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digitalmars.D - Calling C function with static array includes length and pointer

reply mwarning <moritzwarning web.de> writes:
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
Jun 21 2010
next sibling parent reply =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
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
Jun 21 2010
parent reply mwarning <moritzwarning web.de> writes:
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.
Jun 21 2010
parent =?UTF-8?B?QWxpIMOHZWhyZWxp?= <acehreli yahoo.com> writes:
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
Jun 21 2010
prev sibling parent reply torhu <no spam.invalid> writes:
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.
Jun 21 2010
parent mwarning <moritzwarning web.de> writes:
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.
Jun 21 2010