specifies the file descriptor, the events it is interested in (EPOLLIN for readability, EPOLLOUT for writability, etc.), and a user-defined data associated with the file descriptor.

struct epoll_event event;
event.events = EPOLLIN | EPOLLOUT;  // Interested in readability and writability
event.data.fd = my_file_descriptor; // File descriptor to monitor

epoll_ctl(epoll_fd, EPOLL_CTL_ADD, my_file_descriptor, &event);

Wait for Events:

The application enters a loop where it calls epoll_wait to wait for events. This call blocks until one or more registered file descriptors become ready or until a timeout occurs.

#define MAX_EVENTS 10
struct epoll_event events[MAX_EVENTS];

int num_events = epoll_wait(epoll_fd, events, MAX_EVENTS, timeout_ms);

Modify or Remove File Descriptors:

The application can dynamically modify or remove file descriptors from the epoll set using the epoll_ctl system call. For example, to modify events for an existing file descriptor:

struct epoll_event new_event;
new_event.events = EPOLLOUT;  // Modify to be interested in writability

epoll_ctl(epoll_fd, EPOLL_CTL_MOD, my_file_descriptor, &new_event);

To remove a file descriptor from the epoll set:

epoll_ctl(epoll_fd, EPOLL_CTL_DEL, my_file_descriptor, NULL);

How does epoll Checking Ready File Descriptors?

See Answer

After epoll_wait returns, the application iterates through the returned events to identify which file descriptors are ready and for what types of events.

for (int i = 0; i < num_events; ++i) {
        if (events[i].events & EPOLLIN) {
                // File descriptor i is ready for reading
        }

        if (events[i].events & EPOLLOUT) {
                // File descriptor i is ready for writing
        }
        // Check other events if needed (e.g., EPOLLERR, EPOLLHUP)
}

What does it mean if epoll 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

  • Epoll create1

  • Epoll_ctl

  • Epoll_wait

  • 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));

• epoll_create1() creating an epoll instance using epoll_create1, The size parameter is an advisory hint for the kernel regarding the number of file descriptors expected to be monitored, For example,

epoll_fd = epoll_create1(0);

• epoll_ctl() After creating an epoll instance, file descriptors are added to it using epoll_ctl, For example,

ret = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, server_socket, &event);

• epoll_wait() The application then enters a loop where it waits for events using epoll_wait, For example,

ready_fds = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);

• 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 <sys/epoll.h>

#define BUFFER_SIZE 1024
#define MAX_EVENTS 5

int server_socket = -1;
int epoll_fd = -1;

static void sigint_handler(int signo)
{
  (void)close(epoll_fd);
  (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 ready_fds;
  int ret;
  struct sockaddr_in
  server_addr,
  client_addr;
  char buffer[BUFFER_SIZE];
  struct epoll_event 
  events[MAX_EVENTS];
  struct epoll_event event;

  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;
  }

  epoll_fd = epoll_create1(0);
 
  if (epoll_fd < 0) {
    perror("Epoll creation failed");
    exit(EXIT_FAILURE);
  }
	
  event.events = EPOLLIN;
  event.data.fd = server_socket;
  
  ret = epoll_ctl(epoll_fd, 
  EPOLL_CTL_ADD, server_socket, 
  &event);
 
  if (ret < 0) {
    perror("Epoll_ctl failed");
    (void)close(epoll_fd);
    (void)close(server_socket);
    return -3;
  }

  while (1) {
    ready_fds = epoll_wait(epoll_fd, 
    events, MAX_EVENTS, -1);
		
    if (ready_fds < 0) {
	perror("Epoll wait failed");
	(void)close(epoll_fd);
	(void)close(server_socket);
   	break;
    }
    
    if (events[0].data.fd == server_socket) {
	recv_send(buffer, &client_addr);
    }	
  }    

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

Step 3: Compile and Execute Server.c

$ gcc -o server server.c

$ sudo ./server 8080

Port: 8080
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

  • Epoll create1

  • Epoll_ctl

  • Epoll_wait

  • 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);

• epoll_create1() creating an epoll instance using epoll_create1, The size parameter is an advisory hint for the kernel regarding the number of file descriptors expected to be monitored, For example,

epoll_fd = epoll_create1(0);

• epoll_ctl() After creating an epoll instance, file descriptors are added to it using epoll_ctl, For example,

ret = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_socket, &event);

• epoll_wait() The application then enters a loop where it waits for events using epoll_wait, For example,

ready_fds = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);

• 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 <sys/epoll.h>

#define BUFFER_SIZE 1024
#define MAX_EVENTS 2

int client_socket = -1;
int epoll_fd = -1;

static void sigint_handler(int signo)
{
  (void)close(epoll_fd);
  (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 ready_fds;
  int ret;
  struct epoll_event
  events[MAX_EVENTS];
  struct sockaddr_in 
  server_addr;
  char buffer[BUFFER_SIZE];
  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;
  }

  epoll_fd = epoll_create1(0);

  if (epoll_fd < 0) {
   perror("Epoll creation failed");
   (void)close(client_socket);
   return -2;
  }

  struct epoll_event event;

  event.events = EPOLLIN | EPOLLET;
  event.data.fd = client_socket;
  ret = epoll_ctl(epoll_fd, EPOLL_CTL_ADD,
  client_socket, &event);

  if (ret < 0) {
       perror("Epoll_ctl failed");
       (void)close(client_socket);
       return -3;
  }

  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;
    }

    ready_fds = epoll_wait(epoll_fd,
                events,
                MAX_EVENTS, -1);

    if (ready_fds < 0) {
      perror("Epoll wait failed");
      break;
    }

    if (events[0].data.fd ==
    client_socket) {
      recv_data(buffer);
    }
  }

  (void)close(epoll_fd);
  (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
^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
epoll	handles a set of file descriptors with different states, such as reading, writing, and exceptions, by using the struct epoll_event structure and the associated event flags..
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

  • IPV4 UDP server client program with Poll system call

• Current topic

  • IPV4 UDP server client program with Epoll system call

• Next topic

  • af_unix_tcp

• Other sockets

  • 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