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

[Jere <jhb.chat@gmail.com>]

On Monday, March 25, 2019 at 1:46:07 AM UTC-4, matthewbr...@gmail.com wrote:
> 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

So when you do sizeof() in C it returns the number of bytes. When
you do 'Size in Ada, it gives the number of bits.  8bits * 8bytes
is 64 bits, so everything looks correct. They are returning the same
effective size in both C and Ada.