What does the /private prefix on an iOS file path indicate?

Question

I have a bug when my app runs on the iPhone but not when it runs on the simulator. I was using the length of the home directory path to extract the relative path of a file in /Documents. Unfortunately this doesn't always work correctly on the iPhone because the prefix "/private" is being added to the home path. However, with or without the prefix, the same file is referenced ok. The following code demonstrates this inconsistency. What is the purpose of "/private" and when is it supplied by iOS?

- (IBAction)testHomepath:(id)sender {     NSFileManager *fmgr = [NSFileManager defaultManager];     NSString  *homePath = [NSString stringWithFormat:@"%@/Documents",NSHomeDirectory()];     NSString  *dirPath  = [homePath stringByAppendingPathComponent:@"TempDir"];     NSURL     *dirURL   = [NSURL fileURLWithPath:dirPath];     NSString  *filePath = [dirPath  stringByAppendingPathComponent:@"test.jpg"];     [fmgr createDirectoryAtPath:dirPath withIntermediateDirectories:NO attributes:nil error:nil];     [fmgr createFileAtPath:filePath contents:nil attributes:nil];     NSArray *keys  = [[NSArray alloc] initWithObjects:NSURLNameKey,nil];     NSArray *files = [fmgr contentsOfDirectoryAtURL:dirURL includingPropertiesForKeys:keys options:0 error:nil];     NSURL *f1 = (files.count>0)? [files objectAtIndex:0] : 0;     NSURL *f2 = (files.count>1)? [files objectAtIndex:1] : 0;     bool   b0 = [fmgr fileExistsAtPath:filePath];     bool   b1 = [fmgr fileExistsAtPath:f1.path];     bool   b2 = [fmgr fileExistsAtPath:f2.path];      NSLog(@"File exists=%d at path:%@",b0,filePath);     NSLog(@"File exists=%d at path:%@",b1,f1.path);     NSLog(@"File exists=%d at path:%@",b2,f2.path); } 

The following is written to the log when running on the iPhone. I manually spaced the output to show the difference between lines 1 and 2.

2013-02-20 16:31:26.615 Test1[4059:907] File exists=1 at path:        /var/mobile/Applications/558B5D82-ACEB-457D-8A70-E6E00DB3A484/Documents/TempDir/test.jpg 2013-02-20 16:31:26.622 Test1[4059:907] File exists=1 at path:/private/var/mobile/Applications/558B5D82-ACEB-457D-8A70-E6E00DB3A484/Documents/TempDir/test.jpg 2013-02-20 16:31:26.628 Test1[4059:907] File exists=0 at path:(null) 

The following is written to the log when running on the simulator (no "/private"):

2013-02-20 16:50:38.730 Test1[7224:c07] File exists=1 at path:/Users/kenm/Library/Application Support/iPhone Simulator/6.1/Applications/C6FDE177-958C-4BF5-8770-A4D3FBD281F1/Documents/TempDir/test.jpg 2013-02-20 16:50:38.732 Test1[7224:c07] File exists=1 at path:/Users/kenm/Library/Application Support/iPhone Simulator/6.1/Applications/C6FDE177-958C-4BF5-8770-A4D3FBD281F1/Documents/TempDir/.DS_Store 2013-02-20 16:50:38.733 Test1[7224:c07] File exists=1 at path:/Users/kenm/Library/Application Support/iPhone Simulator/6.1/Applications/C6FDE177-958C-4BF5-8770-A4D3FBD281F1/Documents/TempDir/test.jpg 

Answer 1

I tried this from the debugger and discovered that URLByResolvingSymlinksInPath "fixes" the /private/ addition.

(lldb) p (NSURL *)[NSURL fileURLWithPath:@"/private/var" isDirectory:YES] (NSURL *) $1 = 0x1fd9fc20 @"file://localhost/private/var/" (lldb) po [$1 URLByResolvingSymlinksInPath] $2 = 0x1fda0190 file://localhost/var/  (lldb) p (NSURL *)[NSURL fileURLWithPath:@"/var" isDirectory:YES] (NSURL *) $7 = 0x1fd9fee0 @"file://localhost/var/" (lldb) po [$7 URLByResolvingSymlinksInPath] $8 = 0x1fda2f50 file://localhost/var/ 

as you can see, file://localhost/var is what we really want here.

Because of this, it seemed obvious that /private/var is a symlink to /var. However, @Kevin-Ballard points out that is not true. I confirmed that he is correct, and /var is the symlink to /private/var (sigh)

(lldb) p (NSDictionary *)[[NSFileManager defaultManager] attributesOfItemAtPath:@"/var" error:nil] (NSDictionary *) $3 = 0x1fda11b0 13 key/value pairs (lldb) po $3 $3 = 0x1fda11b0 {     ...     NSFileType = NSFileTypeSymbolicLink; }  (lldb) p (NSDictionary *)[[NSFileManager defaultManager]   attributesOfItemAtPath:@"/private/var" error:nil] (NSDictionary *) $5 = 0x1fda4820 14 key/value pairs (lldb) po $5 $5 = 0x1fda4820 {     ...     NSFileType = NSFileTypeDirectory; } 

So URLByResolvingSymlinksInPath is doing something funny here, but now we know. For this particular problem, URLByResolvingSymlinksInPath still sounds like a good solution that works for both the simulator and the device and should continue to work in the future if something changes.