prctl(2)

prctl(2) 💅

okay bestie, pull up a chair, we're in the System Calls Manual now and this one is a whole vibe. prctl() is the "one syscall to rule them all" situation — it's a giant grab-bag of process-and-thread operations all crammed into a single entry point. no cap, it's giving "swiss army knife that also does your taxes." 🔥

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NAME ✨

prctl — operations on a process or thread

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LIBRARY 🎀

Standard C library (libc, -lc)

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SYNOPSIS 🦋

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

int prctl(int op, ...);

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DESCRIPTION 🌸

so basically: prctl() manipulates various aspects of the behavior of the calling thread or process. that's the whole assignment, and it is so much. 💀

you call prctl() with a first argument that describes what you want to do, and then further arguments whose significance depends entirely on that first one. so like, the first arg is the main character and everyone else is just supporting cast. main character energy fr fr.

the first argument can be any one of these icons 👑:

• PR_CAP_AMBIENT
• PR_CAPBSET_READ
• PR_CAPBSET_DROP
• PR_SET_CHILD_SUBREAPER
• PR_GET_CHILD_SUBREAPER
• PR_SET_DUMPABLE
• PR_GET_DUMPABLE
• PR_SET_ENDIAN
• PR_GET_ENDIAN
• PR_SET_FP_MODE
• PR_GET_FP_MODE
• PR_SET_FPEMU
• PR_GET_FPEMU
• PR_SET_FPEXC
• PR_GET_FPEXC
• PR_SET_IO_FLUSHER
• PR_GET_IO_FLUSHER
• PR_SET_KEEPCAPS
• PR_GET_KEEPCAPS
• PR_MCE_KILL
• PR_MCE_KILL_GET
• PR_SET_MM
• PR_SET_VMA
• PR_MPX_ENABLE_MANAGEMENT
• PR_MPX_DISABLE_MANAGEMENT
• PR_SET_NAME
• PR_GET_NAME
• PR_SET_NO_NEW_PRIVS
• PR_GET_NO_NEW_PRIVS
• PR_PAC_RESET_KEYS
• PR_SET_PDEATHSIG
• PR_GET_PDEATHSIG
• PR_SET_PTRACER
• PR_SET_SECCOMP
• PR_GET_SECCOMP
• PR_SET_SECUREBITS
• PR_GET_SECUREBITS
• PR_GET_SPECULATION_CTRL
• PR_SET_SPECULATION_CTRL
• PR_SVE_SET_VL
• PR_SVE_GET_VL
• PR_SET_SYSCALL_USER_DISPATCH
• PR_SET_TAGGED_ADDR_CTRL
• PR_GET_TAGGED_ADDR_CTRL
• PR_TASK_PERF_EVENTS_DISABLE
• PR_TASK_PERF_EVENTS_ENABLE
• PR_SET_THP_DISABLE
• PR_GET_THP_DISABLE
• PR_GET_TID_ADDRESS
• PR_SET_TIMERSLACK
• PR_GET_TIMERSLACK
• PR_SET_TIMING
• PR_GET_TIMING
• PR_SET_TSC
• PR_GET_TSC
• PR_SET_UNALIGN
• PR_GET_UNALIGN
• PR_GET_AUXV
• PR_SET_MDWE
• PR_GET_MDWE
• PR_RISCV_SET_ICACHE_FLUSH_CTX
• PR_FUTEX_HASH

like. sixty-three vibes. pick one. 🫶

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RETURN VALUE 🌈

• on success, a nonnegative value comes back 💖
• on error, -1 comes back, and errno gets set to indicate the error 😭

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ERRORS ⭐

• EINVAL — the value of op is not recognized, or not supported on this system.
• EINVAL — an unused argument is nonzero.

🙈 yes, that's two separate EINVAL situations, and yes, they're listed separately in the original. we don't flatten lore in this house.

