wc: line-buffer the printed counts

* src/wc.c (main): Set stdout to line buffered mode
to ensure parallel running instances don't intersperse
their output.  This adds 6.5% to the run time in the worst case
of many zero length files, but has neglible impact for
standard sized files.
* tests/misc/wc-parallel: New test for atomic output.
* tests/Makefile.am: Reference it.
* NEWS: Mention the fix
This is similar to commit 710fe413, 20-10-2009,
"md5sum, sha*sum, sum: line-buffer the printed checksums"

1 files changed, 4 insertions, 0 deletions

diff --git a/src/wc.c b/src/wc.c
index 52e899e97..48b5a4e39 100644
--- a/src/wc.c
+++ b/src/wc.c
@@ -598,6 +598,10 @@ main (int argc, char **argv)
 
   atexit (close_stdout);
 
+  /* Line buffer stdout to ensure lines are written atomically and immediately
+     so that processes running in parallel do not intersperse their output.  */
+  setvbuf (stdout, NULL, _IOLBF, 0);
+
   print_lines = print_words = print_chars = print_bytes = false;
   print_linelength = false;
   total_lines = total_words = total_chars = total_bytes = max_line_length = 0;