1 files changed, 7 insertions, 1 deletions
diff --git a/io.c b/io.c
index 8d8bfe172b..12ea578dde 100644
--- a/io.c
+++ b/io.c
@@ -320,9 +320,15 @@ io_ungetc(VALUE str, rb_io_t *fptr)
 	    fptr->rbuf_capa = 8192;
         fptr->rbuf = ALLOC_N(char, fptr->rbuf_capa);
     }
-    if (fptr->rbuf_off < len) {
+    if (fptr->rbuf_capa < len + fptr->rbuf_len) {
 	rb_raise(rb_eIOError, "ungetc failed");
     }
+    if (fptr->rbuf_off < len) {
+        MEMMOVE(fptr->rbuf+fptr->rbuf_capa-fptr->rbuf_len,
+                fptr->rbuf+fptr->rbuf_off,
+                char, fptr->rbuf_len);
+        fptr->rbuf_off = fptr->rbuf_capa-fptr->rbuf_len;
+    }
     fptr->rbuf_off-=len;
     fptr->rbuf_len+=len;
     MEMMOVE(fptr->rbuf+fptr->rbuf_off, RSTRING_PTR(str), char, len);