From: Jim Meyering <jim@meyering.net>
Date: Tue, 22 Aug 2000 11:12:01 +0000 (+0000)
Subject: Don't even try to use lseek on character devices.
X-Git-Tag: FILEUTILS-4_0z~126
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=ba9af72e94e7441723352a30fba4847654881071;p=thirdparty%2Fcoreutils.git

Don't even try to use lseek on character devices.
(buggy_lseek_support): New function.
(skip): Use it.
Reported by Martin Gallant via Michael Stone.
---

diff --git a/src/dd.c b/src/dd.c
index 2a89b6b014..ef5a9ed626 100644
--- a/src/dd.c
+++ b/src/dd.c
@@ -718,6 +718,37 @@ swab_buffer (unsigned char *buf, size_t *nread)
   return ++bufstart;
 }
 
+/* Return nonzero iff the file referenced by FDESC is of a type for
+   which lseek's return value is known to be invalid on some systems.
+   Otherwise, return zero.
+   For example, return nonzero if FDESC references a character device
+   (on any system) because the lseek on many Linux systems incorrectly
+   returns `0' for tape devices, even though the function fails to
+   perform the requested operation.  In that case, lseek should return
+   nonzero and set errno.  */
+
+static int
+buggy_lseek_support (int fdesc)
+{
+  /* We have to resort to this because on some systems, lseek doesn't work
+     on some special files but doesn't return an error, either.
+     In particular, the Linux tape drivers are a problem.
+     For example, when I did the following using dd-4.0y or earlier on a
+     Linux-2.2.17 system with a Exabyte SCSI tape drive:
+
+       dev=/dev/nst0
+       reset='mt -f $dev rewind; mt -f $dev fsf 1'
+       eval $reset; dd if=$dev bs=32k of=out1
+       eval $reset; dd if=$dev bs=32k of=out2 skip=1
+
+     the resulting files, out1 and out2, would compare equal.  */
+
+  struct stat stats;
+
+  return (fstat (fdesc, &stats) == 0
+	  && (S_ISCHR (stats.st_mode)));
+}
+
 /* Throw away RECORDS blocks of BLOCKSIZE bytes on file descriptor FDESC,
    which is open with read permission for FILE.  Store up to BLOCKSIZE
    bytes of the data at a time in BUF, if necessary. */
@@ -732,6 +763,7 @@ skip (int fdesc, char *file, uintmax_t records, size_t blocksize,
      operation, fall back on using read.  */
   o = records * blocksize;
   if (o / blocksize != records
+      || buggy_lseek_support (fdesc)
       || lseek (fdesc, o, SEEK_SET) == -1)
     {
       while (records-- > 0)