strnlen_user(9)

💖 strnlen_user(9) 💖

okay bestie, welcome to the strnlen_user glow-up ✨ this one lives in the Kernel API Manual and honestly? it's giving "small function, big opinions." let's get into it 🫶

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🎀 NAME 🎀

strnlen_user — get the size of a user string INCLUDING the final NUL. literally what it says on the tin 💅

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✨ SYNOPSIS ✨

long strnlen_user (const char __user *str , long count );

like yes, that's the whole signature, she's compact 👑

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🌸 ARGUMENTS 🌸

• str — the string you want measured, bestie. that's it, that's the vibe 📏
• count — the maximum count (including the NUL character). so like, the ceiling, the limit, the "don't go past this" energy 🚧

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🦋 CONTEXT 🦋

User context only. and heads up — this function may sleep if pagefaults are enabled 😴 so don't be calling it from somewhere weird, iykyk.

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💜 DESCRIPTION 💜

okay so the actual job: get the size of a NUL-terminated string in user space.

It returns the size of the string INCLUDING the terminating NUL. cute, right?

But — if the string is too long, it returns a number larger than count. so the user has to check the return value against > count 👀 do not skip the vibe check on that return value, fr fr.

On exception (or an invalid count), it returns 0. 😭

⚠️ NOTE! You should basically never use this function. like, genuinely. There is almost never any valid case for using the length of a user space string, since the string can be changed at any time by other threads 💀 it's a moving target, bestie. Use strncpy_from_user instead to get a stable copy of the string. that's the main character move 👑

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🌈 SEE ALSO 🌈

• Kernel file ./lib/strnlen_user.c 🍓

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and that's the whole page, no cap ✨ this little icon comes to you from ./lib, October 2026 edition 🧚‍♀️ go forth and use strncpy_from_user instead, bestie, ily 💖🎀🦋