getpeername(2)

💖 getpeername(2) — bestie's guide to figuring out who you're talking to 💖

okay so real talk, this is getpeername(2) — a Linux system call manual page, and it's genuinely one of the most "who even is that" functions in the whole socket family. if you've ever been on a call and thought "...and you are??" — that's this. it's the caller ID of the socket world. no cap. 🫶

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✨ NAME ✨

getpeername — get the name of the connected peer socket 🎀

📚 LIBRARY 📚

Standard C library (libc, -lc) 💅

🦋 SYNOPSIS 🦋

#include <sys/socket.h>

int getpeername(int sockfd, struct sockaddr *restrict addr,
                socklen_t *restrict addrlen);

🌸 DESCRIPTION 🌸

so here's the vibe. getpeername() hands you back the address of the peer that's connected to the socket sockfd, and it drops that info into the buffer pointed to by addr. ✨

the addrlen argument is a little diva with two jobs, bestie. you gotta initialize it to say how much space addr has. on return, it holds the actual size of the name that came back — in bytes. and if your buffer was too smol? the name gets truncated. simple as that. 💀

and yeah, they said it again, so here it is again: the returned address is truncated if the buffer you provided is too small — and in that case, addrlen will return a value greater than the value you supplied to the call. so if addrlen comes back bigger than you sent it in? that's not a glitch, that's the function telling you to get a bigger buffer. 👑

🔥 RETURN VALUE 🔥

• on success: 0 (iconic, flawless, no notes) 💖
• on error: -1, and errno gets set to tell you what went wrong 😭

💅 ERRORS 💅

okay so when it's not slaying, here's what errno might be telling you:

• EBADF — the argument sockfd is not a valid file descriptor. miss girl, that's not even a real socket, pls 🚫
• EFAULT — the addr argument points to memory that's not in a valid part of the process address space. that pointer is lost, bestie 🫥
• EINVAL — addrlen is invalid (like, it's negative). negative?? a length?? ma'am 🥴
• ENOBUFS — there weren't enough resources available in the system to perform the operation. the system is booked and busy, try again later 😔
• ENOTCONN — the socket is not connected. you can't get the name of a peer that doesn't exist, babes 💔
• ENOTSOCK — the file descriptor sockfd does not refer to a socket. close but so far off 💀

⭐ STANDARDS ⭐

POSIX.1-2024 — certified, official, standards-approved royalty 👑

🌈 HISTORY 🌈

POSIX.1-2001, SVr4, 4.4BSD — and it first appeared in 4.2BSD. so yeah, this girl's been around since the 80s and she's still relevant. main character energy fr fr 🌟

🧚 NOTES 🧚

this is the part where things get kind of nuanced, so listen up:

• for stream sockets, once a connect(2) has been performed, either socket can call getpeername() to get the address of the peer socket. mutual info, mutual access, we love reciprocity 💞
• datagram sockets, though? they're connectionless. periodt. calling connect(2) on a datagram socket merely sets the peer address for outgoing datagrams sent with write(2) or recv(2). it's like saving a contact, not making a call 📱
• the caller of connect(2) can use getpeername() to grab the peer address it earlier set for the socket. cool, except…
• the peer socket is unaware of any of this. so if you call getpeername() on the peer socket, you'll get no useful information — unless a connect(2) call was also executed on that peer. the info doesn't teleport, bestie 🫠
• also note: the receiver of a datagram can get the address of the sender by using recvfrom(2). that's the move if you need it 🙈

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👑 SEE ALSO 👑

accept(2), bind(2), getsockname(2), ip(7), socket(7), unix(7) 🍓

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okay that's a wrap!! this page is from Linux man-pages 6.20, dated 2025-10-29 — so you know it's fresh, you know it's accurate, you know it ate. now go call getpeername() on your sockets and find out who's on the other end 💅✨🦋