diff --git a/core-buffer/test/test_BinaryWriter.cpp b/core-buffer/test/test_BinaryWriter.cpp
index 23e9308..240403e 100644
--- a/core-buffer/test/test_BinaryWriter.cpp
+++ b/core-buffer/test/test_BinaryWriter.cpp
@@ -1,20 +1,86 @@
 #include <BinaryWriter.hpp>
+#include "BufferUtils.hpp"
+#include <fcntl.h>
 #include "gtest/gtest.h"
 
 TEST(BinaryWriter, basic_interaction)
 {
-    auto device_name = "test_device";
     auto root_folder = ".";
+    auto device_name = "test_device";
+    uint64_t pulse_id = 5;
 
     BinaryWriter writer(device_name, root_folder);
 
     JFFileFormat frame_data;
-    uint64_t pulse_id = 1;
-    uint64_t frame_id = 2;
-    uint32_t daq_rec = 3;
-    uint16_t n_recv_packets = 4;
+    frame_data.pulse_id = 1;
+    frame_data.frame_id = 2;
+    frame_data.daq_rec = 3;
+    frame_data.n_recv_packets = 4;
 
     ASSERT_EQ(frame_data.FORMAT_MARKER, JF_FORMAT_START_BYTE);
 
     writer.write(5, &frame_data);
+
+    auto output_filename =
+            BufferUtils::get_filename(root_folder, device_name, pulse_id);
+
+    auto read_fd = open(output_filename.c_str(), O_RDONLY);
+    ASSERT_NE(read_fd, -1);
+
+    auto file_frame_index = BufferUtils::get_file_frame_index(pulse_id);
+
+    JFFileFormat read_data;
+
+    ::lseek(read_fd, file_frame_index * sizeof(JFFileFormat), SEEK_SET);
+    ::read(read_fd, &read_data, sizeof(JFFileFormat));
+
+    ASSERT_EQ(frame_data.FORMAT_MARKER, JF_FORMAT_START_BYTE);
+    ASSERT_EQ(frame_data.FORMAT_MARKER, read_data.FORMAT_MARKER);
+    ASSERT_EQ(frame_data.pulse_id, read_data.pulse_id);
+    ASSERT_EQ(frame_data.frame_id, read_data.frame_id);
+    ASSERT_EQ(frame_data.daq_rec, read_data.daq_rec);
+    ASSERT_EQ(frame_data.n_recv_packets, read_data.n_recv_packets);
 }
+
+TEST(BinaryWriter, test_format_marker)
+{
+    auto root_folder = ".";
+    auto device_name = "test_device";
+    uint64_t pulse_id = 5;
+
+    BinaryWriter writer(device_name, root_folder);
+
+    JFFileFormat frame_data;
+    frame_data.pulse_id = 1;
+    frame_data.frame_id = 2;
+    frame_data.daq_rec = 3;
+    frame_data.n_recv_packets = 4;
+
+    writer.write(5, &frame_data);
+
+    auto output_filename =
+            BufferUtils::get_filename(root_folder, device_name, pulse_id);
+
+    auto read_fd = open(output_filename.c_str(), O_RDONLY);
+    ASSERT_NE(read_fd, -1);
+
+    auto file_frame_index = BufferUtils::get_file_frame_index(pulse_id);
+
+    JFFileFormat read_data;
+
+    // One frame before should be empty.
+    ::lseek(read_fd, (file_frame_index-1) * sizeof(JFFileFormat), SEEK_SET);
+    ::read(read_fd, &read_data, sizeof(JFFileFormat));
+    ASSERT_NE(read_data.FORMAT_MARKER, JF_FORMAT_START_BYTE);
+
+    // One frame after should be empty as well.
+    ::lseek(read_fd, (file_frame_index+1) * sizeof(JFFileFormat), SEEK_SET);
+    ::read(read_fd, &read_data, sizeof(JFFileFormat));
+    ASSERT_NE(read_data.FORMAT_MARKER, JF_FORMAT_START_BYTE);
+
+    // But this frame should be here.
+    ::lseek(read_fd, (file_frame_index) * sizeof(JFFileFormat), SEEK_SET);
+    ::read(read_fd, &read_data, sizeof(JFFileFormat));
+    ASSERT_EQ(read_data.FORMAT_MARKER, JF_FORMAT_START_BYTE);
+
+}
\ No newline at end of file