listen(2)

✨ listen(2) — the socket glow-up era ✨

okay bestie so this is a page from the System Calls Manual, it's called listen and it lives in section 2, and honestly? it's giving main character energy for every server you've ever run. 💅

🌸 LIBRARY 🌸

Standard C library (libc, -lc). yes queen, it's just the plain libc, no extra drama.

🦋 SYNOPSIS 🦋

#include <sys/socket.h>

int listen(int sockfd, int backlog);

💖 DESCRIPTION 💖

So listen() takes the socket that sockfd points at and marks it as a passive socket, meaning it's now a socket that's gonna sit there and accept incoming connection requests using accept(2). it's literally the "i'm open for business" toggle. ✨

The sockfd argument is a file descriptor that refers to a socket of type SOCK_STREAM or SOCK_SEQPACKET. no other types, those two only, slay.

The backlog argument defines the maximum length to which the queue of pending connections for sockfd may grow. like, how long can the line outside the club get before the bouncer is like "sorry babe, capacity." If a connection request arrives when the queue is full, the client may receive an error with an indication of ECONNREFUSED or, if the underlying protocol supports retransmission, the request may be ignored so that a later reattempt at connection succeeds. so it's either a hard no or a "try again later bestie", depending on the protocol. 🙈

🎀 RETURN VALUE 🎀

On success, zero is returned. On error, -1 is returned, and errno is set to indicate the error. that's it, that's the whole vibe check.

🌈 ERRORS 🌈

• EADDRINUSE — Another socket is already listening on the same port. she's taken, sorry.
• EADDRINUSE — (Internet domain sockets) The socket referred to by sockfd had not previously been bound to an address and, upon attempting to bind it to an ephemeral port, it was determined that all port numbers in the ephemeral port range are currently in use. See the discussion of /proc/sys/net/ipv4/ip_local_port_range in ip(7). yeah this one hits twice, same name, different lore — that's on purpose, don't come for me.
• EBADF — The argument sockfd is not a valid file descriptor. not a real fd, blocked. 💀
• ENOTSOCK — The file descriptor sockfd does not refer to a socket. it's an fd but it's not that kind of fd. imposter behavior.
• EOPNOTSUPP — The socket is not of a type that supports the listen() operation. wrong socket type, no listen for you.

👑 STANDARDS 👑

POSIX.1-2024. periodt.

🧚 HISTORY 🧚

4.2BSD, POSIX.1-2001. been iconic for literal decades.

🔥 NOTES 🔥

To accept connections, the following steps are performed — this is the sacred sequence, memorize it:

1. A socket is created with socket(2).
2. The socket is bound to a local address using bind(2), so that other sockets may be connect(2)ed to it.
3. A willingness to accept incoming connections and a queue limit for incoming connections are specified with listen().
4. Connections are accepted with accept(2).

---

The behavior of the backlog argument on TCP sockets changed with Linux 2.2. Now it specifies the queue length for completely established sockets waiting to be accepted, instead of the number of incomplete connection requests. The maximum length of the queue for incomplete sockets can be set using /proc/sys/net/ipv4/tcp_max_syn_backlog. When syncookies are enabled there is no logical maximum length and this setting is ignored. See tcp(7) for more information. so if you're out here trusting your backlog number with syncookies on… it's giving ignored, bestie. 😭

If the backlog argument is greater than the value in /proc/sys/net/core/somaxconn, then it is silently capped to that value. silently. no warning, no note, just vibes. Since Linux 5.4, the default in this file is 4096; in earlier kernels, the default value is 128. Before Linux 2.4.25, this limit was a hard coded value, SOMAXCONN, with the value 128.

⚠️ no cap but also literally cap — if you pass a huge backlog, the kernel just quietly clamps it and you'll never know. check your somaxconn, iykyk.

🍓 EXAMPLES 🍓

See bind(2). yeah the example lives over there, go say hi. 🫶

⭐ SEE ALSO ⭐

accept(2), bind(2), connect(2), socket(2), socket(7)

---

okay that's the whole page, hope this was helpful and not confusing, mwah 💜

and for the credits moment: this was the Linux man-pages 6.20 era, dated 2026-02-11, entry listen(2). obsessed with her honestly, she ate. ✨🎀🦋