IPV4 UDP server client program with Select system call
=======================================================
.. tab-set::
.. tab-item:: IPv4 AF_INET UDP
* In this program, you are going to learn
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* How to create a Socket ?
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* How to bind a socket ?
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* How to send a data ?
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* How to recv a data ?
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* How to use socket APIs ?
* `socket <https://man7.org/linux/man-pages/man2/socket.2.html>`_
* `bind <https://man7.org/linux/man-pages/man2/bind.2.html>`_
* `sendto <https://man7.org/linux/man-pages/man2/send.2.html>`_
* `recvfrom <https://man7.org/linux/man-pages/man2/recv.2.html>`_
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Topics in this section,
* :ref:`IPV4 AF_INET UDP SOCKET FAQs <select_ipv4_af_inet_udp_socket>`
* :ref:`Step 1: Sequence Diagram for SERVER.c <select_ipv4_af_inet_udp_socket_server_sequence_diagram>`
* :ref:`Step 2: Program for Server.c <select_ipv4_af_inet_udp_socket_server_code>`
* :ref:`Step 3: Compile and Execute Server.c <select_ipv4_af_inet_udp_socket_server_side_compile_and_execute>`
* :ref:`Step 4: Sequence Diagram for CLIENT.c <select_ipv4_af_inet_udp_socket_client_sequence_diagram>`
* :ref:`Step 5: Program for Client.c <select_ipv4_af_inet_udp_socket_client_code>`
* :ref:`Step 6: Compile and Execute Client.c <select_ipv4_af_inet_udp_socket_client_side_compile_and_execute>`
* :ref:`Summary <select_ipv4_af_inet_udp_Summary>`
.. _select_ipv4_af_inet_udp_socket:
.. tab-set::
.. tab-item:: IPV4 AF_INET UDP SOCKET : FAQs
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Let us answer few basic questions in this socket
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What does socket(AF_INET SOCK_DGRAM,IPPROTO_UDP) create?
.. dropdown:: See Answer
A UDP socket
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What does AF_INET signify in socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)?
.. dropdown:: See Answer
IPv4 address family
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Why might you choose SOCK_DGRAM when creating a socket?
.. dropdown:: See Answer
For connectionless communication
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What is the purpose of IPPROTO_UDP in socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)?
.. dropdown:: See Answer
It is optional and can be omitted.
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How does a UDP socket differ from a TCP socket created using socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)?
.. dropdown:: See Answer
UDP is connectionless, while TCP is connection-oriented,
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What is the typical use case for a UDP socket created with socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)?
.. dropdown:: See Answer
DNS resolution
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How would you bind a specific IP address and port to a UDP socket created using socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)?
.. dropdown:: See Answer
Use the bind() function after creating the socket.
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What is the purpose of the select system call in network programming?
.. dropdown:: See Answer
To block and wait for activity on one or more file descriptors.
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How does select help in handling multiple sockets efficiently?
.. dropdown:: See Answer
It provides a way to wait for readiness on multiple sockets without blocking the entire program.
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What types of file descriptors can be monitored using select?
.. dropdown:: See Answer
sockets, files, timerfd, socketpair, message_queue, Namedpipes and shared_memory.
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What is the significance of the timeout parameter in the select function?
.. dropdown:: See Answer
It specifies the maximum duration to wait for any file descriptor to become ready.
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How do you handle errors when using the select system call?
.. dropdown:: See Answer
Check the return value for -1 to detect errors, Use perror to print error messages.
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How does select handle a set of file descriptors with different states (e.g., reading, writing, exception)?
.. dropdown:: 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);``
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How does select Checking Ready File Descriptors?
.. dropdown:: See Answer
After select returns, you can check the sets to determine which file descriptors are ready for the specified conditions.
.. code-block:: c
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
}
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What does it mean if select returns 0?
.. dropdown:: See Answer
No file descriptors are ready within the specified timeout.
.. _select_ipv4_af_inet_udp_socket_server_sequence_diagram:
.. tab-set::
.. tab-item:: Step 1: Sequence Diagram for SERVER.c
.. plantuml::
@startuml
!theme spacelab
start
:socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
:bind(server_socket, (struct sockaddr*)&server_addr, sizeof(server_addr));
while (while(1)) is (yes)
:FD_ZERO(&read_fds);
:FD_SET(server_socket, &read_fds);
:select(server_socket + 1, &read_fds, NULL, NULL, NULL);
if (FD_ISSET(server_socket, &read_fds)) then (yes)
:recvfrom(server_socket, buffer, BUFFER_SIZE, 0, (struct sockaddr*)&client_addr, &client_addr_len);
:sendto(server_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, client_addr_len);
else (no)
endif
endwhile (CTRL+c)
:(void)close(server_socket);
stop
@enduml
.. _select_ipv4_af_inet_udp_socket_server_code:
.. tab-set::
.. tab-item:: 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,
.. code-block:: c
server_socket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
* ``bind()`` is used to associate the socket with a specific address and port. For example,
.. code-block:: c
ret = bind(server_socket, (struct sockaddr*)&server_addr, sizeof(server_addr));
* ``select()`` is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,
.. code-block:: c
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,
.. code-block:: c
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,
.. code-block:: c
ret = sendto(server_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, client_addr_len);
* ``close`` is used to close the socket To free up system resources associated with the socket. For example,
.. code-block:: c
(void)close(server_socket);
* See the full program below,
.. literalinclude:: IPV4_AF_INET_UDP/server/server.c
:language: c
:emphasize-lines: 69, 70, 71, 72, 85, 86, 87, 88, 89, 129, 130, 131, 138, 139, 140, 154, 155, 168
.. _select_ipv4_af_inet_udp_socket_server_side_compile_and_execute:
.. tab-set::
.. tab-item:: Step 3: Compile and Execute server.c
.. code-block:: c
:linenos:
:emphasize-lines: 1, 3
$ gcc -o server server.c
$ sudo ./server 8080
Port: 8080
UDP listining
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
Sent buffer = HELLO
Received: HI!
