bindresvport(3)

✨ bindresvport ✨

okay bestie, we're looking at bindresvport(3) from the Library Functions Manual and honestly? it's giving "tiny function, big privileges" 💅

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🎀 NAME 🎀

bindresvport — bind a socket to a privileged IP port 👑

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📚 LIBRARY 📚

Standard C library (libc, -lc) 🫶

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🦋 SYNOPSIS 🦋

#include <sys/types.h>
#include <netinet/in.h>

int bindresvport(int sockfd, struct sockaddr_in *sin);

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💖 DESCRIPTION 💖

so bindresvport() is here to bind the socket that's referenced by the file descriptor sockfd to a privileged anonymous IP port — and by that we mean a port number that's just... arbitrarily selected from the range 512 to 1023. she picks for you, you don't pick for her, periodt ✨

if the bind(2) that bindresvport() performs actually goes through, and sin is not NULL, then sin->sin_port hands you back the port number that was actually allocated. slay 🎀

now, sin can be NULL, in which case sin->sin_family is implicitly taken to be AF_INET. but lowkey there's a catch: in that case, bindresvport() has literally no way to return the port number actually allocated 😭. (you can still get that info later using getsockname(2), so it's not the end of the world bestie)

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🌸 RETURN VALUE 🌸

• she returns 0 on success 💖
• otherwise -1 is returned, and errno is set to indicate the error 🙈

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💀 ERRORS 💀

bindresvport() can fail for any of the same reasons as bind(2). on top of that, these errors can also happen:

• EACCES — the calling process was not privileged. (on Linux specifically: the calling process did not have the CAP_NET_BIND_SERVICE capability in the user namespace governing its network namespace.) not me getting denied at the door 💅
• EADDRINUSE — all privileged ports are in use. every single one. she's booked, fully, no vacancy 😭
• EAFNOSUPPORT (EPFNOSUPPORT in glibc 2.7 and earlier) — sin is not NULL and sin->sin_family is not AF_INET. wrong family, we don't serve that here 🚫

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🌈 ATTRIBUTES 🌈

for an explanation of the terms used in this section, see attributes(7). here's the vibe check:

• Interface: bindresvport()
• Attribute: Thread safety
• Value: glibc >= 2.17: MT-Safe; glibc < 2.17: MT-Unsafe

and the lore behind that: the bindresvport() function uses a static variable that was not protected by a lock before glibc 2.17, which made the function MT-Unsafe. she was messy before the glow-up, ngl 💀

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⭐ VERSIONS ⭐

present on the BSDs, Solaris, and many other systems. she gets around, iconic behavior 👑

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🧚 NOTES 🧚

unlike some bindresvport() implementations, the glibc implementation ignores any value that the caller supplies in sin->sin_port. like... she didn't ask, she doesn't care, she chooses 😌

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💜 STANDARDS 💜

BSD.

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🔥 SEE ALSO 🔥

bind(2), getsockname(2)

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fr fr, that's the whole page. go bind something privileged, but stay safe out there 💅✨

signing off with the receipts: this is Linux man-pages 6.20, dated 2026-02-08, and you were reading bindresvport(3) 🎀🦋🌸