What syscalls does a UDP server need?

I previously described the syscalls needed for a multi-client TCP server. There are at least eight: socket, bind, listen, select, accept, recv, send, and close. This could be used to make an echo server: clients can open a TCP connection, and whatever bytes they send get echoed back.

Now let’s look at the simplest UDP server, and again make an echo server: clients can send a UDP datagram, and whatever they send gets echoed back.

udp_fd = socket(UDP): “OS, please create a UDP socket, and give me a file descriptor referencing it”.
bind(udp_fd, 9999): “OS, please point UDP port 9999 to the new UDP socket”.
msg, from = recvfrom(udp_fd): “OS, please put me to sleep until a UDP datagram is received, then give it to me”.
sendto(udp_fd, msg, from): “OS, please send this UDP datagram from my port to this remote address and port”.
close(udp_fd): “OS, I’m done listening for new datagrams. Stop listening on that port, and remove this file descriptor”.

Compared to the TCP server, we of course don’t get the features of TCP: messages are unreliable, unordered, and nothing monitors network congestion.

But a benefit is simplicity: we have no listen, select, or accept. We do not have to manage multiple connections, deal with splitting byte streams into individual messages, or work with select/kqueue/epoll.

The other benefit is flexibility: our communication may not fit into the TCP model very well.

Tagged #udp, #system-calls, #c, #programming, #networking, #posix.

👋 I'm Jim, a full-stack product engineer. Want to build an amazing product and a profitable business? Read more about me or Get in touch!

Similar posts

What syscalls does a TCP server need?

A minimal TCP server in C uses the socket, bind, listen, accept, recv, send, and close syscalls to manage connections. 2016-12-14

How do I call a program from C?

To call a program from C, use `fork` then `execve`. There is no more direct way! 2017-02-07

How do I use fork in C?

fork duplicates the current process. It returns 0 in the child process. In the parent process, it returns the child’s new process id. 2017-02-06

How do I use execve in C?

execve replaces the current process with a new one. It takes a path, an argument array, and an environment array. The process never returns unless execve fails. 2017-02-05

What is lsof?

lsof lists open system resources, including pipes, sockets, and yes, files. It shows their type, owner, and location. 2017-02-20

How to write a TCP server with the kqueue API

Kqueue is a more efficient alternative to select for managing multiple TCP connections, providing a publish-subscribe model for tracking events in the kernel. 2016-12-18

Similar posts

What syscalls does a TCP server need?

A minimal TCP server in C uses the socket, bind, listen, accept, recv, send, and close syscalls to manage connections. 2016-12-14

How do I call a program from C?

To call a program from C, use `fork` then `execve`. There is no more direct way! 2017-02-07

How do I use fork in C?

fork duplicates the current process. It returns 0 in the child process. In the parent process, it returns the child’s new process id. 2017-02-06

How do I use execve in C?

execve replaces the current process with a new one. It takes a path, an argument array, and an environment array. The process never returns unless execve fails. 2017-02-05

What are the domain and type arguments to the socket system call?

The domain and type arguments to socket() describe the protocol family and socket type, respectively. The protocol argument specifies the actual protocol to use, which may be inferred from the domain and type. 2017-02-27

How to write a TCP server with the kqueue API

Kqueue is a more efficient alternative to select for managing multiple TCP connections, providing a publish-subscribe model for tracking events in the kernel. 2016-12-18

More by Jim

What does the dot do in JavaScript?

foo.bar, foo.bar(), or foo.bar = baz - what do they mean? A deep dive into prototypical inheritance and getters/setters. 2020-11-01

Smear phishing: a new Android vulnerability

Trick Android to display an SMS as coming from any contact. Convincing phishing vuln, but still unpatched. 2020-08-06

A probabilistic pub quiz for nerds

A “true or false” quiz where you respond with your confidence level, and the optimal strategy is to report your true belief. 2020-04-26

Time is running out to catch COVID-19

Simulation shows it’s rational to deliberately infect yourself with COVID-19 early on to get treatment, but after healthcare capacity is exceeded, it’s better to avoid infection. Includes interactive parameters and visualizations. 2020-03-14

The inception bar: a new phishing method

A new phishing technique that displays a fake URL bar in Chrome for mobile. A key innovation is the “scroll jail” that traps the user in a fake browser. 2019-04-27

The hacker hype cycle

I got started with simple web development, but because enamored with increasingly esoteric programming concepts, leading to a “trough of hipster technologies” before returning to more productive work. 2019-03-23

Project C-43: the lost origins of asymmetric crypto

Bob invents asymmetric cryptography by playing loud white noise to obscure Alice’s message, which he can cancel out but an eavesdropper cannot. This idea, published in 1944 by Walter Koenig Jr., is the forgotten origin of asymmetric crypto. 2019-02-16

How Hacker News stays interesting

Hacker News buried my post on conspiracy theories in my family due to overheated discussion, not censorship. Moderation keeps the site focused on interesting technical content. 2019-01-26

My parents are Flat-Earthers

For decades, my parents have been working up to Flat-Earther beliefs. From Egyptology to Jehovah’s Witnesses to theories that human built the Moon billions of years in the future. Surprisingly, it doesn’t affect their successful lives very much. For me, it’s a fun family pastime. 2019-01-20

The dots do matter: how to scam a Gmail user

Gmail’s “dots don’t matter” feature lets scammers create an account on, say, Netflix, with your email address but different dots. Results in convincing phishing emails. 2018-04-07

The sorry state of OpenSSL usability

OpenSSL’s inadequate documentation, confusing key formats, and deprecated interfaces make it difficult to use, despite its importance. 2017-12-02

I hate telephones

I hate telephones. Some rational reasons: lack of authentication, no spam filtering, forced synchronous communication. But also just a visceral fear. 2017-11-08

The Three Ts of Time, Thought and Typing: measuring cost on the web

Businesses often tout “free” services, but the real costs come in terms of time, thought, and typing required from users. Reducing these “Three Ts” is key to improving sign-up flows and increasing conversions. 2017-10-26

Granddad died today

Granddad died. The unspoken practice of death-by-dehydration in the NHS. The Liverpool Care Pathway. Assisted dying in the UK. The importance of planning in end-of-life care. 2017-05-19

How do I call a program in C, setting up standard pipes?

A C function to create a new process, set up its standard input/output/error pipes, and return a struct containing the process ID and pipe file descriptors. 2017-02-17

Your syntax highlighter is wrong

Syntax highlighters make value judgments about code. Most highlighters judge that comments are cruft, and try to hide them. Most diff viewers judge that code deletions are bad. 2014-05-11

This page copyright James Fisher 2016. Content is not associated with my employer. Found an error? Edit this page.