diff --git a/src/util/memstream.c b/src/util/memstream.c
index afbf13fb7b8..bd909b78ebf 100644
--- a/src/util/memstream.c
+++ b/src/util/memstream.c
@@ -53,6 +53,8 @@ u_memstream_open(struct u_memstream *mem, char **bufp, size_t *sizep)
          success = f != NULL;
          if (success)
          {
+            *bufp = NULL;
+            *sizep = 0;
             mem->f = f;
             mem->bufp = bufp;
             mem->sizep = sizep;
@@ -68,27 +70,45 @@ u_memstream_open(struct u_memstream *mem, char **bufp, size_t *sizep)
 #endif
 }
 
+#ifdef _WIN32
+static void
+u_memstream_update_buffer(struct u_memstream *mem)
+{
+   FILE *const f = mem->f;
+   long size = ftell(f);
+   if (size > 0) {
+      /* reserve space for the null terminator as well */
+      if (*mem->bufp == NULL || *mem->sizep < size + 1)
+         *mem->bufp = realloc(*mem->bufp, size + 1);
+
+      fseek(f, 0, SEEK_SET);
+      fread(*mem->bufp, 1, size, f);
+      (*mem->bufp)[size] = '\0';
+   }
+   *mem->sizep = size;
+}
+#endif /* _WIN32 */
+
 void
 u_memstream_close(struct u_memstream *mem)
 {
    FILE *const f = mem->f;
 
 #ifdef _WIN32
-   long size = ftell(f);
-   if (size > 0) {
-      /* reserve space for the null terminator */
-      char *buf = malloc(size + 1);
-      fseek(f, 0, SEEK_SET);
-      fread(buf, 1, size, f);
-      /* insert null terminator */
-      buf[size] = '\0';
-
-      *mem->bufp = buf;
-      *mem->sizep = size;
-   }
-
+   u_memstream_update_buffer(mem);
    remove(mem->temp);
 #endif
 
    fclose(f);
 }
+
+int
+u_memstream_flush(struct u_memstream *mem)
+{
+#ifndef _WIN32
+   return fflush(mem->f);
+#else
+   u_memstream_update_buffer(mem);
+   return 0;
+#endif /* _WIN32 */
+}
diff --git a/src/util/memstream.h b/src/util/memstream.h
index 4dcb4a04d20..42f374cd69b 100644
--- a/src/util/memstream.h
+++ b/src/util/memstream.h
@@ -65,6 +65,9 @@ u_memstream_get(const struct u_memstream *mem)
    return mem->f;
 }
 
+int
+u_memstream_flush(struct u_memstream *mem);
+
 #ifdef __cplusplus
 }
 #endif