[apple/xnu.git] / bsd / man / man2 / exchangedata.2

.\" 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
.
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 
.Fn rename
or
.Fn renamex_np
to conduct safe-save operations instead. 
.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 .
This includes APFS volumes.
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 ,
.Xr rename 2
.
.Sh HISTORY
A
.Fn exchangedata
function call appeared in Darwin 1.3.1 (Mac OS X version 10.0).
.
It was deprecated in macOS 10.13.
Commit	Line	Data
91447636 A	1	.\" Copyright (c) 2003 Apple Computer, Inc. All rights reserved.
	2	.\"
	3	.\" The contents of this file constitute Original Code as defined in and
	4	.\" are subject to the Apple Public Source License Version 1.1 (the
	5	.\" "License"). You may not use this file except in compliance with the
	6	.\" License. Please obtain a copy of the License at
	7	.\" http://www.apple.com/publicsource and read it before using this file.
	8	.\"
	9	.\" This Original Code and all software distributed under the License are
	10	.\" distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
	11	.\" EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
	12	.\" INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
	13	.\" FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT. Please see the
	14	.\" License for the specific language governing rights and limitations
	15	.\" under the License.
	16	.\"
	17	.\" @(#)exchangedata.2
	18	.
	19	.Dd December 15, 2003
	20	.Dt EXCHANGEDATA 2
	21	.Os Darwin
	22	.Sh NAME
	23	.Nm exchangedata
	24	.Nd atomically exchange data between two files
	25	.Sh SYNOPSIS
	26	.Fd #include <unistd.h>
6d2010ae	27	.Fd #include <sys/attr.h>
91447636	28	.Ft int
6d2010ae	29	.Fn exchangedata "const char * path1" "const char * path2" "unsigned int options"
91447636 A	30	.
	31	.Sh DESCRIPTION
	32	The
	33	.Fn exchangedata
	34	function swaps the contents of the files referenced by
	35	.Fa path1
	36	and
	37	.Fa path2
	38	in an atomic fashion.
	39	That is, all concurrent processes will either see the pre-exchanged state or the
	40	post-exchanged state; they can never see the files in an inconsistent state.
	41	The data in all forks is swapped in this way.
	42	The
	43	.Fa options
	44	parameter lets you control specific aspects of the function's behaviour.
	45	.Pp
	46	.
	47	Open file descriptors follow the swapped data.
	48	Thus, a descriptor that previously referenced
	49	.Fa path1
	50	will now reference the data that's accessible via
	51	.Fa path2 ,
	52	and vice versa.
	53	.Pp
	54	.
	55	In general, the file attributes (metadata) are not exchanged.
	56	Specifically, the object identifier attributes (that is, the
	57	.Dv ATTR_CMN_OBJID
	58	and
	59	.Dv ATTR_CMN_OBJPERMANENTID
	60	attributes as defined by the
	61	.Xr getattrlist 2
	62	function) are not swapped.
	63	An exception to this general rule is that the modification time attribute (
	64	.Dv ATTR_CMN_MODTIME
	65	) is swapped.
	66	.Pp
	67	.
	68	When combined, these features allow you to implement a 'safe save' function that
	69	does not break references to the file (for example, aliases).
	70	You first save the new contents to a temporary file and then
	71	exchange the data of the original file and the temporary.
	72	Programs that reference the file via an object identifier will continue to
	73	reference the original file, but now it has the new data.
	74	.Pp
	75	.
d9a64523 A	76	WARNING: This system call is largely supported only by HFS and AFP file systems. Many other
	77	file systems, including APFS, do not support it. Further, it is not supported on iOS, tvOS, or watchOS.
	78	It is recommended that callers refer
	79	instead to
	80	.Fn rename
	81	or
	82	.Fn renamex_np
	83	to conduct safe-save operations instead.
	84	.Pp
	85	.
91447636 A	86	.\" path1 and path2 parameters
	87	.
	88	The
	89	.Fa path1
	90	and
	91	.Fa path2
	92	parameters must both reference valid files.
	93	All directories listed in the path names leading to these files must be
	94	searchable.
	95	You must have write access to the files.
	96	.Pp
	97	.
	98	.\" options parameter
	99	.
	100	The
	101	.Fa options
	102	parameter is a bit set that controls the behaviour of
	103	.Fn exchangedata .
	104	The following option bits are defined.
	105	.
	106	.Bl -tag -width FSOPT_NOFOLLOW
	107	.
	108	.It FSOPT_NOFOLLOW
	109	If this bit is set,
	110	.Fn exchangedata
	111	will not follow a symlink if it occurs as
	112	the last component of
	113	.Fa path1
	114	or
	115	.Fa path2 .
	116	.
	117	.El
	118	.
	119	.Sh RETURN VALUES
	120	Upon successful completion a value of 0 is returned.
	121	Otherwise, a value of -1 is returned and
	122	.Va errno
	123	is set to indicate the error.
	124	.
	125	.Sh COMPATIBILITY
	126	Not all volumes support
	127	.Fn exchangedata .
d9a64523	128	This includes APFS volumes.
91447636 A	129	You can test whether a volume supports
	130	.Fn exchangedata
	131	by using
	132	.Xr getattrlist 2
	133	to get the volume capabilities attribute
	134	.Dv ATTR_VOL_CAPABILITIES ,
	135	and then testing the
	136	.Dv VOL_CAP_INT_EXCHANGEDATA
	137	flag.
	138	.Pp
	139	.
	140	.Sh ERRORS
	141	.Fn exchangedata
	142	will fail if:
	143	.Bl -tag -width Er
	144	.
	145	.It Bq Er ENOTSUP
	146	The volume does not support
	147	.Fn exchangedata .
	148	.
	149	.It Bq Er ENOTDIR
	150	A component of the path prefix is not a directory.
	151	.
	152	.It Bq Er ENAMETOOLONG
	153	A component of a path name exceeded
	154	.Dv NAME_MAX
	155	characters, or an entire path name exceeded
	156	.Dv PATH_MAX
	157	characters.
	158	.
	159	.It Bq Er ENOENT
	160	Either file does not exist.
	161	.
	162	.It Bq Er EACCES
	163	Search permission is denied for a component of the path prefix.
	164	.
	165	.It Bq Er ELOOP
	166	Too many symbolic links were encountered in translating the pathname.
	167	.
	168	.It Bq Er EFAULT
	169	.Fa path1
	170	or
	171	.Em path2
	172	points to an invalid address.
	173	.
	174	.It Bq Er EXDEV
	175	.Fa path1
	176	and
	177	.Em path2
	178	are on different volumes (mounted file systems).
	179	.
	180	.It Bq Er EINVAL
	181	.Fa path1
	182	or
	183	.Em path2
	184	reference the same file.
	185	.
	186	.It Bq Er EINVAL
	187	You try to exchange something other than a regular file (for example, a directory).
	188	.
	189	.It Bq Er EIO
	190	An I/O error occurred while reading from or writing to the file system.
	191	.El
	192	.Pp
193	.
194	.Sh SEE ALSO
195	.
5ba3f43e A	196	.Xr getattrlist 2 ,
5ba3f43e A	197	.Xr rename 2
91447636 A	198	.
	199	.Sh HISTORY
	200	A
	201	.Fn exchangedata
	202	function call appeared in Darwin 1.3.1 (Mac OS X version 10.0).
	203	.
5ba3f43e	204	It was deprecated in macOS 10.13.