What is the Cocoa method for doing the Carbon FSExchangeObjectsCompat call? - macos

Question

There was this great function in the old MoreFilesX, FSExchangeObjectsCompat, that "exchanges the data between two files". It was typically used as part of a safe-save approach, where a temp file was written out, then FSExchangeObjectsCompat was called to exchange the newly-saved temp file with the old "original" file. It preserved all the metadata, privileges, etc.
I'm seeing a failure with this function on High Sierra, on APFS volumes, which never failed on HFS+ volumes. Not a big surprise -- many of those calls are deprecated.
But what is the Cocoa NSFileManager method of doing the same thing?

Answer 1

You want -[NSFileManager replaceItemAtURL:withItemAtURL:backupItemName:options:resultingItemURL:error:].

Answer 2

You can do something similar using lower-level functions. Here's code I wrote to be used with a pre-10.12 SDK. You can make it somewhat simpler if you compile against the 10.12 SDK or later, and even simpler if you have a deployment target that is 10.12 or later.
#ifndef RENAME_SWAP
#define RENAME_SWAP 0x00000002
#endif
/*!
#function ExchangeFiles
#abstract Given full paths to two files on the same volume,
swap their contents.
#discussion This is often part of a safe-save strategy.
#param inOldFile Full path to a file.
#param inNewFile Full path to a file.
#result 0 if all went well, -1 otherwise.
*/
int ExchangeFiles( const char* inOldFile, const char* inNewFile )
{
int result = -1;
static dispatch_once_t sOnce = 0;
static renameFuncType sRenameFunc = NULL;
// Try to get a function pointer to renamex_np, which is available in OS 10.12 and later.
dispatch_once( &sOnce,
^{
sRenameFunc = (renameFuncType) dlsym( RTLD_DEFAULT, "renamex_np" );
});
// renamex_np is only available on OS 10.12 and later, and does not work on HFS+ volumes
// but does work on APFS volumes. Being the latest and greatest, we try it first.
if (sRenameFunc != NULL)
{
result = (*sRenameFunc)( inOldFile, inNewFile, RENAME_SWAP );
}
if (result != 0)
{
// exchangedata is an older function that works on HFS+ but not APFS.
result = exchangedata( inOldFile, inNewFile, 0 );
}
if (result != 0)
{
// Neither function worked, we must go old school.
std::string nameTemplate( inOldFile );
nameTemplate += "-swapXXXX";
// Make a mutable copy of the template
std::vector<char> workPath( nameTemplate.size() + 1 );
memcpy( &workPath[0], nameTemplate.c_str(), nameTemplate.size() + 1 );
mktemp( &workPath[0] );
std::string tempPath( &workPath[0] );
// Make the old file have a temporary name
result = rename( inOldFile, tempPath.c_str() );
// Put the new file data under the old name.
if (result == 0)
{
result = rename( inNewFile, inOldFile );
}
// Put the old data under the new name.
if (result == 0)
{
result = rename( tempPath.c_str(), inNewFile );
}
}
return result;
}