^CCaught sigINT!
.. _select_ipv4_af_inet_udp_socket_client_sequence_diagram:
.. tab-set::
.. tab-item:: Step 4 : Sequence Diagram for CLIENT.c
.. plantuml::
@startuml
!theme spacelab
start
:socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
while (while(1)) is (yes)
:sendto(client_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&server_addr, sizeof(server_addr));
:FD_ZERO(&read_fds);
:FD_SET(client_socket, &read_fds);
:select(client_socket + 1, &read_fds, NULL, NULL, NULL);
if (FD_ISSET(client_socket, &read_fds)) then (yes)
:recvfrom(client_socket, buffer, BUFFER_SIZE, 0, NULL, NULL);
else (no)
endif
endwhile (CTRL+c)
:(void)close(client_socket);
stop
@enduml
.. _select_ipv4_af_inet_udp_socket_client_code:
.. tab-set::
.. tab-item:: Step 5: program for client.c
* Create Socket
* Select
* Sendto data_packet
* Recvfrom data_packet
* Close socket
* ``socket`` is used to create a new socket. For example,
.. code-block:: c
client_socket = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP);
* ``select`` is used in network programming to monitor multiple file descriptors (usually sockets) for read, write, or error conditions. For example,
.. code-block:: c
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,
.. code-block:: c
len = recvfrom(client_socket, buffer, BUFFER_SIZE, 0, NULL, NULL);
* ``sendto`` is used to send the encoded message to the specified server address and port using a UDP socket. For example,
.. code-block:: c
ret = sendto(client_socket, buffer, strlen(buffer), 0, (struct sockaddr*)&server_addr, sizeof(server_addr));
* ``close`` is used to close the socket To free up system resources associated with the socket. For example,
.. code-block:: c
(void)close(client_socket);
* See the full program below,
.. literalinclude:: IPV4_AF_INET_UDP/client/client.c
:language: c
:emphasize-lines: 76, 77, 78, 131, 132, 133, 141, 142, 143, 144, 156, 157, 170
.. _select_ipv4_af_inet_udp_socket_client_side_compile_and_execute:
.. tab-set::
.. tab-item:: Step 6: Compile and Execute client.c
.. code-block:: c
:linenos:
:emphasize-lines: 1, 3
$ gcc -o client client.c
$ sudo ./client 8080 127.0.0.1
Port: 8080
IP Address: 127.0.0.1
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
Received: HELLO
sendbuffer = HI!
^CCaught sigINT!
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$ sudo ./server 8080
$ 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.
.. tab-set::
.. tab-item:: Enhanced Socket Flexibility with ``AF_INET`` and ``PF_INET`` Domains
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**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_DGRAM, IPPROTO_UDP)``.
**Working Scenario:**
Despite the change in domain to ``PF_INET``, the socket continues to operate the same way, handling general network data.
.. _select_ipv4_af_inet_udp_Summary:
.. tab-set::
.. tab-item:: 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.
============== ==================================================================================================================
.. card:: See Also
* Previous topic
* :doc:`../../../sockets/ipv4_af_inet_udp/Basic_example/Basic_example`
* Current topic
* :doc:`../../../sockets/ipv4_af_inet_udp/select/select`
* Next topic
* :doc:`../../../sockets/ipv4_af_inet_udp/poll/poll`
* Other sockets
* :doc:`../../../sockets/af_unix_tcp`
* :doc:`../../../sockets/af_unix_udp`
* :doc:`../../../sockets/ipv4_raw_af_inet_icmp`
* :doc:`../../../sockets/ipv4_raw_af_inet_raw`
* :doc:`../../../sockets/ipv4_raw_af_inet_tcp`
* :doc:`../../../sockets/ipv4_raw_af_inet_udp`
* :doc:`../../../sockets/ipv6_af_inet6_tcp`
* :doc:`../../../sockets/ipv6_af_inet6_udp`
* :doc:`../../../sockets/ipv6_raw_af_inet6_icmp`
* :doc:`../../../sockets/ipv6_raw_af_inet6_tcp`
* :doc:`../../../sockets/ipv6_raw_af_inet6_udp`
* :doc:`../../../sockets/ipv6_raw_af_inet6_raw`
* :doc:`../../../sockets/raw_af_packet_raw_htons_ETH_P_ALL`
* :doc:`../../../sockets/raw_af_packet_tcp_htons_ETH_P_ALL`
* :doc:`../../../sockets/raw_af_packet_udp_htons_ETH_P_ALL`
* Other IPCs
* :doc:`../../../Message_queues/Message_queues`
* :doc:`../../../NamedPipes/NamedPipes`
* :doc:`../../../Netlink/Netlink`
* :doc:`../../../Shared_Memory/Shared_Memory`
* :doc:`../../../Shared_Memory_2_FDS/Shared_Memory_2_FDS`
* :doc:`../../../SocketPair/SocketPair`
* :doc:`../../../Timerfd/Timerfd`