add callback example

Author: perazz

example/system/CMakeLists.txt

@@ -3,3 +3,4 @@ ADD_EXAMPLE(process_2)
 ADD_EXAMPLE(process_3)
 ADD_EXAMPLE(process_4)
 ADD_EXAMPLE(process_5)
+ADD_EXAMPLE(process_6)

example/system/example_process_6.f90

@@ -0,0 +1,45 @@
+! Process example 6: Demonstrate callback 
+program example_process_6
+    use stdlib_system, only: process_type, process_ID, run, is_running, kill, elapsed, is_windows, sleep
+    implicit none
+    type(process_type) :: p
+    integer, target :: nfiles
+
+    ! Run process, attach callback function and some data
+    if (is_windows()) then
+        p = run("dir",want_stdout=.true.,callback=get_dir_nfiles)
+    else
+        p = run("ls -l",want_stdout=.true.,callback=get_dir_nfiles,payload=nfiles)
+    endif
+
+    ! On exit, the number of files should have been extracted by the callback function
+    print *, "Current directory has ",nfiles," files"
+    
+    contains
+        
+       ! Custom callback function: retrieve number of files from ls output
+       subroutine get_dir_nfiles(pid, exit_state, stdin, stdout, stderr, payload)
+           integer(process_ID), intent(in) :: pid
+           integer, intent(in) :: exit_state
+           character(len=*), optional, intent(in) :: stdin, stdout, stderr
+           class(*), optional, intent(inout) :: payload    
+           
+           integer :: i
+
+           if (present(payload)) then 
+            
+               select type (nfiles => payload)
+                   type is (integer)
+                       if (present(stdout)) then 
+                           nfiles = count([ (stdout(i:i) == char(10), i=1,len(stdout)) ])
+                       else
+                           nfiles = -1
+                       endif 
+                   class default
+                       error stop 'Wrong payload passed to the process'
+               end select            
+            
+           end if
+       end subroutine get_dir_nfiles     
+
+end program example_process_6