---
name: "msync"
section: "2"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/msync"
---

# msync(2)

synchronize a mapped region

## NAME

**msync** – synchronize a mapped region

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<sys/mman.h\>**

*int*\

**msync**(*void \*addr*, *size\_t len*, *int flags*);

## DESCRIPTION

The **msync**() system call writes any modified pages back to the file system and updates the file modification time. If *len* is 0, all modified pages within the region containing *addr* will be flushed; if *len* is non-zero, only those pages containing *addr* and *len-1* succeeding locations will be examined. The *flags* argument may be specified as follows:

- MS\_ASYNC

  Return immediately

- MS\_SYNC

  Perform synchronous writes

- MS\_INVALIDATE

  Invalidate all cached data

## RETURN VALUES

The **msync()** function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

## ERRORS

The **msync**() system call will fail if:

- \[EBUSY\]

  Some or all of the pages in the specified region are locked and MS\_INVALIDATE is specified.

- \[EINVAL\]

  The *addr* argument is not a multiple of the hardware page size.

- \[ENOMEM\]

  The addresses in the range starting at *addr* and continuing for *len* bytes are outside the range allowed for the address space of a process or specify one or more pages that are not mapped.

- \[EINVAL\]

  The *flags* argument was both MS\_ASYNC and MS\_INVALIDATE. Only one of these flags is allowed.

- \[EIO\]

  An error occurred while writing at least one of the pages in the specified region.

## SEE ALSO

[madvise(2)](/madvise/2.md), [mincore(2)](/mincore/2.md), [mlock(2)](/mlock/2.md), [mprotect(2)](/mprotect/2.md), [munmap(2)](/munmap/2.md)

## HISTORY

The **msync**() system call first appeared in 4.4BSD.

## BUGS

The **msync**() system call is usually not needed since BSD implements a coherent file system buffer cache. However, it may be used to associate dirty VM pages with file system buffers and thus cause them to be flushed to physical media sooner rather than later.