On 09/03/2026 21:53, Laszlo Ersek wrote:
Hello,
with "dd" built from coreutils @ c5fb1c26de05 ("build: update gnulib
submodule to latest", 2026-03-09), with the gnulib submodule advanced to
d5f683434d1a ("doc: Fix documentation that was added today.",
2026-03-09):
(
set -e
ulimit -S -f 1024
trap '' XFSZ
rm -f f
src/dd if=/dev/urandom of=f bs=768K
)
The above command produces the regular file "f" with 1024*1024 bytes in
it (as expected); however, "dd" prints the following to stderr:
dd: error writing 'f': File too large
2+0 records in
1+0 records out
1048576 bytes (1.0 MB, 1.0 MiB) copied, 0.00625178 s, 168 MB/s
The third line ("1+0 records out") is incorrect. It should be "1+1
records out", because the second (final) write outputs 256*1024 bytes.
According to POSIX
<https://pubs.opengroup.org/onlinepubs/9799919799/utilities/dd.html#tag_20_31_11>,
that counts as a partial output block:
On completion, /dd/ shall write the number of input and output blocks
to standard error. In the POSIX locale the following formats shall be
used:
[...]
"%u+%u records out\n", </number of whole output blocks/>, </number of
partial output blocks/>
[...] A partial output block is one that was written with fewer bytes
than specified by the output block size. [...]
In dd_copy(), we have
2321 if (ibuf == obuf) /* If not C_TWOBUFS. */
2322 {
2323 size_t nwritten = iwrite (STDOUT_FILENO, obuf,
n_bytes_read);
2324 w_bytes += nwritten;
2325 if (nwritten != n_bytes_read)
2326 {
2327 error (0, errno, _("error writing %s"), quoteaf
(output_file));
2328 return EXIT_FAILURE;
2329 }
2330 else if (n_bytes_read == input_blocksize)
2331 w_full++;
2332 else
2333 w_partial++;
2334 continue;
2335 }
The first execution of this code outputs a full block ("nwritten" ==
768*1024 bytes). The second execution outputs "nwritten" == 256*1024
bytes, with "n_bytes_read" == 768*1024 bytes; yet "w_partial" is not
incremented.
The condition for reaching "w_partial++" is
nwritten == n_bytes_read &&
n_bytes_read != input_blocksize
which seems correct to me (it means we managed to output everything we
just read, but we couldn't read a full block -- and therefore we also
couldn't write a (-n identically sized) full block). However, a partial
write can also occur when the read was complete.
The write_output() function (which is not used in this reproducer)
counts partial output records differently:
1275 static void
1276 write_output (void)
1277 {
1278 size_t nwritten = iwrite (STDOUT_FILENO, obuf, output_blocksize);
1279 w_bytes += nwritten;
1280 if (nwritten != output_blocksize)
1281 {
1282 error (0, errno, _("writing to %s"), quoteaf (output_file));
1283 if (nwritten != 0)
1284 w_partial++;
1285 quit (EXIT_FAILURE);
1286 }
1287 else
1288 w_full++;
1289 oc = 0;
1290 }
If we detect a short -- but not entirely fruitless -- write in this
function, then we bump "w_partial" between error() and quit().
The symptom is also reproducible by populating a block device with "dd"
such that the last successful write has no room for a full (output)
block. (In that case, ENOSPC is reported, rather than EFBIG.) That's in
fact how I first encountered the problem; the "ignored SIGXFSZ + EFBIG
errno" method is just a more convenient reproducer.
Thanks for the excellent report and reproducer.
This looks to be a bug since the original implementation.
Attached is a fix and test.
Marking this as done.
thanks!
Padraig
From 1c2518816333d408eae11460ff577e78dc8ebf67 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A1draig=20Brady?= <[email protected]>
Date: Wed, 11 Mar 2026 15:39:20 +0000
Subject: [PATCH] dd: always diagnose partial writes on write failure
* src/dd.c (dd_copy): Increment the partial write count upon failure.
* tests/dd/partial-write.sh: Add a new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the bug fix.
Fixes https://bugs.gnu.org/80583
---
NEWS | 4 ++++
src/dd.c | 2 ++
tests/dd/partial-write.sh | 33 +++++++++++++++++++++++++++++++++
tests/local.mk | 1 +
4 files changed, 40 insertions(+)
create mode 100755 tests/dd/partial-write.sh
diff --git a/NEWS b/NEWS
index 97ed7e721..17532a0e5 100644
--- a/NEWS
+++ b/NEWS
@@ -4,6 +4,10 @@ GNU coreutils NEWS -*- outline -*-
** Bug fixes
+ 'dd' now always diagnoses partial writes correctly upon write failure.
+ Previously it may have indicated that only full writes were performed.
+ [This bug was present in "the beginning".]
+
'fold' will no longer truncate output when encountering 0xFF bytes.
[bug introduced in coreutils-9.8]
diff --git a/src/dd.c b/src/dd.c
index 07b1c6445..5451b0ac9 100644
--- a/src/dd.c
+++ b/src/dd.c
@@ -2264,6 +2264,8 @@ dd_copy (void)
if (nwritten != n_bytes_read)
{
diagnose (errno, _("error writing %s"), quoteaf (output_file));
+ if (nwritten != 0)
+ w_partial++;
return EXIT_FAILURE;
}
else if (n_bytes_read == input_blocksize)
diff --git a/tests/dd/partial-write.sh b/tests/dd/partial-write.sh
new file mode 100755
index 000000000..7b2859472
--- /dev/null
+++ b/tests/dd/partial-write.sh
@@ -0,0 +1,33 @@
+#!/bin/sh
+# Ensure partial writes are properly diagnosed
+
+# Copyright (C) 2026 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ dd
+
+(
+ ulimit -S -f 1024 || skip_ 'unable to set file size ulimit'
+ trap '' XFSZ || skip_ 'unable to ignore SIGXFSZ'
+ dd if=/dev/zero of=f bs=768K count=2 2>err
+ echo $? > dd.ret
+)
+
+if test "$(cat dd.ret)" = 1; then
+ grep -F '+1 records out' err || { cat err; fail=1; }
+fi
+
+Exit $fail
diff --git a/tests/local.mk b/tests/local.mk
index 15766df16..f9cbf9a5d 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -612,6 +612,7 @@ all_tests = \
tests/dd/nocache_eof.sh \
tests/dd/nocache_fail.sh \
tests/dd/not-rewound.sh \
+ tests/dd/partial-write.sh \
tests/dd/reblock.sh \
tests/dd/skip-seek.pl \
tests/dd/skip-seek2.sh \
--
2.53.0
