---
name: "exchangedata"
section: "2"
platform: "macos"
package: "xnu"
version: "12377.121.6"
release: "macOS 26.5"
platforms: ["macos"]
url: "https://new.carta.tech/exchangedata"
---

# exchangedata(2)

atomically exchange data between two files

## NAME

**exchangedata** – atomically exchange data between two files

## SYNOPSIS

**#include \<unistd.h\>**

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

*int*\

**exchangedata**(*const char \* path1*, *const char \* path2*, *unsigned int options*);

## DESCRIPTION

The **exchangedata**() function swaps the contents of the files referenced by  *path1* and  *path2* in an atomic fashion.  That is, all concurrent processes will either see the pre-exchanged state or the  post-exchanged state; they can never see the files in an inconsistent state. The data in all forks is swapped in this way.  The  *options* parameter lets you control specific aspects of the function's behaviour.

Open file descriptors follow the swapped data. Thus, a descriptor that previously referenced  *path1* will now reference the data that's accessible via  *path2*, and vice versa.

In general, the file attributes (metadata) are not exchanged. Specifically, the object identifier attributes (that is, the  ATTR\_CMN\_OBJID and ATTR\_CMN\_OBJPERMANENTID attributes as defined by the  [getattrlist(2)](/getattrlist/2.md) function) are not swapped. An exception to this general rule is that the modification time attribute ( ATTR\_CMN\_MODTIME ) is swapped.

When combined, these features allow you to implement a 'safe save' function that  does not break references to the file (for example, aliases). You first save the new contents to a temporary file and then  exchange the data of the original file and the temporary. Programs that reference the file via an object identifier will continue to  reference the original file, but now it has the new data.

WARNING: This system call is largely supported only by HFS and AFP file systems.  Many other file systems, including APFS, do not support it.  Further, it is not supported on iOS, tvOS, or watchOS.  It is recommended that callers refer instead to  **rename**() or **renamex\_np**() to conduct safe-save operations instead.

The *path1* and  *path2* parameters must both reference valid files. All directories listed in the path names leading to these files must be  searchable. You must have write access to the files.

The *options* parameter is a bit set that controls the behaviour of **exchangedata**(). The following option bits are defined:

- FSOPT\_NOFOLLOW

  If this bit is set,  **exchangedata**() will not follow a symlink if it occurs as  the last component of *path1* or  *path2*.

- FSOPT\_NOFOLLOW\_ANY

  If this bit is set, **exchangedata**() will not follow a symlink if it occurs as the last component of *path1* or *path2*. In addition, if a symbolic link is encountered before the final component, an error is returned.

- FSOPT\_UNIQUE

  If this bit is set, the path lookup will fail if either target file has multiple hard links.

## RETURN VALUES

Upon successful completion a value of 0 is returned. Otherwise, a value of -1 is returned and *errno* is set to indicate the error.

## COMPATIBILITY

Not all volumes support  **exchangedata**(). This includes APFS volumes. You can test whether a volume supports  **exchangedata**() by using  [getattrlist(2)](/getattrlist/2.md) to get the volume capabilities attribute  ATTR\_VOL\_CAPABILITIES, and then testing the  VOL\_CAP\_INT\_EXCHANGEDATA flag.

## ERRORS

**exchangedata**() will fail if:

- \[ENOTSUP\]

  The volume does not support **exchangedata**().

- \[ENOTDIR\]

  A component of the path prefix is not a directory.

- \[ENAMETOOLONG\]

  A component of a path name exceeded  NAME\_MAX characters, or an entire path name exceeded  PATH\_MAX characters.

- \[ENOENT\]

  Either file does not exist.

- \[EACCES\]

  Search permission is denied for a component of the path prefix.

- \[ELOOP\]

  Too many symbolic links were encountered in translating the pathname.

- \[EFAULT\]

  *path1* or *path2* points to an invalid address.

- \[EXDEV\]

  *path1* and *path2* are on different volumes (mounted file systems).

- \[EINVAL\]

  *path1* or *path2* reference the same file.

- \[EINVAL\]

  You try to exchange something other than a regular file (for example, a directory).

- \[EIO\]

  An I/O error occurred while reading from or writing to the file system.

- \[ENOTCAPABLE\]

  if FSOPT\_UNIQUE was passed and either *path1* or *path2* has multiple hard links.

## SEE ALSO

[getattrlist(2)](/getattrlist/2.md), [rename(2)](/rename/2.md)

## HISTORY

A **exchangedata**() function call appeared in Darwin 1.3.1 (Mac OS X version 10.0). It was deprecated in macOS 10.13.