af unix tcp server client program with Epoll system call

AF_UNIX 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,

• AF_UNIX 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

AF_UNIX TCP SOCKET : FAQs

Let us answer few basic questions in this socket

What does AF_UNIX represent in the socket call?

See Answer

AF_UNIX represents the Unix domain socket family. It is used for communication between processes on the same machine, using file system paths as addresses.

Why use SOCK_STREAM as the socket type?

See Answer

SOCK_STREAM indicates a stream-oriented socket, providing a reliable, connection-oriented communication channel.

What does the third parameter (0) indicate in socket(AF_UNIX, SOCK_STREAM, 0)?

See Answer

Default protocol selected by the system.

How can I set up a server and client using AF_UNIX sockets?

See Answer

To set up a server and client,

create a socket using socket(AF_UNIX, SOCK_STREAM, 0),

bind the server to an address,

isten for incoming connections, and establish connections from clients.

Communication occurs through read and write operations on the established connections.

How should errors in the socket call be handled?

See Answer

Check the return value, handle errors using appropriate mechanisms.

What is the role of the file system path in AF_UNIX sockets?

See Answer

The file system path serves as the address for Unix domain sockets. It enables processes to locate and connect to the socket. he path is set in the sun_path field of the struct sockaddr_un structure.

What is the role of the file system path in AF_UNIX sockets?

See Answer

Address.

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 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,

server_fd = socket(AF_UNIX, SOCK_STREAM, 0);

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

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

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

ret = listen(server_fd, MAX_CLIENTS);

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

client_fd = accept(server_fd, NULL, NULL);

• 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);

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

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

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

ret = send(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(client_fd);

• See the full program below,

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

#define MAX_EVENTS 5
#define SOCKET_PATH "/tmp/my_socket"

int server_fd = -1;
int client_fd = -1;
int epoll_fd = -1;

static void sigint_handler(int signo)
{
  unlink(SOCKET_PATH);
  (void)close(epoll_fd);
  (void)close(server_fd);
  (void)close(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 recv_send(char *buffer)
{
  int len, ret;

  memset(buffer, 0,
  sizeof(buffer));
  len = recv(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(client_fd, buffer,
    strlen(buffer), 0);

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

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

int main(void) 
{
  int ready_fds; 
  int ret;
  struct sockaddr_un 
  server_addr, 
  client_addr;
  char buffer[1024];
  struct epoll_event 
  events[MAX_EVENTS];
  struct epoll_event event;

  register_signal_handler(SIGINT,
  sigint_handler);
  
  memset(&server_addr, 0, 
  sizeof(struct sockaddr_un));
  server_addr.sun_family = AF_UNIX;
  strncpy(server_addr.sun_path, 
  SOCKET_PATH, 
  sizeof(server_addr.sun_path) - 1);
  unlink(SOCKET_PATH);

  server_fd = socket(AF_UNIX, 
              SOCK_STREAM, 0);

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

  ret = bind(server_fd, 
  (struct sockaddr*)&server_addr, 
  sizeof(struct sockaddr_un));
  
  if (ret < 0) {
    perror("bind");
    (void)close(server_fd);
    return -2;
  }

  ret = listen(server_fd, 
  MAX_EVENTS);
  
  if (ret < 0)
  {
    perror("listen");
    (void)close(server_fd);
    return -3;
  }

  printf("server listening\n");

  client_fd = accept(server_fd, 
  NULL, NULL);

  if (client_fd < 0) {
    perror("accept");
    (void)close(server_fd);
    return -4;
  }

  printf("Connection accepted\n");
  
  epoll_fd = epoll_create1(0);
 
  if (epoll_fd < 0) {
    perror("Epoll failed");
    exit(EXIT_FAILURE);
  }

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

  if (ret < 0) {
    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 == 
        client_fd) {
      recv_send(buffer);
    }
  }
  
  (void)close(client_fd);
  (void)close(server_fd);
  return 0;
}

Step 3: Compile and Execute Server.c

$ gcc -o server server.c

$ sudo ./server

server 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_UNIX, SOCK_STREAM, 0);

• 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*)&server_addr, sizeof(struct sockaddr_un));

• 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 <sys/un.h>
#include <signal.h>
#include <sys/epoll.h>
#include <netinet/in.h>

#define MAX_EVENTS 3
#define SOCKET_PATH "/tmp/my_socket"

int client_fd = -1;
int epoll_fd = -1;

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

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(void) 
{
  int ready_fds; 
  int cli_connect;
  int ret;
  struct sockaddr_un 
  server_addr;
  char buffer[1024];
  struct epoll_event 
  events[MAX_EVENTS];
  
  register_signal_handler(SIGINT,
  sigint_handler);

  memset(&server_addr, 0, 
  sizeof(struct sockaddr_un));
  server_addr.sun_family = AF_UNIX;
  strncpy(server_addr.sun_path, 
  SOCKET_PATH, 
  sizeof(server_addr.sun_path) - 1);

  client_fd = socket(AF_UNIX, 
              SOCK_STREAM, 0);

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

  cli_connect = connect(client_fd, 
  (struct sockaddr*)&server_addr, 
  sizeof(struct sockaddr_un));

  if (cli_connect < 0) {
     perror("connect");
     (void)close(client_fd);
     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

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
^CCaught sigINT!

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

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• Current topic

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