IPV4 TCP server client program with Epoll system call

IPv4 AF_INET TCP

• In this program, you are going to learn

• How to create a Socket ?

• How to bind a socket ?

• How to listen a socket ?

• How to connect a socket ?

• How to accept a socket ?

• How to send a data ?

• How to recv a data ?

• How to use socket APIs ?

  • socket

  • bind

  • listen

  • epoll_create1

  • epoll_ctl

  • epoll_wait

  • connect

  • accept

  • send

  • recv

Topics in this section,

• IPV4 AF_INET TCP 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 TCP SOCKET : FAQs

Let us answer few basic questions in this socket

What does the AF_INET parameter represent in the socket() function call?

See Answer

Address Family for IPv4

Why is SOCK_STREAM used as the second parameter in the socket() function call?

See Answer

It indicates that the socket will use a reliable, connection-oriented communication (TCP).

What is the default protocol for a TCP socket created with socket(AF_INET, SOCK_STREAM, ?? )?

See Answer

IPPROTO_TCP

What is the primary characteristic of TCP communication?

See Answer

Connection-oriented and reliable

What does a return value of -1 indicate in socket functions?

See Answer

In many socket programming APIs, a return value of -1 usually indicates an error. Functions often set an error code that can be retrieved using perror (or an equivalent mechanism) to determine the specific nature of the error.

Why might bind() or listen() fail in socket programming?

See Answer

bind() might fail if the specified address is already in use, or if the process lacks the necessary permissions. listen() might fail if the socket is not bound, or the operating system limit for pending connections is reached.

How should you handle errors when using accept() in socket programming?

See Answer

Check the return value and handle errors appropriately

Why is it important to check the return value of send() and recv() in socket programming?

See Answer

It detects issues such as network errors or closed connections.

What is a common practice for handling timeouts in socket programming?

See Answer

Use non-blocking sockets with functions like epoll() or poll().

Can you use a TCP socket (SOCK_STREAM) for sending and receiving data concurrently between a client and server?

See Answer

Yes, TCP sockets support bidirectional communication.

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

  • Listen Socket

  • Accept Socket

  • Epoll create1

  • Epoll_ctl

  • Epoll_wait

  • Recv data_packet

  • Send data_packet

  • Close socket

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

tcp_server_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

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

ret = bind(tcp_server_fd, (struct sockaddr*)&tcp_addr, sizeof(tcp_addr));

• listen() is used to set up a socket to accept incoming connections. For example,

ret = listen(tcp_server_fd, MAX_CLIENTS);

• accept() is used in network programming on the server side to accept a connection request from a client. For example,

tcp_client_fd = accept(tcp_server_fd, (struct sockaddr*) &tcp_addr, &tcp_addr_len);

• 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, tcp_server_fd, &event);

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

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

• recv is used in network programming to receive data from a connected socket. For example,

len = recv(tcp_client_fd, buffer, sizeof(buffer) - 1, 0);

• send is used in network programming to send data over a connected socket. For example,

ret = send(tcp_client_fd, buffer, strlen(buffer), 0);

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

(void)close(tcp_client_fd);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <sys/un.h>
#include <signal.h>
#include <sys/epoll.h>

#define MAX_EVENTS 5

int tcp_server_fd = -1;
int tcp_client_fd = -1;

static void sigint_handler(int signo)
{
  (void)close(tcp_server_fd);
  (void)close(tcp_client_fd);
  sleep(2);
  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)
{
  int len, ret;

  memset(buffer, 0, 
  sizeof(buffer));
  len = recv(tcp_client_fd, buffer,
  sizeof(buffer) - 1, 0);

  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';
    printf("Sentbuffer = %s\n", 
    buffer);

    ret = send(tcp_client_fd, buffer,
    strlen(buffer), 0);

    if (ret < 0) {
       perror("send error\n");
       (void)close(tcp_client_fd);
       (void)close(tcp_server_fd);
       exit(EXIT_FAILURE);
    }

  } else if (len < 0) {
      perror("recv");
      (void)close(tcp_client_fd);
      (void)close(tcp_server_fd);
      exit(EXIT_FAILURE);
   }
}

int main(int argc, char *argv[]) 
{
  int epoll_fd; 
  int ready_fds;
  int ret;
  struct sockaddr_in 
  tcp_addr;
  char buffer[1024];
  struct epoll_event 
  events[MAX_EVENTS];

  socklen_t tcp_addr_len = sizeof(
  tcp_addr);
  
  register_signal_handler(SIGINT,
  sigint_handler);

  if (argc != 2) {
    printf("%s <port-number>", 
    argv[0]);
    return -1;
  }

