af unix tcp server and client program with poll 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

  • 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 a common practice for handling timeouts in socket programming?

See Answer

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

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 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

  • Listen Socket

  • Accept Socket

  • Poll

  • Recv data_packet

  • Send data_packet

  • Close socket

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

tcp_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);

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

ret = poll(fds, MAX_CLIENTS + 1, -1);

• 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);

• 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 <poll.h>
#include <netinet/in.h> //protocol "IPPROTO_TCP"

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

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

static void sigint_handler(int signo)
{
  unlink(SOCKET_PATH);
  (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 len, ret;
  struct sockaddr_un 
  server_addr,
  client_addr;
  char buffer[1024];
  struct pollfd 
  fds[MAX_CLIENTS + 1];

  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_CLIENTS);
  
  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 == -1) {
   perror("accept");
   (void)close(server_fd);
   return -4;
 }
 
 printf("connection accepted = %d\n", 
 client_fd);

 memset(fds, 0, 
 sizeof(fds));

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

  while (1) {

    ret = poll(fds, MAX_CLIENTS + 1, -1);

    if (ret == -1) {
      perror("poll");
      break;
    }

    if (fds[0].revents & POLLIN) {  
        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 = 4
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

  • Poll

  • Recv data_packet

  • Send 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));

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

ret = poll(fds, 2, -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,

Step 6: Compile and Execute Client.c

$ gcc -o client client.c

$ sudo ./client

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.
poll	Monitor multiple file descriptors (usually sockets) for read, write, or error conditions.
recv	Receive data from a connected socket.
send	Send data over a connected socket.

See Also

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  • //TODO ipv6_raw_af_inet6_icmp

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