git.saurik.com Git - apple/xnu.git/blame_incremental

... / ...

Commit	Line	Data
	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>
	27	.Ft int
	28	.Fn exchangedata "const char * path1" "const char * path2" "unsigned long options"
	29	.
	30	.Sh DESCRIPTION
	31	The
	32	.Fn exchangedata
	33	function swaps the contents of the files referenced by
	34	.Fa path1
	35	and
	36	.Fa path2
	37	in an atomic fashion.
	38	That is, all concurrent processes will either see the pre-exchanged state or the
	39	post-exchanged state; they can never see the files in an inconsistent state.
	40	The data in all forks is swapped in this way.
	41	The
	42	.Fa options
	43	parameter lets you control specific aspects of the function's behaviour.
	44	.Pp
	45	.
	46	Open file descriptors follow the swapped data.
	47	Thus, a descriptor that previously referenced
	48	.Fa path1
	49	will now reference the data that's accessible via
	50	.Fa path2 ,
	51	and vice versa.
	52	.Pp
	53	.
	54	In general, the file attributes (metadata) are not exchanged.
	55	Specifically, the object identifier attributes (that is, the
	56	.Dv ATTR_CMN_OBJID
	57	and
	58	.Dv ATTR_CMN_OBJPERMANENTID
	59	attributes as defined by the
	60	.Xr getattrlist 2
	61	function) are not swapped.
	62	An exception to this general rule is that the modification time attribute (
	63	.Dv ATTR_CMN_MODTIME
	64	) is swapped.
	65	.Pp
	66	.
	67	When combined, these features allow you to implement a 'safe save' function that
	68	does not break references to the file (for example, aliases).
	69	You first save the new contents to a temporary file and then
	70	exchange the data of the original file and the temporary.
	71	Programs that reference the file via an object identifier will continue to
	72	reference the original file, but now it has the new data.
	73	.Pp
	74	.
	75	.\" path1 and path2 parameters
	76	.
	77	The
	78	.Fa path1
	79	and
	80	.Fa path2
	81	parameters must both reference valid files.
	82	All directories listed in the path names leading to these files must be
	83	searchable.
	84	You must have write access to the files.
	85	.Pp
	86	.
	87	.\" options parameter
	88	.
	89	The
	90	.Fa options
	91	parameter is a bit set that controls the behaviour of
	92	.Fn exchangedata .
	93	The following option bits are defined.
	94	.
	95	.Bl -tag -width FSOPT_NOFOLLOW
	96	.
	97	.It FSOPT_NOFOLLOW
	98	If this bit is set,
	99	.Fn exchangedata
	100	will not follow a symlink if it occurs as
	101	the last component of
	102	.Fa path1
	103	or
	104	.Fa path2 .
	105	.
	106	.El
	107	.
	108	.Sh RETURN VALUES
	109	Upon successful completion a value of 0 is returned.
	110	Otherwise, a value of -1 is returned and
	111	.Va errno
	112	is set to indicate the error.
	113	.
	114	.Sh COMPATIBILITY
	115	Not all volumes support
	116	.Fn exchangedata .
	117	You can test whether a volume supports
	118	.Fn exchangedata
	119	by using
	120	.Xr getattrlist 2
	121	to get the volume capabilities attribute
	122	.Dv ATTR_VOL_CAPABILITIES ,
	123	and then testing the
	124	.Dv VOL_CAP_INT_EXCHANGEDATA
	125	flag.
	126	.Pp
	127	.
	128	.Sh ERRORS
	129	.Fn exchangedata
	130	will fail if:
	131	.Bl -tag -width Er
	132	.
	133	.It Bq Er ENOTSUP
	134	The volume does not support
	135	.Fn exchangedata .
	136	.
	137	.It Bq Er ENOTDIR
	138	A component of the path prefix is not a directory.
	139	.
	140	.It Bq Er ENAMETOOLONG
	141	A component of a path name exceeded
	142	.Dv NAME_MAX
	143	characters, or an entire path name exceeded
	144	.Dv PATH_MAX
	145	characters.
	146	.
	147	.It Bq Er ENOENT
	148	Either file does not exist.
	149	.
	150	.It Bq Er EACCES
	151	Search permission is denied for a component of the path prefix.
	152	.
	153	.It Bq Er ELOOP
	154	Too many symbolic links were encountered in translating the pathname.
	155	.
	156	.It Bq Er EFAULT
	157	.Fa path1
	158	or
	159	.Em path2
	160	points to an invalid address.
	161	.
	162	.It Bq Er EXDEV
	163	.Fa path1
	164	and
	165	.Em path2
	166	are on different volumes (mounted file systems).
	167	.
	168	.It Bq Er EINVAL
	169	.Fa path1
	170	or
	171	.Em path2
	172	reference the same file.
	173	.
	174	.It Bq Er EINVAL
	175	You try to exchange something other than a regular file (for example, a directory).
	176	.
	177	.It Bq Er EIO
	178	An I/O error occurred while reading from or writing to the file system.
	179	.El
	180	.Pp
	181	.
	182	.Sh SEE ALSO
	183	.
	184	.Xr getattrlist 2
	185	.
	186	.Sh HISTORY
	187	A
	188	.Fn exchangedata
	189	function call appeared in Darwin 1.3.1 (Mac OS X version 10.0).
	190	.