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Message-ID: <4B0F5C70.7010102@analog.com>
Date: Fri, 27 Nov 2009 04:58:00 -0000
From: Jie Zhang <jie.zhang@analog.com>
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To: yunfeng zhang <zyf.zeroos@gmail.com>
CC: Richard Henderson <rth@redhat.com>, gcc@gcc.gnu.org,   Andrew Haley <aph@redhat.com>
Subject: Re: No .got section in ELF
References: <4df04b840911231758md5a545el2d417b663af1647f@mail.gmail.com> 	 <4df04b840911251824j71fa1d33i2cecc9481617dda5@mail.gmail.com> 	 <4B0DF1EC.30001@redhat.com> <4df04b840911252204g33297e18m8e189c62a0662378@mail.gmail.com>
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On 11/26/2009 02:04 PM, yunfeng zhang wrote:
> The result is the same
>
> #include<stdio.h>
>
> extern int g __attribute__((visibility("hidden")));
> int g;
>
> int foo(int a, int b)
> {
>          g = a + b;
>          printf("%x, %x",&g, foo);
>          return g;
> }
>
> load and call `foo' in the library, an outputting (with vdso) is
>      cc15bc, cc03fc
> and open f.map
>      0x15bc, 0x3fc
>
> It shows Linux simply maps the library to memory *using* library segment layout.
>
> Using e.cc to call it
>
> #include<exception>
> #include<typeinfo>
> #include<cstddef>
> #include<dlfcn.h>
> #include<stdio.h>
>
> int main(void)
> {
>                  void* handle = dlopen("./f.so", RTLD_NOW);
>                  typedef int (*gso)(int, int);
>                  gso f;
>                  *(void**) (&f) = dlsym(handle, "foo");
>                  f(1, 2);
>          return 0;
> }
>
You got the bad test case. Please try the following:

$ cat f.c
#include <stdio.h>
int g;

int foo(int a, int b)
{
   g = a + b;
   printf("&g = 0x%x, foo = 0x%x\n", &g, foo);
   return g;
}

$ cat e.c
int g;
extern int foo(int a, int b);

int main(void)
{
   foo(1, 2);
   return 0;
}

$ gcc -shared -fPIC -Wl,-soname,./libf.so,-Map,f.map -o libf.so f.c
$ gcc -o e e.c -ldl -L. -lf
$ ./e
&g = 0x600a30, foo = 0x294a2614

Then comment out the "int g;" in e.c. and do the same steps as above:

$ ./e
&g = 0x58294948, foo = 0x58094614

You can see that "C-A" is *not* a constant. Your premise is wrong.


Jie