Writing an auxiliary function that returns a null terminated array of C strings

Question

Writing an auxiliary function that returns a null terminated array of C strings

vmagnin opened this issue a year ago · 4 comments

Some GTK functions expect a null terminated array of C strings. For example, in examples/bazaar.f90 we have this piece of code for gtk_about_dialog_set_authors():

	character(len=14, kind=c_char), dimension(2), parameter :: authors = &
										  & ["Vincent MAGNIN", "James TAPPIN  "]
	character(kind=c_char), dimension(:), allocatable :: string
	character(kind=c_char), pointer, dimension(:) :: credit
	type(c_ptr), dimension(:), allocatable :: c_ptr_array
	...
	! To add authors we need a pointer toward a null terminated array of strings.
	! This code comes from src/gtk-hl-dialog.f90:
	allocate(c_ptr_array(size(authors)+1))

	do i = 1, size(authors)
	  call f_c_string(authors(i), string)
	  allocate(credit(size(string)))
	  ! A Fortran pointer toward the Fortran string:
	  credit(:) = string(:)
	  ! Store the C address in the array:
	  c_ptr_array(i) = c_loc(credit(1))
	  nullify(credit)
	end do
	! The array must be null terminated:
	c_ptr_array(size(authors)+1) = c_null_ptr
	! https://docs.gtk.org/gtk3/method.AboutDialog.set_authors.html
	call gtk_about_dialog_set_authors(dialog, c_ptr_array)
	deallocate(c_ptr_array)

It would be interesting to write in src/gtk-sup.f90 an auxiliary function that returns such a null terminated array of C strings. It would be called by examples/bazaar.f90, examples/menubar.f90 and src/gtk-hl-dialog.f90.

Answer 1 · 2023-09-15T13:44:22.000Z

Hi, I've not had much time to look at GTK-Fortran lately, and also I'm not really keen to get involved with GTK4.

I think that would be semi-trivial.
I'm also looking at a way of doing a c→f string conversion that gets the lengths from the null character rather than from the length returned by strlen as that can cause trouble if there are multibyte characters in the string.

Answer 2 · 2023-09-15T15:08:16.000Z

Hi James @jtappin

string conversion functions don't depend on the GTK version, so such a function could be added in the gtk3 and gtk4 branches. Those functions are still in src/gtk-sup.f90:
https://github.com/vmagnin/gtk-fortran/blob/gtk4/src/gtk-sup.f90

I have noticed that in GLib there is that function:
https://docs.gtk.org/glib/func.utf8_strlen.html

And I know that modern Fortran can support UTF8 (ISO/IEC 10646), but the standard says that it may be implemented or not by the compiler.

Answer 3 · 2023-09-22T09:48:45.000Z

Here is a patch to add an interface that will return an array of NULL terminated strings if CSTR argument is a 2-D allocatable array.
sup.txt

I think that's actually how I originally did it, but changed it because test widgets want a single string with line feeds to separate the lines.

P.S. Having a quick play about, I think that actually what I need for the automatic c→f converter is to use strlen and override what the text widget tells me about the length of the text.

Answer 4 · 2023-09-22T16:48:54.000Z

Here is a patch to add an interface that will return an array of NULL terminated strings if CSTR argument is a 2-D allocatable array.
sup.txt

Thanks a lot @jtappin
I have applied the patch in the gtk4 branch and backported the code in the gtk3 branch. When I will have some time, I will try to use that function in the gtk4 examples where it can be useful.