ACCF_DATA(9)

✨ ACCF_DATA ✨

okay bestie, gather round, because today we're talking about accf_data — the accept filter that literally just makes your server wait for the vibe to arrive before it lets anybody in. 💅

she lives in section 9 of the kernel developer's manual, and honestly? she's giving patient queen. 👑

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🌸 NAME 🌸

accf_data — buffer incoming connections until data arrives 💖

like... that's it. that's the whole job. she holds the door but doesn't open it until someone actually shows up with something to say.

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🎀 SYNOPSIS 🎀

options INET
options ACCEPT_FILTER_DATA
kldload accf_data

three little lines, three ways to get her into your life. no cap. ✨

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🦋 DESCRIPTION 🦋

so here's the tea ☕ — this is a filter, and you place it on a socket. specifically, a socket that's gonna be using accept() to receive incoming connections. that's her whole scene.

what she does: she prevents the application from receiving the connected descriptor via accept() until data actually arrives on the connection. 🫶 like, no data, no descriptor. she's not handing over the goods to a ghost. periodt.

so instead of your app waking up to a connection that's just... sitting there. silently. doing nothing. (💀 the worst kind of guest), she makes it wait until there's actually something to read.

also!! the ACCEPT_FILTER_DATA kernel option is also a module, which means you can enable it at runtime via kldload(8) — but only if the INET option has been compiled into the kernel. so check that first, bestie, otherwise she's not coming to the party. 🙈

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🌈 EXAMPLES 🌈

okay so assuming ACCEPT_FILTER_DATA has been included in the kernel config file, or the accf_data module has been loaded, this is how you enable the data accept filter on the socket sok. look at her go:

struct accept_filter_arg afa;

bzero(&afa, sizeof(afa));
strcpy(afa.af_name, "dataready");
setsockopt(sok, SOL_SOCKET, SO_ACCEPTFILTER, &afa, sizeof(afa));

see that "dataready" string? that's her name at the setsockopt party. 🎀 zero the struct, copy the name, call the setsockopt, done. iconic behaviour, honestly. no notes. 👑

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💜 SEE ALSO 💜

• setsockopt(2)
• accept_filter(9)
• accf_dns(9)
• accf_http(9)

the whole friend group fr fr 🫶 go say hi.

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⭐ HISTORY ⭐

the accept filter mechanism and the accf_data filter were introduced in FreeBSD 4.0. so she's been around. she's a veteran. she's got lore. 🌸

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👑 AUTHORS 👑

this manual page and the filter were written by Alfred Perlstein. thank you for your service king 🫡✨

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lowkey important reminder: she only shows up if INET is compiled into your kernel. don't forget that part, bestie, or you'll be sitting there wondering why nothing works. 😭

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and that's the whole page!! written on November 15, 2000, coming at you from Debian — yeah, this documentation is older than most of us and she's still serving. 💖✨ bye bestie, go buffer some connections 🦋