af unix udp server client program with Select 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 select system call in network programming?

See Answer

To block and wait for activity on one or more file descriptors.

How does select help in handling multiple sockets efficiently?

See Answer

It provides a way to wait for readiness on multiple sockets without blocking the entire program.

What types of file descriptors can be monitored using select?

See Answer

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

What is the significance of the timeout parameter in the select function?

See Answer

It specifies the maximum duration to wait for any file descriptor to become ready.

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

See Answer

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

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

See Answer

Preparing File Descriptor Sets:

select(readfds, writefds, exceptfds);

Setting Up Readiness Conditions:

If you are interested in monitoring file descriptors for readability, you add them to the readfds set.

FD_ZERO(&readfds);

FD_SET(fd1, &readfds);

Setting Up Writability Conditions:

If you are interested in monitoring file descriptors for writability, you add them to the writefds set.

FD_ZERO(&writefds);

FD_SET(fd2, &writefds);

Setting Up Exceptional Conditions:

If you are interested in monitoring file descriptors for exceptional conditions, you add them to the exceptfds set.

FD_ZERO(&exceptfds);

FD_SET(fd3, &exceptfds);

How does select Checking Ready File Descriptors?

See Answer

After select returns, you can check the sets to determine which file descriptors are ready for the specified conditions.

if (FD_ISSET(fd1, &readfds)) {
        // fd1 is ready for reading
}
if (FD_ISSET(fd3, &writefds)) {
        // fd2 is ready for writing
}
if (FD_ISSET(fd4, &exceptfds)) {
        // fd3 has an exceptional condition
}

What does it mean if select 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

  • Select

  • 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));

• select() is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,

ret = select(server_socket + 1, &read_fds, NULL, NULL, NULL);

• 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>

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

int server_socket = -1;

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 register_signal_handler(
int signum,
void (*handler)(int))
{
  if (signal(signum, handler) ==
  SIG_ERR) {
     printf("Cannot handle signal\n");
     exit(EXIT_FAILURE);
  }
}

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);
    }
}

int main(void) 
{
  struct sockaddr_un 
  unix_address,
  client_address;
  fd_set read_fds;
  int ret;
  char buffer[BUFFER_SIZE];

  register_signal_handler(SIGINT,
  sigint_handler);

  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);

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

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

  while (1) {
    FD_ZERO(&read_fds);
    FD_SET(server_socket, &read_fds);

    ret = select(server_socket + 1, 
    &read_fds, NULL, NULL, NULL);
    
    if (ret < 0) {
      perror("select");
      break;
    }

    if (FD_ISSET(server_socket, 
        &read_fds)) {
        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 == -1) {
           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
Received: Hello, Server!
sentbufffer = HELLO
^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

  • 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));

• select is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,

ret = select(client_socket + 1, &read_fds, NULL, NULL, NULL);

• 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);

• 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));

• 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>

#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;
  fd_set read_fds;

  register_signal_handler(SIGINT,
  sigint_handler);

  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);

  client_socket = socket(AF_UNIX, 
                  SOCK_DGRAM, 0);

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

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

  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);
    }

    FD_ZERO(&read_fds);
    FD_SET(client_socket, &read_fds);

    ret = select(client_socket + 1, 
    &read_fds, NULL, NULL, NULL);

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

    if (FD_ISSET(client_socket, 
       &read_fds)) {
       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
^CCaught sigINT!

Summary

Socket API	Learning
socket	Create a new socket
bind	Associate the socket with a specific address and port
select	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

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  • //TODO ipv6_raw_af_inet6_icmp

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