PR_SVE_SET_VL(2const)

💖 PR_SVE_SET_VL(2const) 💖

okay bestie, this is a page from the 2const corner of the Linux man-pages universe, and it's about SVE vector lengths on arm64. buckle up 🎀

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✨ NAME ✨

PR_SVE_SET_VL — set the thread's SVE vector length 🦋

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📚 LIBRARY 📚

Standard C library (libc, -lc) — the classic 💅

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🧚 SYNOPSIS 🧚

#include <linux/prctl.h>  /* Definition of PR_* constants */
#include <sys/prctl.h>

int prctl(PR_SVE_SET_VL, unsigned long val);

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🌸 DESCRIPTION 🌸

So, you're configuring the thread's SVE vector length, as specified by val. that's the whole vibe.

The bits of val that correspond to PR_SVE_VL_LEN_MASK must be set to the desired vector length in bytes. and here's the cute part — it's interpreted as an upper bound: the kernel will pick the greatest available vector length that does not exceed the value you specified. so if you're feeling ✨extra✨ and you want the absolute maximum, you set the PR_SVE_VL_LEN_MASK bits to SVE_VL_MAX (which lives in <asm/sigcontext.h>) and you get the max supported vector length. periodt.

Also, the other bits of val must be set to one of these flag combos:

• 0L — do the change immediately. but at the next execve(2) in the thread, the vector length gets reset to the value configured in /proc/sys/abi/sve_default_vector_length. so it's giving temporary 😭
• PR_SVE_VL_INHERIT — do the change immediately, and subsequent execve(2) calls will preserve the new vector length. long-term commitment, we love to see it 🫶
• PR_SVE_SET_VL_ONEXEC — defer the change so it happens at the next execve(2) in the thread. further execve(2) calls will reset the vector length to the value configured in /proc/sys/abi/sve_default_vector_length. one-time thing, no strings 💀
• PR_SVE_SET_VL_ONEXEC | PR_SVE_VL_INHERIT — defer the change so it happens at the next execve(2) in the thread, and further execve(2) calls will preserve the new vector length. delayed and permanent, main character energy 👑

In all cases, any previously pending deferred change is canceled. gone. cancelled, as the kids say 🚫

On success, a nonnegative value is returned that describes the selected configuration. if PR_SVE_SET_VL_ONEXEC was included in val, then the configuration described by the return value will take effect at the next execve(2). otherwise, the configuration is already in effect when the PR_SVE_SET_VL call returns. either way, the value is encoded in the exact same way as the return value of PR_SVE_GET_VL. and heads up: there is no explicit flag in the return value corresponding to PR_SVE_SET_VL_ONEXEC. iykyk 🙈

The configuration (including any pending deferred change) is inherited across fork(2) and clone(2). it runs in the family 💜

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💫 RETURN VALUE 💫

On success, PR_SVE_SET_VL returns the nonnegative value described above. on error, -1 is returned, and errno is set to indicate the error. classic. 😤

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🚨 ERRORS 🚨

• EINVAL — SVE is not available on this platform. not today, bestie 💀
• EINVAL — the value in the bits of val corresponding to PR_SVE_VL_LEN_MASK is outside the range [SVE_VL_MIN, SVE_VL_MAX] or is not a multiple of 16.
• EINVAL — the other bits of val are invalid or unsupported.

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🍓 FILES 🍓

• /proc/sys/abi/sve_default_vector_length

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🌈 STANDARDS 🌈

Linux. arm64 only. sorry x86 girlies, this one's not for you 🎀

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🕰️ HISTORY 🕰️

Linux 4.15 (arm64). vintage but iconic ✨

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⚠️ CAVEATS ⚠️

Because the compiler or run-time environment may be using SVE, using this call without the PR_SVE_SET_VL_ONEXEC flag may crash the calling process. 😭

The conditions for using it safely are complex and system-dependent. so like… don't use it unless you really know what you are doing. fr fr. this is not a vibe-check-and-see situation, this is a read-the-docs-three-times situation 🙏

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👀 SEE ALSO 👀

• prctl(2)
• PR_SVE_GET_VL(2const)

For more information, see the kernel source file Documentation/arch/arm64/sve.rst.

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and that's the whole page, bestie! this one's from Linux man-pages 6.20, dated 2026-02-08, and it's PR_SVE_SET_VL(2const). go forth and vector responsibly 💅✨🦋