getentropy(3)

🎀 getentropy(3) — bestie, let's get you some random bytes 💅

okay so this is the manual page for getentropy, a lil function from the Library Functions Manual, and it is honestly one of the most wholesome functions in libc because it just... gives you randomness. like, that's the whole bit. no drama, no config, no surprises. ✨

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🌸 NAME 🌸

getentropy — fill a buffer with random bytes 🎲

📚 LIBRARY 📚

Standard C library (libc, -lc)

✨ SYNOPSIS ✨

#include <unistd.h>

int getentropy(size_t length;
               void buffer[length], size_t length);

🌈 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

getentropy():
    _DEFAULT_SOURCE

yeah bestie, that's the whole secret handshake. define _DEFAULT_SOURCE and glibc will let you in. 🫶

💖 DESCRIPTION 💖

The getentropy() function writes length bytes of high-quality random data to the buffer starting at the location pointed to by buffer. it's giving "cryptographically respectable, no notes" energy, fr fr. 🔥

The maximum permitted value for the length argument is 256. that's the ceiling. do not test it. 💀

A successful call to getentropy() always provides the requested number of bytes of entropy. not "maybe", not "some", not "best effort" — always. periodt. 👑

🦋 RETURN VALUE 🦋

• On success, this function returns zero. 🎉
• On error, -1 is returned, and errno is set to indicate the error.

🙈 ERRORS 🙈

• EFAULT — part or all of the buffer specified by buffer and length is not in valid addressable memory. girl, that memory is not there. 😭
• EIO — length is greater than 256. (yeah, both of the EIOs in this list are real, don't merge them, i'm watching 👀)
• EIO — an unspecified error occurred while trying to overwrite buffer with random data.
• ENOSYS — this kernel version does not implement the getrandom(2) system call required to implement this function. ancient kernel behavior honestly. 💀

⭐ STANDARDS ⭐

POSIX.1-2024. she's standard, she's official, she's on the list. 💅

🧚 HISTORY 🧚

OpenBSD 5.6, glibc 2.25, FreeBSD 12.0, POSIX.1-2024. a whole timeline. iconic of her honestly. 🌸

👑 NOTES 👑

The getentropy() function is implemented using getrandom(2). that's the machinery under the hood, iykyk. 🔧

Whereas the glibc wrapper makes getrandom(2) a cancelation point, getentropy() is not a cancelation point. write that down, bestie, it matters. 📝

getentropy() is also declared in <sys/random.h>. (No feature test macro need be defined to obtain the declaration from that header file.) so if you're pulling it from there, no _DEFAULT_SOURCE ritual required. ✨

A call to getentropy() may block if the system has just booted and the kernel has not yet collected enough randomness to initialize the entropy pool. In this case, getentropy() will keep blocking even if a signal is handled, and will return only once the entropy pool has been initialized.

like... she will wait. no cap. signals cannot sway her. she returns when the vibes are ready and not one second sooner. 🫶

🌟 SEE ALSO 🌟

getrandom(2), urandom(4), random(7)

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and that's the page, bestie!! 🎀 this one came from Linux man-pages 6.20, dated 2026-04-21 — so go forth, fill your buffers with exactly 256 bytes or fewer of certified premium randomness, and slay. 💖💅✨