differences between Ada and C in gnat forcing me to use C instead of Ada

[matthewbrentmccarty@gmail.com]

Hi all:

I really want to use Ada.  But gnat is forcing me to use C.  Basically, I wanted to interface with my scanner using SANE (Scanner Access Now Easy).  Using my "real" Debian machine since gnat doesn't work under Windows 10 (Debian subsystem for Linux), I created the ada specs from the C headers and the sizes in C and Ada are different.  For example:

In C, this code:
printf("sizeof(char *)=%d\n", sizeof(char *));

prints "sizeof(char *)=8"


and this code:
printf("sizeof(SANE_Device)=%d\n", sizeof(SANE_Device));

prints "sizeof(SANE_Device)=32".  In C, the header defines it this way:

typedef char SANE_Char;
typedef const SANE_Char *SANE_String_Const;

typedef struct
  {
    SANE_String_Const name;	/* unique device name */
    SANE_String_Const vendor;	/* device vendor string */
    SANE_String_Const model;	/* device model name */
    SANE_String_Const type;	/* device type (e.g., "flatbed scanner") */
  }
SANE_Device;

which makes sense to me since 8*4 = 32 (sizeof(char *) * 4 elements in the record.  But on the Ada side, I get the following code:

Text_Io.Put_Line ("sane_sane_h.SANE_Device'Size=>" & 
    Integer'Image (sane_sane_h.SANE_Device'Size));

to print "sane_sane_h.SANE_Device'Size=> 256"

And sure enough "Integer'Image (InterfaceS.C.Strings.chars_ptr'Size))" prints out 64.  Now, how can we interface to a C library passing around the 'Address of an array of pointers to records if the elements in the record are different sizes.  No wonder, I get back garbage when calling sane_get_devices. How is it that "sizeof(char *) is 8 in C and InterfaceS.C.Strings.chars_ptr'Size is 64 in Ada.  When I try to rep spec it, I get

   --size for "SANE_String_Const" too small, minimum allowed is 64
   --
   --for SANE_String_Const'Size use 8;

and

--for SANE_Device'Size use 32;                                     
-- causes compiler error "size for "SANE_Device" too small, minimum allowed is 256 
--       for SANE_Device use record
--          name at 0 range 0 .. 7;
--          vendor at 1 range 0 .. 7;
--          model at 2 range 0 .. 7;
--          c_type at 3 range 0 .. 7;
--       end record;

And sure enough Standard'Address_Size is  64.  Sorry to be so long winded. I guess I'll write C code.  Arggggh!

Regards,
Matthew McCarty

PS: Here are the steps.

So, I installed the development libraries using:

%apt-get install sane sane-utils libsane-extras xsane

which gave me

/usr/include/sane/sane.h


I then ran

%g++ -c -fdump-ada-spec -C /usr/include/sane/sane.h

which includes the following types and record:


with Interfaces.C; use Interfaces.C;
...
...
subtype SANE_Char is char;  -- /usr/include/sane/sane.h:46
...
type SANE_String is access all SANE_Char;  -- /usr/include/sane/sane.h:47
...
-- unique device name  
type SANE_Device is record
   name : SANE_String_Const;  -- /usr/include/sane/sane.h:104
   vendor : SANE_String_Const;  -- /usr/include/sane/sane.h:105
   model : SANE_String_Const;  -- /usr/include/sane/sane.h:106
   c_type : SANE_String_Const;  -- /usr/include/sane/sane.h:107
end record;
pragma Convention (C_Pass_By_Copy, SANE_Device);  -- /usr/include/sane/sane.h:109

function sane_get_devices (device_list : System.Address; local_only : SANE_Bool) return SANE_Status;  -- /usr/include/sane/sane.h:221
pragma Import (C, sane_get_devices, "sane_get_devices");