On Sat Nov 19 11, Tim Kientzle wrote:
>
> On Nov 18, 2011, at 12:31 PM, Alexander Best wrote:
>
> > On Fri Nov 18 11, Tim Kientzle wrote:
> >>
> >> Take a look at
> >>
> >> http://libarchive.googlecode.com/svn/trunk/libarchive/archive_read_open_filename.c
> >>
> >> Especially the comments about detecting "disk-like" devices.
> >> I rewrote a bunch of this code to introduce an explicit
> >> notion of "strategy" so that we could optimize access
> >> to a variety of different devices.
> >>
> >> This code has a notion of "disk-like" file descriptors and
> >> some optimizations for such. There are some comments
> >> in there outlining similar optimizations that could be made
> >> for "tape-like" or "socket-like" devices.
> >
> > great you posted that file as reference. i believe most of the stuff done
> > there
> > should actually be done within lseek().
>
> Libarchive runs on a lot of systems other than FreeBSD.
> FreeBSD is not the only Unix-like system with this issue,
> so that code isn't going to go out of libarchive regardless.
>
> If you think those same ideas can be used in dd or hd
> to speed them up, please send your patches.
i'd like to propose the followup patch for hexdump(1). basically,
the logic behind is is this:
1) if the file argument is a fifo, pipe or socket -- goto 4)
2) if the file argument is a tape drive -- goto 4)
3) for all other cases try fseeko(), if that fails -- goto 4)
4) use getchar()
you should notice a dramtic increase in speed from something like the
following:
'hexdump -s 500m -n32 /dev/random'
cheers.
alex
>
> The key point: You cannot unconditionally call lseek()
> to skip over data. Instead, treat lseek() as an optimization
> that can be used under some circumstances. The
> question then becomes one of figuring out when
> that optimization can be enabled.
>
> Tim
>
diff --git a/usr.bin/hexdump/display.c b/usr.bin/hexdump/display.c
index 991509d..8c8b065 100644
--- a/usr.bin/hexdump/display.c
+++ b/usr.bin/hexdump/display.c
@@ -35,8 +35,10 @@ static char sccsid[] = "@(#)display.c 8.1 (Berkeley)
6/6/93";
#include <sys/cdefs.h>
__FBSDID("$FreeBSD$");
+#include <sys/ioctl.h>
#include <sys/param.h>
#include <sys/stat.h>
+#include <sys/conf.h>
#include <ctype.h>
#include <err.h>
@@ -368,7 +370,7 @@ next(char **argv)
void
doskip(const char *fname, int statok)
{
- int cnt;
+ int type;
struct stat sb;
if (statok) {
@@ -380,16 +382,38 @@ doskip(const char *fname, int statok)
return;
}
}
- if (S_ISREG(sb.st_mode)) {
- if (fseeko(stdin, skip, SEEK_SET))
+ if (S_ISFIFO(sb.st_mode) || S_ISSOCK(sb.st_mode)) {
+ noseek();
+ return;
+ }
+ if (S_ISCHR(sb.st_mode) || S_ISBLK(sb.st_mode)) {
+ if (ioctl(fileno(stdin), FIODTYPE, &type))
err(1, "%s", fname);
- address += skip;
- skip = 0;
- } else {
- for (cnt = 0; cnt < skip; ++cnt)
- if (getchar() == EOF)
- break;
- address += cnt;
- skip -= cnt;
+ /*
+ * Most tape drives don't support seeking,
+ * yet fseeko() would succeed.
+ */
+ if (type & D_TAPE) {
+ noseek();
+ return;
+ }
+ }
+ if (fseeko(stdin, skip, SEEK_SET)) {
+ noseek();
+ return;
}
+ address += skip;
+ skip = 0;
+}
+
+void
+noseek(void)
+{
+ int count;
+
+ for (count = 0; count < skip; ++count)
+ if (getchar() == EOF)
+ break;
+ address += count;
+ skip -= count;
}
diff --git a/usr.bin/hexdump/hexdump.h b/usr.bin/hexdump/hexdump.h
index be85bd9..1d4bb85 100644
--- a/usr.bin/hexdump/hexdump.h
+++ b/usr.bin/hexdump/hexdump.h
@@ -97,6 +97,7 @@ u_char *get(void);
void newsyntax(int, char ***);
int next(char **);
void nomem(void);
+void noseek(void);
void oldsyntax(int, char ***);
size_t peek(u_char *, size_t);
void rewrite(FS *);
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