Message-ID: <CAKgNAkhZSb0UfZz7N+gKRQD0r7gnC_3-VCB_ORio8ojczbCDvQ@mail.gmail.com>
Date:	Sun, 18 Sep 2011 09:07:50 +0200
From:	Michael Kerrisk <mtk.manpages@...il.com>
To:	Josef Bacik <josef@...hat.com>
Cc:	linux-man@...r.kernel.org, Eric Blake <eblake@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Man page doc for SEEK_DATA/SEEK_HOLE

Hello Josef,

For Linux 3.1, you've added SEEK_HOLE + SEEK_DATA. I've attempted to
document these as below for the lseek.2 man page. Could you please
review?

Thanks,

Michael


diff --git a/man2/lseek.2 b/man2/lseek.2
index 26943e2..941ea08 100644
--- a/man2/lseek.2
+++ b/man2/lseek.2
@@ -85,6 +85,69 @@ of the file (but this does not change the size of the file).
 If data is later written at this point, subsequent reads of the data
 in the gap (a "hole") return null bytes (\(aq\\0\(aq) until
 data is actually written into the gap.
+.SS Seeking file data and holes
+Since version 3.1, Linux supports the following additional values for
+.IR whence :
+.TP
+.B SEEK_DATA
+Adjust the file offset to the next region
+in the file greater than or equal to
+.I offset
+containing data.
+If
+.I offset
+points to data,
+then the file offset is set to
+.IR offset .
+.TP
+.B SEEK_HOLE
+Adjust the file offset to the next hole in the file
+greater than or equal to
+.IR offset .

+If
+.I offset
+points into the middle of a hole,
+then the file offset is set to
+.IR offset .
+If there is no hole past
+.IR offset ,
+then the file offset is adjusted to the end of the file
+(i.e., there is an implicit hole at the end of any file).
+.PP
+In both of the above cases,
+.BR lseek ()
+fails if
+.I offset
+points past the end of the file.
+
+These operations allow applications to map holes in a sparsely
+allocated file.
+This can be useful for applications such as file backup tools,
+which can save space when creating backups and preserve holes,
+if they have a mechanism for discovering holes.
+
+For the purposes of these operations, a hole is a sequence of zeroes that
+(normally) has not been allocated in the underlying file storage.
+However, a file system is not obliged to report holes,
+so these operations are not a guaranteed mechanism for
+mapping the storage space actually allocated to a file.
+(Furthermore, a sequence of zeroes that actually has been written
+to the underlying storage normally won't be reported as a hole.)
+In the simplest implementation,
+a file system can support the operations by making
+.BR SEEK_HOLE
+always return the offset of the end of the file,
+and making
+.BR SEEK_DATA
+always return
+return
+.IR offset
+(i.e., even if the location referred to by
+.I offset
+is a hole,
+it can be considered to consist of data that is a sequence of zeroes).
+.\" https://lkml.org/lkml/2011/4/22/79
+.\" http://lwn.net/Articles/440255/
+.\" http://blogs.oracle.com/bonwick/entry/seek_hole_and_seek_data

 .SH "RETURN VALUE"
 Upon successful completion,
 .BR lseek ()
@@ -101,11 +164,14 @@ is not an open file descriptor.
 .TP
 .B EINVAL
 .I whence
-is not one of
-.BR SEEK_SET ,
-.BR SEEK_CUR ,
-.BR SEEK_END ;
-or the resulting file offset would be negative,
+is not valid (this error may be returned if
+.I whence
+is
+.BR SEEK_DATA
+or
+.BR SEEK_HOLE
+and the underlying file system does not support the operation).
+Or: the resulting file offset would be negative,
 or beyond the end of a seekable device.
 .\" Some systems may allow negative offsets for character devices
 .\" and/or for remote file systems.
@@ -118,8 +184,21 @@ The resulting file offset cannot be represented in an
 .B ESPIPE
 .I fd
 is associated with a pipe, socket, or FIFO.
+.TP
+.B ENXIO
+.I whence
+is
+.B SEEK_DATA
+or
+.BR SEEK_HOLE ,
+and the current file offset is beyond the end of the file.
 .SH "CONFORMING TO"
 SVr4, 4.3BSD, POSIX.1-2001.
+
+.BR SEEK_DATA
+and
+.BR SEEK_HOLE
+are nonstandard extensions also present in Solaris.
 .SH NOTES
 Some devices are incapable of seeking and POSIX does not specify which
 devices must support
--
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