Can it be that bitmap is a "sdl_surface pointer" and sdl_save_bmp requires a "sdl_surface"??

No, the runtime library is reporting that bitmap is not associated, meaning the initial call to SDL_Create_RGB_Surface_From failed, returning a null (or, in Fortran, unassociated) pointer.

No, the actual problem here is that the fortran-sdl2 library contains a bug.  The Fortran binding for this function is incorrect.  If you look at the GitHub version of the Fortran interface, you'll notice that they've bound the C function not to SDL_CreateRGBSurfaceFrom, but, rather, SDL_CreateRGBSurface.  It appears to be a copypasta error.

I've already submitted a pull request to correct the issue.  In the meantime, I'll try to build a new version of the package to fix this.

subroutine save_image2()

use opengl_glu
use opengl_glut
use common_window
use, intrinsic :: iso_fortran_env, only: i1 => int8, i8 => int64, r8 => real64,&
                  stderr => error_unit, stdout => output_unit
use :: sdl2
use :: sdl2_image
use :: SDL2_mixer
use :: SDL2_ttf
   
implicit none
integer(kind=GLbyte), dimension(:), pointer::image
type(sdl_surface),pointer :: bitmap
integer :: idepth,ipitch,mm
allocate(image(iwidth*iheight*4))  ! assumes 4 bytes (RGB + alpha per pixel)

call glReadPixels(0,0,iwidth,iheight,GL_RGBA,GL_UNSIGNED_BYTE,c_loc(image))
idepth=32
ipitch=4*iwidth
!ipitch=0
bitmap => SDL_Create_RGB_Surface_From(c_loc(image),iwidth,iheight,idepth,&
   ipitch, int(z'000000ff',kind=i8),int(z'0000ff00',kind=i8),&
           int(z'00ff0000',kind=i8),int(z'ff000000',kind=i8))
       
mm=sdl_save_bmp(bitmap,"test.bmp")

endsubroutine save_image2

However I still get the same error:
At line 27 of file .\save_image2.f90
Fortran runtime error: Pointer actual argument 'bitmap' is not associated
Error termination.

Can it be that bitmap is a "sdl_surface pointer" and sdl_save_bmp requires a "sdl_surface"??

If I fill out ipitch=0 the code runs, but the saved file is empty

One issue I can see is that pulling the pixels from OpenGL, you've requested the format as GL_RGB, which will not include an alpha channel.  Your SDL call, though, assumes an alpha channel is present, hence the 4 bytes per pixel.  You probably want to use GL_RGBA on the call to glReadPixels. 

Additionally, your call to glReadPixels is using a problematic pointer for image data.  The function definition is actually expecting a type(c_ptr) that points to a region of memory that can store the image.  What you really should have here instead is a Fortran pointer to allocated memory. 

The declaration should be simply:

integer(kind=GLbyte), dimension(:), pointer::image

Next, before using it for anything, you need to allocate it to the image size:

allocate(image(iwidth*iheight*4))  ! assumes 4 bytes (RGB + alpha per pixel)

Finally, on your OpenGL call, you need to provide the C location of the array:

call glReadPixels(0,0,iwidth,iheight,GL_RGBA,GL_UNSIGNED_BYTE,C_LOC(image))

Likewise, on the call to SDL:

bitmap => SDL_Create_RGB_Surface_From(C_LOC(image),iwidth,iheight,idepth,ipitch,&
         int(z'000000ff',kind=i8),int(z'0000ff00',kind=i8),&
         int(z'00ff0000',kind=i8),int(z'ff000000',kind=i8))

I haven't run the code, though.  Perhaps that will improve things.

the routine that should do the job reads:

subroutine save_image2()

use opengl_glu
use opengl_glut
use common_window
use, intrinsic :: iso_fortran_env, only: i1 => int8, i8 => int64, r8 => real64,&
                  stderr => error_unit, stdout => output_unit
use :: sdl2
use :: sdl2_image
use :: SDL2_mixer
use :: SDL2_ttf
   
implicit none
type(c_ptr),dimension(0:iwidth*iheight*3) :: image
type(sdl_surface),pointer :: bitmap
integer :: idepth,ipitch,mm

call glReadPixels(0,0,iwidth,iheight,GL_RGB,GL_UNSIGNED_BYTE,image)
idepth=32
ipitch=4*iwidth
!ipitch=0
bitmap => SDL_Create_RGB_Surface_From(image,iwidth,iheight,idepth,ipitch,&
         int(z'000000ff',kind=i8),int(z'0000ff00',kind=i8),&
         int(z'00ff0000',kind=i8),int(z'ff000000',kind=i8))
mm=sdl_save_bmp(bitmap,"test.bmp")

endsubroutine save_image2

If I fill out ipitch = 0, I get the empty bmp file
if I fill out ipitch = 4*iwidth I get an error (see below)

At line 28 of file .\save_image2.f90
Fortran runtime error: Pointer actual argument 'bitmap' is not associated
Error termination.

The window sizes iwidth and iheight are obtained via the common block "common_window"

I hope I provided enough information

==============================================================================
Generating Makefile... Okay
==============================================================================
Processing default resource
Generating atb_ex.dll.exe
C:/Users/csitt/AppData/Local/sfpm/64/lib\libfortran-sdl2.a(sdl2.o): In function `__sdl2_MOD_sdl_log_warn':
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
C:\Users\jeffr\Workspace\packages\fortran_sdl2\fortran-sdl2/src/sdl2.f90:587: undefined reference to `SDL_LogWarn'
C:/Users/csitt/AppData/Local/sfpm/64/lib\libfortran-sdl2.a(sdl2.o): In function `__sdl2_MOD_sdl_log_verbose':
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
C:\Users\jeffr\Workspace\packages\fortran_sdl2\fortran-sdl2/src/sdl2.f90:578: undefined reference to `SDL_LogVerbose'
C:/Users/csitt/AppData/Local/sfpm/64/lib\libfortran-sdl2.a(sdl2.o): In function `__sdl2_MOD_sdl_log_info':
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
C:\Users\jeffr\Workspace\packages\fortran_sdl2\fortran-sdl2/src/sdl2.f90:569: undefined reference to `SDL_LogInfo'
C:/Users/csitt/AppData/Local/sfpm/64/lib\libfortran-sdl2.a(sdl2.o): In function `__sdl2_MOD_sdl_log_error':
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
C:\Users\jeffr\Workspace\packages\fortran_sdl2\fortran-sdl2/src/sdl2.f90:560: undefined reference to `SDL_LogError'
C:/Users/csitt/AppData/Local/sfpm/64/lib\libfortran-sdl2.a(sdl2.o): In function `__sdl2_MOD_sdl_log_debug':

What I actually want is to save an opengl image to a file as a bitmap, jpg or another format. Perhaps somebody has a solution??

- Simple DirectMedia Layer
- SDL2_image
- SDL2_mixer
- SDL2_ttf

I'm not sure it would upgrade those automatically. 

You say you've "uninstalled and reinstalled opengl," but I'm not sure I understand what package you're talking about.  glclearcolor is available in the sdl2 module, and it doesn't require any other imports.  I'm not sure if you're calling it directly or not.

I do not know how to solve this.
I uninstalled and reinstalled opengl, but it does not help.

One word of warning: the Fortran library name has changed, which could cause an existing project's linking step to fail.  The library flag is now:

-lfortran-sdl2

Let me know if there's any problem!