diff --git a/sim/mips/sky-pke.c b/sim/mips/sky-pke.c
index 804b37ba34c..f6dafec929c 100644
--- a/sim/mips/sky-pke.c
+++ b/sim/mips/sky-pke.c
@@ -898,7 +898,7 @@ pke_pc_advance(struct pke_device* me, int num_words)
          if (( me->fifo_trace_file == NULL) &&
              ( me->fifo_trace_file_name != NULL ))
            sky_open_file (&me->fifo_trace_file, me->fifo_trace_file_name,
-                         (char *) NULL);
+                         (char *) NULL, _IOLBF );
 
          /* assert complete classification */
               ASSERT(fq->word_class[3] != wc_unknown);
diff --git a/sim/mips/sky-pke.h b/sim/mips/sky-pke.h
index 27347e1ef55..df804510c3a 100644
--- a/sim/mips/sky-pke.h
+++ b/sim/mips/sky-pke.h
@@ -338,7 +338,7 @@ do { \
                  ((me)->fifo_trace_file_name != NULL )) \
                sky_open_file (&((me)->fifo_trace_file), \
                               (me)->fifo_trace_file_name, \
-                              (char *) NULL); \
+                              (char *) NULL, _IOLBF ); \
              fprintf (((me)->fifo_trace_file != NULL) ? \
                       (me)->fifo_trace_file : stdout, \
                       "# Reg %s:%s = 0x%x\n", #reg, #flag, (unsigned)(value)); \
@@ -472,7 +472,7 @@ int read_pke_pcx (struct pke_device *device, void *buf);
                  ((me)->fifo_trace_file_name != NULL )) \
                sky_open_file (&((me)->fifo_trace_file), \
                               (me)->fifo_trace_file_name, \
-                              (char *) NULL); \
+                              (char *) NULL, _IOLBF ); \
              fprintf (((me)->fifo_trace_file != NULL) ? \
                       (me)->fifo_trace_file : stdout, \
                       "# Write %2d bytes  to  ", size); \