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VERSIONS 🧚

IRIX has a prctl() system call too — and fun fact, it was also introduced in Linux 2.1.44 as irix_prctl on the MIPS architecture 🍓 — with prototype:

ptrdiff_t prctl(int op, int arg2, int arg3);

and operations to get the maximum number of processes per user, get the maximum number of processors the calling process can use, find out whether a specified process is currently blocked, get or set the maximum stack size, and so on. the and-so-on is doing so much heavy lifting there bestie 💀

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STANDARDS 🦋

Linux.

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HISTORY 🎀

Linux 2.1.57, glibc 2.0.6

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CAVEATS ⚠️

listen. listen. this part matters. 👑

the prototype of the libc wrapper uses a variadic argument list. that makes it necessary to pass the arguments with the right width. when you're passing numeric constants, like 0, use a suffix: 0L. do it. don't be cute about it. 💅

careless use of some prctl() operations can confuse the user-space run-time environment, so these operations should be used with care. like, genuinely. be careful. this is not a drill. 😭

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SEE ALSO ✨

signal(2), PR_CAP_AMBIENT(2const), PR_CAPBSET_READ(2const), PR_CAPBSET_DROP(2const), PR_SET_CHILD_SUBREAPER(2const), PR_GET_CHILD_SUBREAPER(2const), PR_SET_DUMPABLE(2const), PR_GET_DUMPABLE(2const), PR_SET_ENDIAN(2const), PR_GET_ENDIAN(2const), PR_SET_FP_MODE(2const), PR_GET_FP_MODE(2const), PR_SET_FPEMU(2const), PR_GET_FPEMU(2const), PR_SET_FPEXC(2const), PR_GET_FPEXC(2const), PR_SET_IO_FLUSHER(2const), PR_GET_IO_FLUSHER(2const), PR_SET_KEEPCAPS(2const), PR_GET_KEEPCAPS(2const), PR_MCE_KILL(2const), PR_MCE_KILL_GET(2const), PR_SET_MM(2const), PR_SET_VMA(2const), PR_MPX_ENABLE_MANAGEMENT(2const), PR_MPX_DISABLE_MANAGEMENT(2const), PR_SET_NAME(2const), PR_GET_NAME(2const), PR_SET_NO_NEW_PRIVS(2const), PR_GET_NO_NEW_PRIVS(2const), PR_PAC_RESET_KEYS(2const), PR_SET_PDEATHSIG(2const), PR_GET_PDEATHSIG(2const), PR_SET_PTRACER(2const), PR_SET_SECCOMP(2const), PR_GET_SECCOMP(2const), PR_SET_SECUREBITS(2const), PR_GET_SECUREBITS(2const), PR_SET_SPECULATION_CTRL(2const), PR_GET_SPECULATION_CTRL(2const), PR_SVE_SET_VL(2const), PR_SVE_GET_VL(2const), PR_SET_SYSCALL_USER_DISPATCH(2const), PR_SET_TAGGED_ADDR_CTRL(2const), PR_GET_TAGGED_ADDR_CTRL(2const), PR_TASK_PERF_EVENTS_DISABLE(2const), PR_TASK_PERF_EVENTS_ENABLE(2const), PR_SET_THP_DISABLE(2const), PR_GET_THP_DISABLE(2const), PR_GET_TID_ADDRESS(2const), PR_SET_TIMERSLACK(2const), PR_GET_TIMERSLACK(2const), PR_SET_TIMING(2const), PR_GET_TIMING(2const), PR_SET_TSC(2const), PR_GET_TSC(2const), PR_SET_UNALIGN(2const), PR_GET_UNALIGN(2const), PR_GET_AUXV(2const), PR_SET_MDWE(2const), PR_GET_MDWE(2const), PR_RISCV_SET_ICACHE_FLUSH_CTX(2const), PR_FUTEX_HASH(2const), core(5)

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and that's the whole page bestie, you ate that 🫶 this one came from Linux man-pages 6.20, dated 2026-02-08, and it lives in the prctl(2) slot of the System Calls Manual. go forth and pass your arguments with the right width. periodt. 👑💖