if_nametoindex(3)

💖 if_nametoindex(3) — the bestie guide to name ↔ index glow-ups 💖

okay so this is if_nametoindex(3), a Library Functions Manual page from the if_nametoindex girlies, and we're about to learn how network interface names and interface indexes find each other. it's giving matchmaker. it's giving translator. it's giving MT-Safe and unbothered. 🦋

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✨ NAME ✨

if_nametoindex, if_indextoname — mappings between network interface names and indexes

like. the whole point. name goes in, index comes out. index goes in, name comes out. it's a two-way situationship and both directions ate. 💅

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📚 LIBRARY 📚

Standard C library (libc, -lc)

yes bestie, just the standard C library, nothing exotic, no extra credit needed. 🎀

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🫶 SYNOPSIS 🫶

#include <net/if.h>

unsigned int if_nametoindex(const char *ifname);
char *if_indextoname(unsigned int ifindex, char *ifname);

one include, two functions, zero drama. we love a low-maintenance API. ✨

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🌸 DESCRIPTION 🌸

so. if_nametoindex() returns the index of the network interface corresponding to the name ifname. you hand it a name, it hands you back a number. iconic behavior. no cap.

and if_indextoname() does the exact reverse glow-up: it returns the name of the network interface corresponding to the interface index ifindex. the name gets placed in the buffer pointed to by ifname. 🪄

⚠️ okay listen, this part is important: that buffer must allow for the storage of at least IF_NAMESIZE bytes. bestie. do not skimp on the buffer. do not give it a tiny lil buffer and hope for the best. at least IF_NAMESIZE bytes. that's the rule. 🙈

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👑 RETURN VALUE 👑

okay so results, we love results:

• on success, if_nametoindex() returns the index number of the network interface. on error, 0 is returned and errno is set to indicate the error. 🫶
• on success, if_indextoname() returns ifname. on error, NULL is returned and errno is set to indicate the error. 😭

so like, 0 and NULL are the "nope" signals. check them. please. for me. 💜

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💀 ERRORS 💀

if_nametoindex() may fail and set errno if:

• ENODEV — No interface found with given name.

if_indextoname() may fail and set errno if:

• ENXIO — No interface found for the index.

and also, plot twist: if_nametoindex() and if_indextoname() may also fail for any of the errors specified for socket(2) or ioctl(2). so if you're getting a weird errno and you're like "that's not in the list??" — yeah bestie, go check socket(2) and ioctl(2), the receipts are over there. 🔥

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🌈 ATTRIBUTES 🌈

for an explanation of the terms used in this section, see attributes(7). 💖

• Interface: if_nametoindex(), if_indextoname() — Attribute: Thread safety — Value: MT-Safe

translated from table-ese: both of these are MT-Safe, which is the good one. thread-safe. run them from all your threads, they will not fight. periodt. ✨

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⭐ STANDARDS ⭐

POSIX.1-2008, RFC 3493.

certified, standardised, officially that girl. 👑

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🕰️ HISTORY 🕰️

POSIX.1-2001. BSDi.

so it's been around since POSIX.1-2001 and comes from BSDi — an elder icon, honestly, and it still serves. 🌸

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🦋 SEE ALSO 🦋

• getifaddrs(3)
• if_nameindex(3)
• ifconfig(8)

go read those next bestie, they're in the same friend group. 💕

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and that's the whole page!! mwah!! 😘💅

this little masterpiece came to you courtesy of Linux man-pages 6.20, dated 2026-02-08, and it's the if_nametoindex(3) page — go map some interfaces, hydrate, and check your errno. 🎀✨🫶