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digitalmars.D.learn - Are function pointers compile time constants?

reply 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
Feb 20 2011
next sibling parent reply 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
Feb 20 2011
parent reply "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?
Feb 20 2011
parent reply 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
Feb 21 2011
parent 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;
Feb 22 2011
prev sibling next sibling parent d coder <dlang.coder gmail.com> writes:
Thanks Simon.
Feb 20 2011
prev sibling next sibling parent "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
Feb 20 2011
prev sibling parent 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
Feb 26 2011