af unix udp server client program with Poll system call

AF_UNIX 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,

• AF_UNIX 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

AF_UNIX UDP SOCKET : FAQs

Let us answer few basic questions in this socket

What does socket(AF_UNIX, SOCK_DGRAM, 0) do?

See Answer

Creates a Unix domain socket for datagram communication.

How does AF_UNIX differ from AF_INET in sockets?

See Answer

AF_UNIX is for local communication using file system paths, while AF_INET is for network communication using IP addresses.

Why choose SOCK_DGRAM as the socket type?

See Answer

It enables connectionless, datagram-oriented communication.

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

See Answer

The system chooses the default protocol for the specified domain and type.

What role does the file system path play in AF_UNIX datagram sockets?

See Answer

Serves as the address for Unix domain datagram sockets.

How does error handling work with the socket call?

See Answer

Check the return value; if it’s -1, an error occurred. Use appropriate error-handling mechanisms.

How is data flow managed in AF_UNIX datagram sockets?

See Answer

Data is sent and received in discrete, independent units known as datagrams.

Can a Unix domain datagram socket connect to multiple servers?

See Answer

Yes, each datagram is independent, allowing communication with multiple servers.

What happens if the specified file system path in AF_UNIX datagram sockets doesn’t exist?

See Answer

Typically results in an error; the path should exist or be creatable.

Are AF_UNIX datagram sockets suitable for bi-directional communication?

See Answer

Yes, they can handle both sending and receiving data independently.

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_UNIX, SOCK_DGRAM, 0);

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

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

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

ret = poll(fds, 1, -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_address, sizeof(client_address));

• 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 <sys/socket.h>
#include <sys/un.h>
#include <signal.h>
#include <poll.h>

#define SOCKET_PATH "/tmp/udp_server_socket"
#define SOCKET_CLIENT "/tmp/udp_client_socket"
#define BUFFER_SIZE 1024

int server_socket;

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

void recv_data(
char *buffer,
struct sockaddr_un *client_addr)
{
  int len;

  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);
  } else if (len < 0) {
        perror("recvfrom");
        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(void) 
{
  struct sockaddr_un 
  unix_address, 
  client_address;
  socklen_t addr_len = sizeof(
  unix_address);
  int ret;
  char buffer[BUFFER_SIZE];
  struct pollfd fds[1];

  register_signal_handler(SIGINT,
  sigint_handler);

  server_socket = socket(AF_UNIX, 
                  SOCK_DGRAM, 0);
 
  if (server_socket < 0) {
    perror("socket");
    return -1;
  }

  memset(&unix_address, 0, 
  sizeof(unix_address));
  unix_address.sun_family = AF_UNIX;
  strncpy(unix_address.sun_path, 
  SOCKET_PATH, 
  sizeof(unix_address.sun_path) - 1);
  unlink(SOCKET_PATH);

  memset(&client_address, 0, 
  sizeof(client_address));
  client_address.sun_family = AF_UNIX;
  strncpy(client_address.sun_path, 
  SOCKET_CLIENT, 
  sizeof(client_address.sun_path) - 1);

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

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

  while (1) {
    
   ret = poll(fds, 1, -1);

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

    if (fds[0].revents & POLLIN) {  
      recv_data(buffer, &unix_address);
      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_address, 
      sizeof(client_address));
      
      if (ret < 0) {
        perror("sendto error");
        break;
      } else {
        printf("sentbufffer = %s\n",
        buffer);
      }
    }
  }

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

Step 3: Compile and Execute Server.c

$ gcc -o server server.c

$ sudo ./server

Received: Hello, Server!
sentbufffer = HELLO
Received: Hello, Server!
sentbufffer = HELLO
Received: Hello, Server!
sentbufffer = HELLO
Received: Hello, Server!
sentbufffer = HELLO
Received: Hello, Server!
sentbufffer = HELLO
Received: Hello, Server!
sentbufffer = HELLO
Received: Hello, Server!
sentbufffer = HELLO
Received: Hello, Server!
sentbufffer = HELLO
Received: Hello, Server!
sentbufffer = HELLO
^Caught 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

  • Bind

  • Select

  • Sendto data_packet

  • Recvfrom data_packet

  • Close socket

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

client_socket = socket(AF_UNIX, SOCK_DGRAM, 0);

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

ret = bind(client_socket, (struct sockaddr*)&client_address, sizeof(client_address));

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

ret = poll(fds, 1, -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_address, sizeof(server_address));

• 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, sizeof(buffer), 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 <sys/socket.h>
#include <sys/un.h>
#include <signal.h>
#include <poll.h>
#include <netinet/in.h> //protocol "IPPROTO_UDP"

#define SOCKET_PATH "/tmp/udp_server_socket"
#define SOCKET_CLIENT "/tmp/udp_client_socket"

int client_socket = -1;

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

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

  len = recvfrom(client_socket,
  buffer,
  sizeof(buffer), 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(void)
{
  struct sockaddr_un 
  server_address, 
  client_address;
  char buffer[1024];
  const char *message 
  = "Hello, Server!";
  int ret;
  struct pollfd fds[1];

  register_signal_handler(SIGINT,
  sigint_handler);

  client_socket = socket(AF_UNIX, 
                  SOCK_DGRAM, 0);

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

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

  memset(&client_address, 0, 
  sizeof(client_address));
  client_address.sun_family = AF_UNIX;
  strncpy(client_address.sun_path, 
  SOCKET_CLIENT, sizeof(client_address.sun_path) - 1);
  unlink(SOCKET_CLIENT);

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

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

  while(1) {
    ret = sendto(client_socket, 
    message, strlen(message), 0, 
    (struct sockaddr *)&server_address, 
    sizeof(server_address));

    if (ret == -1) {
      perror("sendto error");
      break;
    } else {
      printf(" sentbuffer = %s\n", 
      message);
    }
    
    ret = poll(fds, 1, -1);
   
    if (ret < 0) {
      perror("poll");
      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

sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
sentbuffer = Hello, Server!
Received: HELLO
^CCaught sigINT!

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

  • af unix udp server client program with Select system call

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  • af unix udp server client program with Poll system call

• Next topic

  • af unix udp server client program with Epoll system call

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