uniq: avoid locking standard output when printing line counts with -c

This increases throughput on systems with unlocked stdio functions.
E.g., 2.5x in the following case:

    $ timeout 1m taskset 1 shuf -r -i 0-100 \
        | taskset 2 uniq -c \
        | taskset 3 pv -r > /dev/null
    [ 100MiB/s]
    $ timeout 1m taskset 1 shuf -r -i 0-100 \
        | taskset 2 uniq -c \
        | taskset 3 pv -r > /dev/null
    [ 246MiB/s]

* src/uniq.c: Include assure.h.
(writeline): Convert the value to a string by hand and use fwrite, which
may be unlocked, instead of printf.
* tests/uniq/uniq-c-width.sh: New test to verify the padding logic.
* tests/local.mk (all_tests): Add the test case.
* NEWS: Mention the improvement.

NEWS

@@ -59,6 +59,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   'sort' will now better use available memory and parallel operation
   when reading from unknown sized inputs like pipes.
 
+  'uniq -c' now operates up to 2.5x times faster on systems with unlocked stdio
+  functions.
+
+
 ** Build-related
 
   configure no longer accepts the --with-linux-crypto option, which allowed

src/uniq.c

@@ -23,6 +23,7 @@
 
 #include "system.h"
 #include "argmatch.h"
+#include "assure.h"
 #include "linebuffer.h"
 #include "fadvise.h"
 #include "mcel.h"
@@ -324,7 +325,23 @@ writeline (struct linebuffer const *line,
     return;
 
   if (count_occurrences)
-    printf ("%7jd ", linecount + 1);
+    {
+      char buf[7 + INT_BUFSIZE_BOUND (intmax_t)];
+      char *end = buf + sizeof buf;
+      char *p = end;
+      *--p = ' ';
+      intmax_t i = linecount + 1;
+      affirm (0 < i);
+      do
+        *--p = '0' + i % 10;
+      while ((i /= 10));
+      /* 7 characters, padded with spaces if LINECOUNT + 1 is too small.  */
+      while (end - p < 8)
+        *--p = ' ';
+      const idx_t nbytes = end - p;
+      if (fwrite (p, sizeof (char), nbytes, stdout) != nbytes)
+        write_error ();
+    }
 
   if (fwrite (line->buffer, sizeof (char), line->length, stdout)
       != line->length)

tests/local.mk

@@ -531,6 +531,7 @@ all_tests =					\
   tests/uniq/uniq.pl				\
   tests/uniq/uniq-perf.sh			\
   tests/uniq/uniq-collate.sh			\
+  tests/uniq/uniq-c-width.sh			\
   tests/misc/xattr.sh				\
   tests/misc/yes.sh				\
   tests/tail/wait.sh				\

tests/uniq/uniq-c-width.sh

@@ -0,0 +1,33 @@
+#!/bin/sh
+# Test 'uniq -c' with various integer widths.
+
+# 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/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ uniq
+expensive_
+
+count=$(shuf -i 10000000-99999999 -n 1) || framework_failure_
+
+while test $count -gt 0; do
+  printf '%7d y\n' $count >exp || framework_failure_
+  yes | head -n $count | uniq -c >out 2>err || fail=1
+  compare exp out || fail=1
+  compare /dev/null err || fail=1
+  count=$((count / 10))
+done
+
+Exit $fail