• d coder <dlang.coder gmail.com> Feb 20 2011
• Simon <s.d.hammett gmail.com> Feb 20 2011
• "Nick Sabalausky" <a a.a> Feb 20 2011
• Simon <s.d.hammett gmail.com> Feb 21 2011
• Kagamin <spam here.lot> Feb 22 2011
• d coder <dlang.coder gmail.com> Feb 20 2011
• "Steven Schveighoffer" <schveiguy yahoo.com> Feb 20 2011
• Dan Olson <zans.is.for.cans yahoo.com> Feb 26 2011

d coder <dlang.coder gmail.com> writes:

Greetings

I tried to initialize a struct member with a function pointer, and
found that DMD2 did not like it. Are not function pointers compile
time constants? And why they should not be?

Regards
- Cherry

Simon <s.d.hammett gmail.com> writes:

On 20/02/2011 14:59, d coder wrote:
 Greetings

 I tried to initialize a struct member with a function pointer, and
 found that DMD2 did not like it. Are not function pointers compile
 time constants? And why they should not be?

 Regards
 - Cherry


No a function doesn't have an address until the .exe is loaded into 
memory. And with Address space randomisation on 'doze there is no 
reasonable way to make a function pointer a compile time value.

-- 
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk

"Nick Sabalausky" <a a.a> writes:

"Simon" <s.d.hammett gmail.com> wrote in message 
news:ijrdif$1nn6$1 digitalmars.com...
 On 20/02/2011 14:59, d coder wrote:
 Greetings

 I tried to initialize a struct member with a function pointer, and
 found that DMD2 did not like it. Are not function pointers compile
 time constants? And why they should not be?

 Regards
 - Cherry


 No a function doesn't have an address until the .exe is loaded into 
 memory. And with Address space randomisation on 'doze there is no 
 reasonable way to make a function pointer a compile time value.


I didn't know Windows did that, I thought it was just certain versions of 
Unix/Linux. Do you happen to know which version of Windows was first to have 
it? 

Simon <s.d.hammett gmail.com> writes:

On 21/02/2011 00:24, Steven Schveighoffer wrote:
 On Sun, 20 Feb 2011 16:23:14 -0500, Nick Sabalausky <a a.a> wrote:

 "Simon" <s.d.hammett gmail.com> wrote in message
 news:ijrdif$1nn6$1 digitalmars.com...
 On 20/02/2011 14:59, d coder wrote:
 Greetings

 I tried to initialize a struct member with a function pointer, and
 found that DMD2 did not like it. Are not function pointers compile
 time constants? And why they should not be?

 Regards
 - Cherry


 No a function doesn't have an address until the .exe is loaded into
 memory. And with Address space randomisation on 'doze there is no
 reasonable way to make a function pointer a compile time value.


 I didn't know Windows did that, I thought it was just certain versions of
 Unix/Linux. Do you happen to know which version of Windows was first
 to have
 it?


 Probably the first one with dlls? I don't see how else you could have
 dlls, because you can't just say "this function will always be at
 address 12345, and no other function anyone else ever compiles can take
 that spot".


Vista. For any particular exe, you'll find that it's dependant dlls 
alway get loaded in the same place. That's why buffer overflow bugs 
(where/are) so easy to exploit.

 That being said, I'm not sure why the OP's issue couldn't be solved --
 clearly position independent code works (and that is statically
 created), why couldn't a reference to that function also be created in a
 struct initializer? Is it a limitation of the linker?

 -Steve


You can't use the relative jump of POS code.
The offset to the function will be different at each call site.

You could make function pointers compile time constants if:

You disallow ASR
You disallow them when compiling to a dll
You disallow in-lining of any function of which you take the address
You disallow the linker from rearranging functions
You disallow the linker from merging duplicate functions.
You merge the compiler and linker

And it wouldn't work with template functions anyway as you can get 
multiple copies of those.

That's a lot of stuff to give up.

-- 
My enormous talent is exceeded only by my outrageous laziness.
http://www.ssTk.co.uk

Kagamin <spam here.lot> writes:

Simon Wrote:

 You could make function pointers compile time constants if:
 
 You disallow ASR
 You disallow them when compiling to a dll
 You disallow in-lining of any function of which you take the address
 You disallow the linker from rearranging functions
 You disallow the linker from merging duplicate functions.
 You merge the compiler and linker


Interestingly, this data layout is impossible without rearranging:

byte a;
byte[&c] b;
byte c;

d coder <dlang.coder gmail.com> writes:

Thanks Simon.

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

On Sun, 20 Feb 2011 16:23:14 -0500, Nick Sabalausky <a a.a> wrote:

 "Simon" <s.d.hammett gmail.com> wrote in message
 news:ijrdif$1nn6$1 digitalmars.com...
 On 20/02/2011 14:59, d coder wrote:
 Greetings

 I tried to initialize a struct member with a function pointer, and
 found that DMD2 did not like it. Are not function pointers compile
 time constants? And why they should not be?

 Regards
 - Cherry


 No a function doesn't have an address until the .exe is loaded into
 memory. And with Address space randomisation on 'doze there is no
 reasonable way to make a function pointer a compile time value.


 I didn't know Windows did that, I thought it was just certain versions of
 Unix/Linux. Do you happen to know which version of Windows was first to  
 have
 it?


Probably the first one with dlls?  I don't see how else you could have  
dlls, because you can't just say "this function will always be at address  
12345, and no other function anyone else ever compiles can take that spot".

That being said, I'm not sure why the OP's issue couldn't be solved --  
clearly position independent code works (and that is statically created),  
why couldn't a reference to that function also be created in a struct  
initializer?  Is it a limitation of the linker?

-Steve

Dan Olson <zans.is.for.cans yahoo.com> writes:

d coder <dlang.coder gmail.com> writes:

 Greetings

 I tried to initialize a struct member with a function pointer, and
 found that DMD2 did not like it. Are not function pointers compile
 time constants? And why they should not be?

 Regards
 - Cherry


I just want to point out that this *should* be doable in D.  At compile
time the function's address in the object code is a placeholder that the
linker or loader will fixup based on the function symbol.  This is what
linkers do!  And if an function ptr is used, the function can still be
inlined; just need to keep around the real function reference through
the pointer.

Dan