Issue 4957
Let os.ftruncate raise OSError like documented.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index a1c3444..915d30d 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6633,7 +6633,7 @@
 	res = ftruncate(fd, length);
 	Py_END_ALLOW_THREADS
 	if (res < 0) {
-		PyErr_SetFromErrno(PyExc_IOError);
+		posix_error();
 		return NULL;
 	}
 	Py_INCREF(Py_None);