Teach dd(1) about an option to write sparse files. Can be useful for

things like diskless clients' swap files etc.

Submitted by:	pascal@zuo.dec.com (Pascal Pederiva) (ages ago, with many
						  stylistic changes by me)

4 files changed, 45 insertions, 7 deletions

diff --git a/bin/dd/args.c b/bin/dd/args.c
index c5a14f7..f1e556a 100644
--- a/bin/dd/args.c
+++ b/bin/dd/args.c
@@ -34,7 +34,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- *	$Id: args.c,v 1.8 1997/02/22 14:02:41 peter Exp $
+ *	$Id: args.c,v 1.9 1997/10/08 12:10:33 eivind Exp $
  */
 
 #ifndef lint
@@ -283,6 +283,7 @@ static struct conv {
 	{ "oldascii",	C_ASCII,	C_EBCDIC,	e2a_32V },
 	{ "oldebcdic",	C_EBCDIC,	C_ASCII,	a2e_32V },
 	{ "oldibm",	C_EBCDIC,	C_ASCII,	a2ibm_32V },
+	{ "sparse",	C_SPARSE,	0,		NULL },
 	{ "osync",	C_OSYNC,	C_BS,		NULL },
 	{ "swab",	C_SWAB,		0,		NULL },
 	{ "sync",	C_SYNC,		0,		NULL },
diff --git a/bin/dd/dd.1 b/bin/dd/dd.1
index 157e52e..85aca7b 100644
--- a/bin/dd/dd.1
+++ b/bin/dd/dd.1
@@ -33,7 +33,7 @@
 .\" SUCH DAMAGE.
 .\"
 .\"     @(#)dd.1	8.2 (Berkeley) 1/13/94
-.\"	$Id: dd.1,v 1.4 1997/02/22 14:02:43 peter Exp $
+.\"	$Id: dd.1,v 1.5 1997/04/28 04:07:29 steve Exp $
 .\"
 .Dd January 13, 1994
 .Dt DD 1
@@ -250,6 +250,13 @@ regularly sized blocks to be written.
 This option is incompatible with use of the
 .Cm bs= Ns Ar n
 block size specification.
+.It Cm sparse
+If one or more output blocks would consist solely of
+.Tn NUL
+bytes, try to seek the output file by the required space instead of
+filling them with
+.Tn NULs ,
+resulting in a sparse file.
 .It Cm swab
 Swap every pair of input bytes.
 If an input buffer has an odd number of bytes, the last byte will be
diff --git a/bin/dd/dd.c b/bin/dd/dd.c
index 60d7e67..bf14e16 100644
--- a/bin/dd/dd.c
+++ b/bin/dd/dd.c
@@ -34,7 +34,7 @@
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
- *	$Id: dd.c,v 1.10 1997/02/22 14:02:44 peter Exp $
+ *	$Id: dd.c,v 1.11 1997/08/19 19:46:18 jlemon Exp $
  */
 
 #ifndef lint
@@ -75,6 +75,7 @@ IO	in, out;		/* input/output state */
 STAT	st;			/* statistics */
 void	(*cfunc) __P((void));	/* conversion function */
 u_long	cpy_cnt;		/* # of blocks to copy */
+u_long	pending = 0;		/* pending seek if sparse */
 u_int	ddflags;		/* conversion options */
 u_int	cbsz;			/* conversion block size */
 u_int	files_cnt = 1;		/* # of files to copy */
@@ -347,7 +348,7 @@ dd_close()
 			memset(out.dbp, 0, out.dbsz - out.dbcnt);
 		out.dbcnt = out.dbsz;
 	}
-	if (out.dbcnt)
+	if (out.dbcnt || pending)
 		dd_out(1);
 }
 
@@ -356,7 +357,7 @@ dd_out(force)
 	int force;
 {
 	static int warned;
-	int cnt, n, nw;
+	int cnt, n, nw, i, sparse;
 	u_char *outp;
 
 	/*
@@ -378,7 +379,35 @@ dd_out(force)
 	outp = out.db;
 	for (n = force ? out.dbcnt : out.dbsz;; n = out.dbsz) {
 		for (cnt = n;; cnt -= nw) {
-			nw = write(out.fd, outp, cnt);
+			sparse = 0;
+			if (ddflags & C_SPARSE) {
+				sparse = 1;	/* Is buffer sparse? */
+				for (i = 0; i < cnt; i++)
+					if (outp[i] != 0) {
+						sparse = 0;
+						break;
+					}
+			}
+			if (sparse && !force) {
+				pending += cnt;
+				nw = cnt;
+			} else {
+				if (pending != 0) {
+					if (force)
+						pending--;
+					if (lseek (out.fd, pending, SEEK_CUR) == -1)
+						err(2, "%s: seek error creating sparse file",
+						    out.name);
+					if (force)
+						write(out.fd, outp, 1);
+					pending = 0;
+				}
+				if (cnt)
+					nw = write(out.fd, outp, cnt);
+				else
+					return;
+			}
+
 			if (nw <= 0) {
 				if (nw == 0)
 					errx(1, "%s: end of device", out.name);
diff --git a/bin/dd/dd.h b/bin/dd/dd.h
index 30904d9..477f444 100644
--- a/bin/dd/dd.h
+++ b/bin/dd/dd.h
@@ -35,7 +35,7 @@
  * SUCH DAMAGE.
  *
  *	@(#)dd.h	8.3 (Berkeley) 4/2/94
- *	$Id$
+ *	$Id: dd.h,v 1.6 1997/02/22 14:02:45 peter Exp $
  */
 
 /* Input/output stream state. */
@@ -95,3 +95,4 @@ typedef struct {
 #define	C_UCASE		0x40000
 #define	C_UNBLOCK	0x80000
 #define	C_OSYNC		0x100000
+#define	C_SPARSE	0x200000