loader - why the DSO was loaded at address 0 -

there simple test program,it dlopen()s dso "/lib/libc.so.6" , print address of symbol "printf".

#include <dlfcn.h> #include <stdio.h>  int main() {     void* handle;     void (*func)();     char* error_test;     if (handle = dlopen("/lib/libc.so.6", rtld_now)) {         func = dlsym(handle, "printf");         (*func)("address:%p\n",func);         sleep(-1);         dlclose(handle);         return 0;     }     return -1; } 

when debug "ld_debug=all",it shows that:

[root@localhost glibc_test]# ld_debug=all ld_debug_output=error ./test & [18] 14690 [root@localhost glibc_test]# address:0x2f87b0 

but file "error" shows "/lib/libc.so.6" loaded address 0! that`s why?

 14690: initialize program: ./test  14690:   14690:   14690: transferring control: ./test  14690:   14690: symbol=dlopen;  lookup in file=./test [0]  14690: symbol=dlopen;  lookup in file=/changed_glibc/lib/libdl.so.2 [0]  14690: binding file ./test [0] /changed_glibc/lib/libdl.so.2 [0]: normal symbol `dlopen' [glibc_2.1]  14690:   14690: file=/lib/libc.so.6 [0];  needed ./test [0]  14690: file=/lib/libc.so.6 [0];  generating link map  14690:   dynamic: 0x0043cd7c  base: 0x00000000   size: 0x00191988  14690:     entry: 0x002c5e40  phdr: 0x002af034  phnum:         10 

what`s more,the "/proc/14270/maps" file perform normal,and address of symbol "func"(0x2f87b0) in test program lies in right range(002af000-0043b000).

[root@localhost ~]# cat /proc/14690/maps  002af000-0043b000 r-xp 00000000 fd:00 1714117    /lib/libc-2.12.so 0043b000-0043d000 r--p 0018c000 fd:00 1714117    /lib/libc-2.12.so 0043d000-0043e000 rw-p 0018e000 fd:00 1714117    /lib/libc-2.12.so 0043e000-00441000 rw-p 00000000 00:00 0  

the issue related "prelink",when dso loading, has been prelinked address.