First draft of new "--sync" option.

doc/NEWS.md

@@ -1,4 +1,5 @@
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+ * feature: new "`--sync`" option to flush cache to disk after every write (related to [GH#6](https://github.com/a-j-wood/pv/issues/6), to improve accuracy when writing to slow disks)
  * fix: correct byte prefix size to 2 spaces in rate display, so progress display size remains constant at low transfer rates
  * cleanup: rewrote `configure.in` as per suggestions in newer "`autoconf`" manuals
  * cleanup: replaced `header.in` with one generated by "`autoheader`", moving custom logic to a separate header file "`config-aux.h`"

doc/VERSION

@@ -1 +1 @@
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doc/quickref.1.in

@@ -395,6 +395,13 @@ If a size was specified with
 stop transferring data once that many bytes have been written, instead of
 continuing to the end of input.
 .TP
+.B \-Y, \-\-sync
+After every write operation, synchronise the buffer caches to disk - see
+.BR fdatasync (2).
+This has no effect when the output is a pipe.  Using
+.B \-Y
+may improve the accuracy of the progress bar when writing to a slow disk.
+.TP
 .B \-d PID[:FD], \-\-watchfd PID[:FD]
 Instead of transferring data, watch file descriptor
 .B FD