[Linux-ia64] Pointer to function has a bad value

[Linux-ia64] Pointer to function has a bad value

[Robboy, David G]

Is this a known bug or fixed in a later compiler?  Using gcc version
2.9-ia64-000216-final, assigning the address of a function to a pointer
gives the wrong value.  In the following program, the address of twiddle is
0x40000000000003f0, but the value of the pointer i is 0x4000000000078300.

void twiddle(void)
{
	int i = 34;
}
main()
{
	unsigned long i = (unsigned long)twiddle;
	printf("i=%lx\n", i);
}

 - David Robboy

Re: [Linux-ia64] Pointer to function has a bad value

[Andreas Schwab]

"Robboy, David G" <david.g.robboy@intel.com> writes:

|> Is this a known bug or fixed in a later compiler?  Using gcc version
|> 2.9-ia64-000216-final, assigning the address of a function to a pointer
|> gives the wrong value.  In the following program, the address of twiddle is
|> 0x40000000000003f0, but the value of the pointer i is 0x4000000000078300.

This is correct.  The conversion from pointer to integer is implementation
defined, the only requirement is that if the integer type is wide enough
you'll get the same pointer when casting back to the original pointer
type.

Andreas.

-- 
Andreas Schwab                                  "And now for something
SuSE Labs                                        completely different."
Andreas.Schwab@suse.de
SuSE GmbH, Schanzäckerstr. 10, D-90443 Nürnberg
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Re: [Linux-ia64] Pointer to function has a bad value

[Doug Rabson]

On Thu, 8 Mar 2001, Robboy, David G wrote:

> Is this a known bug or fixed in a later compiler?  Using gcc version
> 2.9-ia64-000216-final, assigning the address of a function to a pointer
> gives the wrong value.  In the following program, the address of twiddle is
> 0x40000000000003f0, but the value of the pointer i is 0x4000000000078300.

For ia64, function pointers are actually pointers to function descriptors.
A function descriptor contains a pointer to the actual function's code and
the value which the 'gp' register should have for that function (this is
part of the ABI support for shared libraries).

A function descriptor looks like this:

	struct ia64_fdesc {
		uint64_t	func;
		uint64_t	gp;
	};

-- 
Doug Rabson				Mail:  dfr@nlsystems.com
					Phone: +44 20 8348 6160