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

  tcp_server_fd = socket(AF_INET, 
                  SOCK_STREAM, 
                  IPPROTO_TCP);
  
  if (tcp_server_fd < 0) {
    perror("socket");
    return -2;
  }

  ret = bind(tcp_server_fd, 
  (struct sockaddr *)&tcp_addr, 
  sizeof(tcp_addr));
  
  if (ret < 0) {
    perror("bind");
    (void)close(tcp_server_fd);
    return -3;
  }

  ret = listen(tcp_server_fd,
  MAX_EVENTS);
  
  if (ret < 0) {
    perror("listen");
    (void)close(tcp_server_fd);
    return -4;
  }
  
  printf("Server is listening\n");

  tcp_client_fd = accept(tcp_server_fd, 
  (struct sockaddr *) &tcp_addr, 
  &tcp_addr_len);

  if (tcp_client_fd < 0) {
    perror("accept");
    (void)close(tcp_server_fd);
    return -5;
  }

  printf("Connection accepted\n");

  epoll_fd = epoll_create1(0);
  
  if (epoll_fd <= 0)
  {
    perror("Epoll creation failed");
    exit(EXIT_FAILURE);
  }

  struct epoll_event event;
  event.events = EPOLLIN;
  event.data.fd = tcp_client_fd;
  
  ret = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, 
  tcp_client_fd, &event);
       
  if (ret  == -1) {
    perror("Epoll_ctl failed");
    exit(EXIT_FAILURE);
  }

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

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

    if (events[0].data.fd == 
    tcp_client_fd) {
      recv_send(buffer);
    }
  }

  (void)close(tcp_client_fd);
  (void)close(tcp_server_fd);

  return 0;
}

Step 3: Compile and Execute Server.c

$ gcc -o server server.c

$ sudo ./server 8080

Port: 8080
Server is listening
Connection accepted
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

  • Connect Socket

  • Epoll create1

  • Epoll_ctl

  • Epoll_wait

  • Send data_packet

  • Recv data_packet

  • Close socket

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

client_fd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

• connect is used in network programming to establish a connection from a client to a server. For example,

cli_connect = connect(client_fd, (struct sockaddr*)&tcp_addr, tcp_addr_len);

• 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_fd, &event);

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

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

• send is used in network programming to send data over a connected socket. For example,

ret = send(client_fd, buffer, strlen(buffer), 0);

• recv is used in network programming to receive data from a connected socket. For example,

len = recv(client_fd, buffer, sizeof(buffer) - 1, 0);

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

(void)close(client_fd);

• See the full program below,

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <sys/un.h>
#include <signal.h>
#include <sys/epoll.h>

#define MAX_EVENTS 2

int client_fd;

static void sigint_handler(int signo)
{
  (void)close(client_fd);
  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 = recv(client_fd, buffer,
  sizeof(buffer) - 1, 0);

  if (len > 0) {
    buffer[len] = '\0';
    (void)printf("Received: %s\n",
    buffer);
    
  } else if (len == 0) {
      printf("Connection closed\n");
      exit(EXIT_FAILURE);
   }
}

void send_data(char *buffer)
{
  int ret;
  
  memset(buffer, 0, 
  sizeof(buffer));
  
  strncpy(buffer, "HI",
  strlen("HI") + 1);
  buffer[strlen(buffer) + 1] = '\0';

  ret = send(client_fd, buffer,
  strlen(buffer), 0);

  if (ret < 0) {
    perror("send error\n");
    (void)close(client_fd);
    exit(EXIT_FAILURE);
  }
  printf("sentbuffer = %s\n", 
  buffer);
}

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 epoll_fd; 
  int ready_fds; 
  int cli_connect;
  int ret;
  struct sockaddr_in 
  tcp_addr;
  char buffer[1024];
  struct epoll_event 
  events[MAX_EVENTS];

  socklen_t tcp_addr_len = sizeof(
  tcp_addr);

  register_signal_handler(SIGINT,
  sigint_handler);

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

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

  client_fd = socket(AF_INET, 
              SOCK_STREAM, 
              IPPROTO_TCP);

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

  cli_connect = connect(client_fd, 
  (struct sockaddr *)&tcp_addr, 
  tcp_addr_len);

  if (cli_connect < 0) {
    perror("connect");
    return -2;
  } else {
    printf("connected\n");
  }  
  
  ret = epoll_fd = epoll_create1(0);
 
  if (ret < 0) {
   perror("Epoll creation failed");
   exit(EXIT_FAILURE);
  }

  struct epoll_event event;
        
  event.events = EPOLLIN | EPOLLET;
  event.data.fd = client_fd;
  ret = epoll_ctl(epoll_fd, EPOLL_CTL_ADD, 
  client_fd, &event);
       
  if (ret  == -1) {
       perror("Epoll_ctl failed");
       exit(EXIT_FAILURE);
  }
    
  while (1) {
    send_data(buffer);

    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_fd) {
      recv_data(buffer);
    }
  }

  (void)close(client_fd);
  
  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
connected
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
sentbuffer = HI
Received: HELLO
^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_STREAM, IPPROTO_TCP).

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
listen	Set up a socket to accept incoming connections.
connect	Establish a connection from a client to a server.
accept	Server side to accept a connection request from a client.
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..
recv	Receive data from a connected socket.
send	Send data over a connected socket.

See Also

• Previous topic

  • IPV4 TCP server client program with Poll system call

• Current topic

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• Next topic

  • ipv4_af_inet_udp

• 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