diff --git a/CMakeLists.txt b/CMakeLists.txt
index af58e3e..0c3e4fe 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -26,6 +26,10 @@ check_symbol_exists(strsep "string.h" HAVE_STRSEP)
 if(HAVE_STRSEP)
   add_definitions(-DHAVE_STRSEP)
 endif()
+check_symbol_exists(fseeko "stdio.h" HAVE_FSEEKO)
+if(HAVE_FSEEKO)
+  add_definitions(-DHAVE_FSEEKO)
+endif()
 
 add_definitions(
   -DPACKAGE_VERSION="${PACKAGE_VERSION}"
diff --git a/src/parsley.cpp b/src/parsley.cpp
index e9d49bd..b218f44 100644
--- a/src/parsley.cpp
+++ b/src/parsley.cpp
@@ -1598,7 +1598,7 @@ void APar_ScanAtoms(const char *path, bool deepscan_REQ) {
           APar_TestCompatibleBrand(file, 0, dataSize);
         }
 
-        fseek(file, jump, SEEK_SET);
+        fseeko(file, jump, SEEK_SET);
 
         while (jump < file_size) {
 
@@ -5718,31 +5718,14 @@ void APar_MergeTempFile(FILE *dest_file,
     if (file_pos + max_buffer <= src_file_size) {
       APar_readX(buffer, src_file, file_pos, max_buffer);
 
-      // fseek(dest_file, dest_position + file_pos, SEEK_SET);
-#if defined(_WIN32)
-      fpos_t file_offset = dest_position + file_pos;
-#elif defined(__GLIBC__)
-      fpos_t file_offset = {0};
-      file_offset.__pos = dest_position + file_pos;
-#else
-      off_t file_offset = dest_position + file_pos;
-#endif
-      fsetpos(dest_file, &file_offset);
+      fseeko(dest_file, dest_position + file_pos, SEEK_SET);
       fwrite(buffer, max_buffer, 1, dest_file);
       file_pos += max_buffer;
 
     } else {
       APar_readX(buffer, src_file, file_pos, src_file_size - file_pos);
       // fprintf(stdout, "buff starts with %s\n", buffer+4);
-#if defined(_WIN32)
-      fpos_t file_offset = dest_position + file_pos;
-#elif defined(__GLIBC__)
-      fpos_t file_offset = {0};
-      file_offset.__pos = dest_position + file_pos;
-#else
-      off_t file_offset = dest_position + file_pos;
-#endif
-      fsetpos(dest_file, &file_offset);
+      fseeko(dest_file, dest_position + file_pos, SEEK_SET);
       fwrite(buffer, src_file_size - file_pos, 1, dest_file);
       file_pos += src_file_size - file_pos;
       break;
diff --git a/src/util.cpp b/src/util.cpp
index 98e302e..cb2c4b6 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -155,26 +155,29 @@ void TestFileExistence(const char *filePath, bool errorOut) {
   }
 }
 
-#if defined(_WIN32)
-
-///////////////////////////////////////////////////////////////////////////////////////
-//                                Win32 functions //
-///////////////////////////////////////////////////////////////////////////////////////
-
 #ifndef HAVE_FSEEKO
+#ifdef _WIN32
+int fseeko(FILE *stream, off_t pos, int whence) {
+  return _fseeki64(stream, pos, whence);
+}
 
-int fseeko(FILE *stream, uint64_t pos, int whence) { // only using SEEK_SET here
-  if (whence == SEEK_SET) {
-    fpos_t fpos = pos;
-    return fsetpos(stream, &fpos);
-  } else {
-    return -1;
-  }
-  return -1;
+off_t ftello(FILE *stream) { return _ftelli64(stream); }
+
+#else
+int fseeko(FILE *stream, off_t pos, int whence) {
+  return fseek(stream, pos, whence);
 }
 
+off_t ftello(FILE *stream) { return ftell(stream); }
+#endif
 #endif
 
+#if defined(_WIN32)
+
+///////////////////////////////////////////////////////////////////////////////////////
+//                                Win32 functions //
+///////////////////////////////////////////////////////////////////////////////////////
+
 /*----------------------
 APar_OpenFileWin32
         utf8_filepath - a pointer to a string (possibly utf8) of the full path
diff --git a/src/util.h b/src/util.h
index 616bbbc..674309a 100644
--- a/src/util.h
+++ b/src/util.h
@@ -47,10 +47,12 @@ FILE *APar_OpenFile(const char *utf8_filepath, const char *file_flags);
 FILE *APar_OpenISOBaseMediaFile(const char *file, bool open); // openSomeFile
 void TestFileExistence(const char *filePath, bool errorOut);
 
-#if defined(_WIN32)
 #ifndef HAVE_FSEEKO
-int fseeko(FILE *stream, uint64_t pos, int whence);
+int fseeko(FILE *stream, off_t pos, int whence);
+off_t ftello(FILE *stream);
 #endif
+
+#if defined(_WIN32)
 HANDLE APar_OpenFileWin32(const char *utf8_filepath,
                           DWORD dwDesiredAccess,
                           DWORD dwShareMode,