1 files changed, 22 insertions, 0 deletions

diff --git a/lib/libc/sys/fsync.2 b/lib/libc/sys/fsync.2
index 234ea6e996ed..1116cf9b3605 100644
--- a/lib/libc/sys/fsync.2
+++ b/lib/libc/sys/fsync.2
@@ -53,6 +53,28 @@ of buffers for the associated file to be written to a disk.
 should be used by programs that require a file to be
 in a known state, for example, in building a simple transaction
 facility.
+.Sh IMPLEMENTATION NOTES
+.Pp
+In the non-threaded library
+.Fn fsync
+is implemented as the
+.Va fsync
+syscall.
+.Pp
+In the threaded library, the
+.Va fsync
+syscall is assembled to
+.Fn _thread_sys_fsync
+and
+.Fn fsync
+is implemented as a function which locks
+.Va fd
+for read and write, then calls
+.Fn _thread_sys_fsync .
+Before returning,
+.Fn fsync
+unlocks
+.Va fd .
 .Sh RETURN VALUES
 A 0 value is returned on success.  A -1 value indicates
 an error.