add example program

Author: perazz

example/system/CMakeLists.txt

@@ -1,4 +1,5 @@
 ADD_EXAMPLE(get_runtime_os)
+ADD_EXAMPLE(null_device)
 ADD_EXAMPLE(os_type)
 ADD_EXAMPLE(process_1)
 ADD_EXAMPLE(process_2)

example/system/example_null_device.f90

@@ -0,0 +1,20 @@
+! Showcase usage of the null device
+program example_null_device
+    use stdlib_system, only: null_device
+    use iso_fortran_env, only: output_unit
+    implicit none
+    integer :: unit 
+    logical :: screen_output = .false.
+
+    if (screen_output) then 
+       unit = output_unit
+    else
+       ! Write to the null device if no screen output is wanted
+       open(newunit=unit,file=null_device())
+    endif     
+
+    write(unit,*) "Hello, world!" 
+
+    if (.not.screen_output) close(unit) 
+
+end program example_null_device