Fortran: How to store value 255 into one byte?

Question

I would like to call a C api function from Fortran. The C function accepts an array of bytes: ``` void image(const void *bitmap, int w, int h); ``` where three successive bytes in `*bitmap` represent an RGB colour tripple and are interpreted as `unsigned char` in C. I want to initialize the bitmap in Fortran and take care of drawing inside C. Current definition in Fortran uses ``` integer*1 rgbImage(6,2) ``` to initialize an image of 2x2 for example, but compiler won't accept assignment ``` rgbImage(1,1) = 255 ``` to get red colour. I've seen hints of using `BYTE`, `UNSIGNED*1`, `LOGICAL*1` for unsigned single bytes, but gfortran (MacPort's gcc 4.4 or 4.6 under Mac OS X) isn't really happy with either of them. I could probably get away by cheating and assigning value `-1` instead of `255`, but that is very uncomfortable to use. The compiler flag `-fno-range-check` helped compile the code, but might not be available in other Fortran compilers and I consider it an ugly solution (I would…

Accepted Answer

My suggestion would be to use `CHARACTER` variables, and use `ACHAR` to set values (and `ICHAR` to convert back to integers as necessary). That should get you what you want and be completely portable. eg,

```
character, dimension(6,2) :: rgbImage

rgbImage(1,1) = achar(255)
```

Updated to add: if you're going to use the Fortran 2003 iso_c_binding stuff to interface to the C routines (highly recommended!) then you might as well make that rgbImage array characters of kind `c_char`, eg

```
character(kind=c_char), dimension(6,2) :: rgbImage
integer(kind=c_int) :: w, h

...
rgbImage(1,1) = achar(255)
...

call image(rgbImage, w, h)
```

where you've defined the interface for the routine

```
 interface
    subroutine image(img, w, h) bind(C)
       use, intrinsic :: iso_c_binding
       implicit none
       integer(kind=c_int), intent(in), value :: w, h
       character(kind=c_char) :: img(:,:)
    end subroutine image
 end interface
```

Fortran: How to store value 255 into one byte?

Solution

Problem