IP_MULTICAST_IF(2const)

💖 IP_MULTICAST_IF — bestie's guide to picking the local device for a multicast socket 💖

okay so real talk, this one's a constant page — like, it's IP_MULTICAST_IF from the <netinet/in.h> squad, aka a 2const page, not a chunky full syscall page. It lives in the Standard C library (libc, link it with -lc). Let's get into it 🦋

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✨ SYNOPSIS ✨

#include <netinet/in.h>  /* Definition of IP* constants */
#include <sys/socket.h>

int setsockopt(int sockfd, IPPROTO_IP, IP_MULTICAST_IF,
               const void val[size], size_t size);
int getsockopt(int sockfd, IPPROTO_IP, IP_MULTICAST_IF,
               struct in_addr *val, sizeof(struct in_addr));

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🎀 DESCRIPTION 🎀

So like, what does it even do? It sets the local device for a multicast socket. That's the whole vibe 💅

When you're calling setsockopt(2), the argument you pass is one of these three cuties:

• an ip_mreqn(2type) structure,
• an ip_mreq(2type) structure — but only since Linux 3.5, slay for that version bump ✨
• or an in_addr(2type) structure.

And here's the iconic part: the kernel figures out which structure you're handing it based on the size you pass in size. It's giving mind-reader energy but make it syscall. 🔮

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💀 ERRORS 💀

ngl there's no giant error list here bestie, it just points you at the real ones:

• See IPPROTO_IP(2const).
• See setsockopt(2).
• See ip(7).

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🌈 HISTORY 🌈

• Linux 1.2. yes she's an OG, been here since the dawn of time, main character energy fr fr 👑

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👑 SEE ALSO 👑

• IPPROTO_IP(2const)
• in_addr(2type)
• ip_mreqn(2type)
• setsockopt(2)
• ip(7)

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lowkey this page is short but it ate, no cap. Now go set that multicast device bestie 🫶

That's the whole thing!! This was pulled straight from Linux man-pages 6.20, dated 2025-11-25 — so if the vibes ever change upstream, check the newer version, mwah 💋💖