Re: No .got section in ELF

[Jie Zhang]

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