Problem Passing Struct to C

[Steven Schveighoffer]

On Fri, 06 May 2011 05:56:17 -0400, Mike Parker <aldacron at gmail.com> wrote:

> Testing out a new binding I knocked up for a C library. One of the  
> functions takes a struct by value. It looks somewhat like this:
>
> struct S {}
> struct Color
> {
>      float r,g,b,a;
> }
>
> extern C void function(S* s, Color color, int x, int y, in char*)  
> draw_text;
>
> Now, there is another function that adjusts color values when making a  
> color. In C, it is sometimes used like so:
>
> draw_text(s, map_color(255, 0, 0, 0), 0, 0, "Blarg");
>
> When I'm calling draw_text like this on the D side, my text output is  
> corrupt. I keep getting weird things like ^^P^, but in the appropriate  
> color. It's consistent, no matter what string I pass, but is different  
> for each color value. If I call draw_text like this:
>
> auto color = map_color(...);
> draw_text(s, color, 0, 0, "Blarg");
>
> It works as expected. Has anyone else seen this, or know of a  
> workaround? I'm going to dig through bugzilla later on and see if it's  
> been reported already, but I'm curious if anyone knows of the cause off  
> hand.
>

There are two possible causes I can think of, depending on how draw_text  
is defined in C.

1. is it truly a function pointer?  That is, does the definition look like  
void (*draw_text)(S* s, ...), or is it just void draw_text(S* s, ...)?

2. It's possible that the extern C (BTW, I thought it had to be  
extern(C)?) is only applying to the symbol name draw_text and not the  
function type.

If you answer no to #1, then your problem is your draw_text definition in  
D.  It should look like this:

extern(C) void draw_text(S * s, Color color, int x, int y, in char *);

If you answer yes to #1, then to check what type the compiler is giving to  
draw text, do:

pragma(msg, (typeof(draw_text)).stringof);

This hopefully tells you that it's extern(C), but if not, that is likely  
the problem.

If that's not it, I have no idea ;)

-Steve