---
name: "msync"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
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 modified whole pages back to the filesystem and updates the file modification time. 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
```

The *MS\_ASYNC* flag is not permitted to be combined with other flags.

## RETURN VALUES

If any errors occur, -1 is returned and errno is set to indicate the error. Otherwise, a 0 value is returned.

## ERRORS

**msync**() will fail if:

- \[EBUSY\]

  Some of the specified addresses are locked and MS\_INVALIDATE is specified.

- \[EINVAL\]

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

- \[EINVAL\]

  *len* is too large, or less than 1.

- \[EINVAL\]

  *flags* is invalid (e.g., it combines MS\_ASYNC with another flag, which is not permitted).

- \[EIO\]

  An I/O error occurs while writing to the file system.

- \[ENOMEM\]

  The specified address range is outside of the address range of the process or includes an unmapped page.

## LEGACY SYNOPSIS

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

The include file ⟨sys/types.h⟩ is necessary.

## SEE ALSO

[madvise(2)](/madvise/2.md), [mincore(2)](/mincore/2.md), [mprotect(2)](/mprotect/2.md), [munmap(2)](/munmap/2.md), [compat(5)](/compat/5.md)

## HISTORY

The **msync**() function first appeared in 4.4BSD.