msync(2)

💾 msync(2) — the "flush ur mmap edits to disk bestie" page

okay so this is the manual page for msync, a system call (that's the (2) in msync(2), iykyk), living in the System Calls Manual. she's the one who makes sure the stuff you scribbled into a memory-mapped file actually makes it to the filesystem. real ones know her.

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✨ LIBRARY ✨

Standard C library (libc, -lc). yeah bestie, she's in libc, no special hunting required. 💅

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🌸 SYNOPSIS 🌸

#include <sys/mman.h>

int msync(size_t length;
          void addr[length], size_t length, int flags);

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🦋 DESCRIPTION 🦋

okay so picture this: you mapped a file into memory with mmap(2) and you've been ✍️ editing it, living your best life. msync() flushes those changes made to the in-core copy of that file back to the filesystem. Without use of this call, there is no guarantee that changes are written back before munmap(2) is called. 🫠 so like… do it, bestie.

To be more precise: the part of the file that corresponds to the memory area starting at addr and having length length is the stuff that gets updated. 🎯

The flags argument should specify exactly one of MS_ASYNC and MS_SYNC, and may additionally include the MS_INVALIDATE bit. (so like, one of them, plus optionally the third one. not both of the first two. we'll get to why. 😭) These bits have the following meanings:

• MS_ASYNC — specifies that an update be scheduled, but the call returns immediately. she's giving "I'll deal with it later" energy 🛋️✨
• MS_SYNC — requests an update and waits for it to complete. main character behavior, she stays until the job is done 👑
• MS_INVALIDATE — asks to invalidate other mappings of the same file, so that they can be updated with the fresh values just written. out with the old cache, in with the new 💫

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💖 RETURN VALUE 💖

On success, zero is returned. (slay 🎀) On error, -1 is returned, and errno is set to indicate the error. (not slay 😭)

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🔥 ERRORS 🔥

• EBUSY — MS_INVALIDATE was specified in flags, and a memory lock exists for the specified address range. locked down, can't touch. 🔒
• EINVAL — addr is not a multiple of PAGESIZE; or any bit other than MS_ASYNC | MS_INVALIDATE | MS_SYNC is set in flags; or both MS_SYNC and MS_ASYNC are set in flags. pick one, bestie, you can't have both 🫶
• ENOMEM — the indicated memory (or part of it) was not mapped. girl what are you even syncing, there's nothing there 💀

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🌈 VERSIONS 🌈

According to POSIX, either MS_SYNC or MS_ASYNC must be specified in flags, and indeed failure to include one of these flags will cause msync() to fail on some systems. ✋😤

However, Linux permits a call to msync() that specifies neither of these flags, with semantics that are (currently) equivalent to specifying MS_ASYNC. (Since Linux 2.6.19, MS_ASYNC is in fact a no-op, since the kernel properly tracks dirty pages and flushes them to storage as necessary. the kernel is doing it for you, she's so competent 🧚)

Notwithstanding the Linux behavior, portable, future-proof applications should ensure that they specify either MS_SYNC or MS_ASYNC in flags. like, Linux will let you be messy but do NOT be messy in prod 🚨

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⭐ STANDARDS ⭐

POSIX.1-2024. certified iconic. 📜

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🎀 HISTORY 🎀

SUSv1, POSIX.1-1996. she's a legend, she's been around.

This call was introduced in Linux 1.3.21, and then used EFAULT instead of ENOMEM. In Linux 2.4.19, this was changed to the POSIX value ENOMEM. glow-up arc complete ✨

On POSIX systems on which msync() is available, both _POSIX_MAPPED_FILES and _POSIX_SYNCHRONIZED_IO are defined in <unistd.h> to a value greater than 0. (See also sysconf(3).) go check them if you don't believe me 🫡

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👑 SEE ALSO 👑

• mmap(2)
• B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and 389–391.

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⚠️ bestie reminder: msync() failing quietly means your changes might not hit the disk before munmap(2). check your return values, fr fr. 🫶

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💜 that's the page, she's written by the Linux man-pages 6.20 girlies and dated 2026-02-11. msync(2), periodt. 💅🌸