.\" Copyright (c) 2003 Apple Computer, Inc. All rights reserved. .\" .\" The contents of this file constitute Original Code as defined in and .\" are subject to the Apple Public Source License Version 1.1 (the .\" "License"). You may not use this file except in compliance with the .\" License. Please obtain a copy of the License at .\" http://www.apple.com/publicsource and read it before using this file. .\" .\" This Original Code and all software distributed under the License are .\" distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER .\" EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES, .\" INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY, .\" FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT. Please see the .\" License for the specific language governing rights and limitations .\" under the License. .\" .\" @(#)exchangedata.2 . .Dd December 15, 2003 .Dt EXCHANGEDATA 2 .Os Darwin .Sh NAME .Nm exchangedata .Nd atomically exchange data between two files .Sh SYNOPSIS .Fd #include <unistd.h> .Fd #include <sys/attr.h> .Ft int .Fn exchangedata "const char * path1" "const char * path2" "unsigned int options" . .Sh DESCRIPTION The .Fn exchangedata function swaps the contents of the files referenced by .Fa path1 and .Fa 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 .Fa options parameter lets you control specific aspects of the function's behaviour. .Pp . Open file descriptors follow the swapped data. Thus, a descriptor that previously referenced .Fa path1 will now reference the data that's accessible via .Fa path2 , and vice versa. .Pp . In general, the file attributes (metadata) are not exchanged. Specifically, the object identifier attributes (that is, the .Dv ATTR_CMN_OBJID and .Dv ATTR_CMN_OBJPERMANENTID attributes as defined by the .Xr getattrlist 2 function) are not swapped. An exception to this general rule is that the modification time attribute ( .Dv ATTR_CMN_MODTIME ) is swapped. .Pp . 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. .Pp . .\" path1 and path2 parameters . The .Fa path1 and .Fa 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. .Pp . .\" options parameter . The .Fa options parameter is a bit set that controls the behaviour of .Fn exchangedata . The following option bits are defined. . .Bl -tag -width FSOPT_NOFOLLOW . .It FSOPT_NOFOLLOW If this bit is set, .Fn exchangedata will not follow a symlink if it occurs as the last component of .Fa path1 or .Fa path2 . . .El . .Sh RETURN VALUES Upon successful completion a value of 0 is returned. Otherwise, a value of -1 is returned and .Va errno is set to indicate the error. . .Sh COMPATIBILITY Not all volumes support .Fn exchangedata . You can test whether a volume supports .Fn exchangedata by using .Xr getattrlist 2 to get the volume capabilities attribute .Dv ATTR_VOL_CAPABILITIES , and then testing the .Dv VOL_CAP_INT_EXCHANGEDATA flag. .Pp . .Sh ERRORS .Fn exchangedata will fail if: .Bl -tag -width Er . .It Bq Er ENOTSUP The volume does not support .Fn exchangedata . . .It Bq Er ENOTDIR A component of the path prefix is not a directory. . .It Bq Er ENAMETOOLONG A component of a path name exceeded .Dv NAME_MAX characters, or an entire path name exceeded .Dv PATH_MAX characters. . .It Bq Er ENOENT Either file does not exist. . .It Bq Er EACCES Search permission is denied for a component of the path prefix. . .It Bq Er ELOOP Too many symbolic links were encountered in translating the pathname. . .It Bq Er EFAULT .Fa path1 or .Em path2 points to an invalid address. . .It Bq Er EXDEV .Fa path1 and .Em path2 are on different volumes (mounted file systems). . .It Bq Er EINVAL .Fa path1 or .Em path2 reference the same file. . .It Bq Er EINVAL You try to exchange something other than a regular file (for example, a directory). . .It Bq Er EIO An I/O error occurred while reading from or writing to the file system. .El .Pp . .Sh SEE ALSO . .Xr getattrlist 2 . .Sh HISTORY A .Fn exchangedata function call appeared in Darwin 1.3.1 (Mac OS X version 10.0). .