ios.fsync   → 0 or nil

Immediately writes all buffered data in ios to disk. Returns nil if the underlying operating system does not support fsync(2). Note that fsync differs from using IO#sync=. The latter ensures that data is flushed from Ruby’s buffers, but doesn’t not guarantee that the underlying operating system actually writes it to disk.

Source Code

/*
*  call-seq:
*     ios.fsync   => 0 or nil
*  
*  Immediately writes all buffered data in <em>ios</em> to disk.
*  Returns <code>nil</code> if the underlying operating system does not
*  support <em>fsync(2)</em>. Note that <code>fsync</code> differs from
*  using <code>IO#sync=</code>. The latter ensures that data is flushed
*  from Ruby's buffers, but doesn't not guarantee that the underlying
*  operating system actually writes it to disk.
*/

static VALUE
rb_io_fsync(io)
   VALUE io;
{
#ifdef HAVE_FSYNC
   OpenFile *fptr;
   FILE *f;

   GetOpenFile(io, fptr);
   f = GetWriteFile(fptr);

   io_fflush(f, fptr);
   if (fsync(fileno(f)) < 0)
       rb_sys_fail(fptr->path);
   return INT2FIX(0);
#else
   rb_notimplement();
   return Qnil;                /* not reached */
#endif
}