diff --git a/photon/libc/sys/linux/fio.c b/photon/libc/sys/linux/fio.c
index ecaa7d9..3a89c4e 100644
--- a/photon/libc/sys/linux/fio.c
+++ b/photon/libc/sys/linux/fio.c
@@ -45,7 +45,9 @@ unsigned int __fio_flush(struct __io_file *f)
         f->err = 1;
     }
 
+    size_t flushed = f->buf_idx;
     f->buf_idx = 0;
+    return flushed;
 }
 
 int __fio_error(struct __io_file *f)