Re: ifunc attribute in C++

[<asomers@xxxxxxxxx>]

On Fri, Jan 30, 2015 at 8:15 AM, Alexander Monakov <amonakov@xxxxxxxxx> wrote:
>
>
> On Fri, 30 Jan 2015, asomers@xxxxxxxxx wrote:
>
>> Does the ifunc attribute work in C++, and if so can somebody please
>> show me a working example?  I found several examples that work in C,
>> but none in C++.  For example, the below file works fine in C:
>
> Yes, it works in C++; the issue you're hitting is that due to C++ name
> mangling, the assembler name of the resolver function (foo_ifunc) is changed,
> and no longer matches what you wrote in the attribute.  Perhaps the easiest
> fix for that is to put the resolver inside an 'extern "C" { ... }' block.
>
> HTH.
> Alexander

Thanks, Alexander.  I thought I had already tried that, but I guess I
put the wrong function(s) in the extern block.  For the record, here
is the program that works:

#include <stdio.h>

typedef int foo_t(int);

int foo (int) __attribute__ ((ifunc ("foo_ifunc")));

static int global = 1;

static int
f1 (int x)
{
  return x;
}

static int
f2 (int x)
{
  return x + 1;
}

extern "C" {

foo_t *
foo_ifunc (void)
{
  return global == 1 ? f1 : f2;
}

}

int
main (int argc, char** argv __attribute__((unused)))
{
  printf ("%d\n", foo(argc));
  return (0);
}

-Alan