IPV4 TCP server client program with Select 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

  • 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 select() 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 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

  • Listen Socket

  • Accept Socket

  • Select

  • 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, 5);

• 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);

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

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

• 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>

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 fdmax = 0; 
  int flag = 0;
  int ret;
  struct sockaddr_in 
  tcp_addr;
  fd_set read_fds;
  char buffer[1024];
  socklen_t tcp_addr_len = sizeof(
  tcp_addr);

  register_signal_handler(SIGINT, 
  sigint_handler);

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

  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 -1;
  }

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

  ret = listen(tcp_server_fd, 5);
  
  if (ret < 0)
  {
    perror("listen");
    (void)close(tcp_server_fd);
    return -3;
  }

  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 -4;
  }

  (void)printf("Server lsiten...\n");

  fdmax = tcp_client_fd;
  
  while (1) {
    FD_ZERO(&read_fds);
    FD_SET(tcp_client_fd, &read_fds);

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

    if (FD_ISSET(tcp_client_fd,
        &read_fds)) {
        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 lsiten...
connection = 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
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

  • Select

  • Recv data_packet

  • Send 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);

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

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

• 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>

int client_fd = -1;

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 cli_connect; 
  int ret;
  struct sockaddr_in 
  tcp_addr;
  fd_set read_fds;
  char buffer[1024];

  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");
  }

  while (1) {	
   send_data(buffer);
   
   FD_ZERO(&read_fds);
   FD_SET(client_fd, &read_fds);

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

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

   if (FD_ISSET(client_fd, 
      &read_fds)) {
      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
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.
select	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

• Previous topic

  • Basic example tcp server and client

• Current topic

  • IPV4 TCP server client program with Select system call

• Next topic

  • IPV4 TCP server client program with Poll system call

• Other sockets

  • ipv4_af_inet_udp

  • 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