diff --git a/src/pyfmi/fmi_util.pxd b/src/pyfmi/fmi_util.pxd
index cbb98c55..ea434fa4 100644
--- a/src/pyfmi/fmi_util.pxd
+++ b/src/pyfmi/fmi_util.pxd
@@ -29,26 +29,14 @@ cimport pyfmi.fmi3 as FMI3
     This is because fseek/ftell is not sufficient as soon as the number of bytes in a result file
     exceed the maximum value for long int.
 """
-IF UNAME_SYSNAME == "Windows":
-    cdef extern from "stdio.h" nogil:
-        ctypedef struct FILE:
-            pass
-        long long _ftelli64(FILE *stream)
-        int _fseeki64(FILE *stream, long long offset, int whence)
-    cdef inline int os_specific_fseek(FILE *stream, long long offset, int whence):
-        return _fseeki64(stream, offset, whence)
-    cdef inline long long os_specific_ftell(FILE *stream):
-        return _ftelli64(stream)
-ELSE:
-    cdef extern from "stdio.h" nogil:
-        ctypedef struct FILE:
-            pass
-        long long ftello(FILE *stream)
-        int fseeko(FILE *stream, long long offset, int whence)
-    cdef inline int os_specific_fseek(FILE *stream, long long offset, int whence):
-        return fseeko(stream, offset, whence)
-    cdef inline long long os_specific_ftell(FILE *stream):
-        return ftello(stream)
+cdef extern from "fmi_util_os.h" nogil:
+    int os_fseek(FILE *stream, long long offset, int whence)
+    long long os_ftell(FILE *stream)
+
+cdef inline int os_specific_fseek(FILE *stream, long long offset, int whence) nogil:
+    return os_fseek(stream, offset, whence)
+cdef inline long long os_specific_ftell(FILE *stream) nogil:
+    return os_ftell(stream)
 
 cdef class DumpDataFMI3:
     cdef np.ndarray time_tmp
diff --git a/src/pyfmi/fmi_util_os.h b/src/pyfmi/fmi_util_os.h
new file mode 100644
index 00000000..bddc698c
--- /dev/null
+++ b/src/pyfmi/fmi_util_os.h
@@ -0,0 +1,14 @@
+#ifndef FMI_UTIL_OS_H
+#define FMI_UTIL_OS_H
+
+#include <stdio.h>
+
+#if defined(_WIN32)
+    #define os_fseek _fseeki64
+    #define os_ftell _ftelli64
+#else
+    #define os_fseek fseeko
+    #define os_ftell ftello
+#endif
+
+#endif
diff --git a/src/pyfmi/master.pyx b/src/pyfmi/master.pyx
index 531de547..5ffde493 100644
--- a/src/pyfmi/master.pyx
+++ b/src/pyfmi/master.pyx
@@ -44,7 +44,7 @@ from pyfmi.fmi2 import FMI2_CONTINUOUS, FMI2_INPUT, FMI2_OUTPUT
 from pyfmi.fmi_util import Graph
 from pyfmi.exceptions import FMUException, InvalidFMUException
 
-IF WITH_OPENMP:
+if WITH_OPENMP:
     cimport openmp
 
 DEF SERIAL   = 0
@@ -1643,7 +1643,7 @@ cdef class Master:
         
         if options["num_threads"] and options["execution"] == "parallel":
             pass
-            IF WITH_OPENMP: 
+            if WITH_OPENMP:
                 openmp.omp_set_num_threads(options["num_threads"])
         if options["step_size"] <= 0.0:
             raise FMUException("The step-size must be greater than zero.")