On 05/02/2023 18:27, Stephane Chazelas wrote:
"wc -c" without filename arguments is meant to read stdin til
EOF and report the number of bytes it has read.
When stdin is on a regular file, GNU wc has that optimisation
whereby it skips the reading, does a pos = lseek(0,0,SEEK_CUR)
to find out its current position within the file, fstat(0) and
reports st_size - pos (assuming st_size > pos).
However, it does not move the position to the end of the file.
That means for instance that:
$ echo test > file
$ { wc -c; wc -c; } < file
5
5
Instead of 5, then 0:
$ { wc -c; cat; } < file
5
test
So the optimisation is incomplete.
It also reports the size of the file even if it could not possibly read it
because it's not open in read mode:
{ wc -c; } 0>> file
5
IMO, it should only do the optimisation if
- fcntl(F_GETFL) to check that the file is opened in O_RDONLY or O_RDWR
- current checks for /proc /sys-like filesystems
- pos > st_size
- lseek(0,st_size,SEEK_POS) is successful.
(that leaves a race window above where it could move the cursor
backward, but I would think that can be ignored as if something
else reads at the same time, there's not much we can expect
anyway).
Yes I agree.
Adjusting would also avoid the following inconsistencies:
$ { wc -c; wc -c; } < file
5
5
$ { wc -l; wc -l; } < file
1
0
$ truncate -s $(getconf PAGESIZE) file
$ { wc -c; wc -c; } < file
4096
0
Hopefully the attached addresses this.
Note it doesn't add the constraint on the input being readable,
which I'll think a bit more about.
cheers,
PádraigFrom 42f72ec424e7eecd6b56c5b6fca5f377ff73795b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A1draig=20Brady?= <p...@draigbrady.com>
Date: Sun, 5 Feb 2023 19:52:31 +0000
Subject: [PATCH] wc: ensure we update file offset
* src/wc.c (wc): Update the offset when not reading,
and do read if we can't update the offset.
* tests/misc/wc-proc.sh: Add a test case.
* NEWS: Mention the bug fix.
Fixes https://bugs.gnu.org/61300
---
NEWS | 4 ++++
src/wc.c | 5 ++++-
tests/misc/wc-proc.sh | 12 ++++++++++++
3 files changed, 20 insertions(+), 1 deletion(-)
diff --git a/NEWS b/NEWS
index b3cde4a01..1cea8cc32 100644
--- a/NEWS
+++ b/NEWS
@@ -57,6 +57,10 @@ GNU coreutils NEWS -*- outline -*-
sized files larger than SIZE_MAX.
[bug introduced in coreutils-8.24]
+ `wc -c` will again correctly update the read offset of inputs.
+ Previously it deduced the size of inputs while leaving the offset unchanged.
+ [bug introduced in coreutils-8.27]
+
** Changes in behavior
Programs now support the new Ronna (R), and Quetta (Q) SI prefixes,
diff --git a/src/wc.c b/src/wc.c
index 5f3ef6eee..de04612e9 100644
--- a/src/wc.c
+++ b/src/wc.c
@@ -446,7 +446,10 @@ wc (int fd, char const *file_x, struct fstatus *fstatus, off_t current_pos)
beyond the end of the file. As in the example above. */
bytes = end_pos < current_pos ? 0 : end_pos - current_pos;
- skip_read = true;
+ if (bytes && 0 <= lseek (fd, bytes, SEEK_CUR))
+ skip_read = true;
+ else
+ bytes = 0;
}
else
{
diff --git a/tests/misc/wc-proc.sh b/tests/misc/wc-proc.sh
index 5eb43b982..2b5026405 100755
--- a/tests/misc/wc-proc.sh
+++ b/tests/misc/wc-proc.sh
@@ -42,6 +42,18 @@ cat <<\EOF > exp
EOF
compare exp out || fail=1
+# Ensure we update the offset even when not reading,
+# which wasn't the case from coreutils-8.27 to coreutils-9.1
+{ wc -c; wc -c; } < no_read > out || fail=1
+{ wc -c; wc -c; } < do_read >> out || fail=1
+cat <<\EOF > exp
+2
+0
+1048576
+0
+EOF
+compare exp out || fail=1
+
# Ensure we don't read too much when reading,
# as was the case on 32 bit systems
# from coreutils-8.24 to coreutils-9.1
--
2.26.2
