Re: Calling C function with static array includes length and pointer

[Ali Çehreli]

mwarning wrote:

> Anyway, the D spec says:
> "Static arrays are value types, but as in C static arrays are passed to
> functions by reference and cannot be returned from functions."
> (http://www.digitalmars.com/d/1.0/arrays.html#static-arrays)

I've been thinking D2. :) In D2, fixed-sized arrays are value types and 
copied to and from functions:


import std.stdio;

int[2] foo(int[2] array)
{
writeln("in foo   : ", &(array[0]));
return array;
}

void main()
{
int[2] array;
writeln("in main 1: ", &(array[0]));
int[2] result = foo(array);
writeln("in main 2: ", &(result[0]));
}

All three addresses are separate in D2.

Ali

Re: Calling C function with static array includes length and pointer

[mwarning]

On Tue, 22 Jun 2010 00:00:27 +0200, torhu wrote:

> On 21.06.2010 23:30, mwarning wrote:
>> Hi,
>>
>> I try to pass a static array to a variadic C function. Looks like the
>> array is passed by values as expected, but the length and pointer are
>> prepended, too. Is this intentional or a bug?
>>
>> http://pastebin.com/6ejFF37j
> 
> I believe this works as intended.  And arrays in D 1 are *not* passed by
> value, ie. the data is not copied.
> 
>  From www.digitalmars.com/d/1.0/arrays.html:
> 
> "a string literal is passed as a (length,pointer) combination to
> variadic parameters"
Thanks, I missed that section!

> In other words, use stack.ptr if you want just the pointer.
I wrap it into a struct, that will do it.

Re: Calling C function with static array includes length and pointer

[torhu]

On 21.06.2010 23:30, mwarning wrote:

Hi,

I try to pass a static array to a variadic C function.
Looks like the array is passed by values as expected,
but the length and pointer are prepended, too.
Is this intentional or a bug?

http://pastebin.com/6ejFF37j


I believe this works as intended.  And arrays in D 1 are *not* passed by 
value, ie. the data is not copied.


From www.digitalmars.com/d/1.0/arrays.html:

"a string literal is passed as a (length,pointer) combination to 
variadic parameters"


In other words, use stack.ptr if you want just the pointer.

Re: Calling C function with static array includes length and pointer

[mwarning]

On Mon, 21 Jun 2010 14:38:50 -0700, Ali Çehreli wrote:

> mwarning wrote:
>> Hi,
>> 
>> I try to pass a static array to a variadic C function. Looks like the
>> array is passed by values as expected, but the length and pointer are
>> prepended, too. Is this intentional or a bug?
>> 
>> http://pastebin.com/6ejFF37j
> 
> Fixed sized arrays don't have ptr and length members:
> 
> import std.stdio;
> 
> void main()
> {
>  int[5] array;
>  writeln(array.sizeof);   // prints 20 writeln(array.ptr, '
>  ', &array); // prints the save value
> }
Hm? You just have proved that they have. :>
But those are not members of a struct/pair instance (as for dynamic 
arrays). They are compile time values.
Whatever, it's me nitpicking. :)

> The code you've pasted has this:
> 
> extern(C) void* foo(uint x, ...)
> {
>  Stdout("x: ")(x).nl;
>  Stdout("y: ")(*(&x + 1)).nl; //outputs 64! return null;
> }
> 
> The expression *(&x + 1) is undefined behavior, because (&x + 1) is not
> valid, i.e. not specified to be accessible by the language.
> 
> Ali
True, the D spec doesn't specify the memory layout.
But the C calling convention does (depending on the platform).

Anyway, the D spec says:
"Static arrays are value types, but as in C static arrays are passed to 
functions by reference and cannot be returned from functions."
(http://www.digitalmars.com/d/1.0/arrays.html#static-arrays)

It seems that D does a little more then just pass by value. It includes 
the pointer and length value as well.

Btw.: smth. told me workaround/solution: wrap the static array in a 
struct.

Re: Calling C function with static array includes length and pointer

[Ali Çehreli]

mwarning wrote:

Hi,

I try to pass a static array to a variadic C function.
Looks like the array is passed by values as expected,
but the length and pointer are prepended, too.
Is this intentional or a bug?

http://pastebin.com/6ejFF37j


Fixed sized arrays don't have ptr and length members:

import std.stdio;

void main()
{
int[5] array;
writeln(array.sizeof);   // prints 20
writeln(array.ptr, ' ', &array); // prints the save value
}

The code you've pasted has this:

extern(C) void* foo(uint x, ...)
{
Stdout("x: ")(x).nl;
Stdout("y: ")(*(&x + 1)).nl; //outputs 64!
return null;
}

The expression *(&x + 1) is undefined behavior, because (&x + 1) is not 
valid, i.e. not specified to be accessible by the language.


Ali

Calling C function with static array includes length and pointer

[mwarning]

Hi,

I try to pass a static array to a variadic C function.
Looks like the array is passed by values as expected,
but the length and pointer are prepended, too.
Is this intentional or a bug?

http://pastebin.com/6ejFF37j