???????п????????????????? .txt ?ε???? 0 ????????е???7????????????? a ???????? %eax ?У?????? a ?????????? %eax ?е??????? g_share ??????????? g_share ???????0(%rip). ?????????????????????????????? g_share ??????????????????????????????????????????????????棬???????????????????????????????????????????????????????????????Щ??????????????????????????????????????????б????????????Щ???????????λ?????????????Щ???????“???λ??”(relocatioin table)??
????-bash-3.00$ objdump -r a.o
????a.o:     file format elf64-x86-64
????RELOCATION RECORDS FOR [.text]:
????OFFSET           TYPE              VALUE
????000000000000000c R_X86_64_PC32     g_share+0xfffffffffffffffc
???????????У???????????????????????У?????? g_share ??????????????? offset ??????????????λ???????е?????TYPE ?????????????????? cpu ???????????????????????????????????????????? type ??????????????? value ??????? type ???????÷????????
?????????????????????????????????????????????????????????ε?????????????????????Щ??????λ?????????????? ????????????????????????У??? g_share(0x40496??)?? g_func(0x4047a??)????????????????????????
-bash-3.00$ gcc -o am a.o main.o
-bash-3.00$ objdump -S am
// skip some of the ouput
extern int g_func(int a);
int main()
{
400468:       55                      push   %rbp
400469:       48 89 e5                mov    %rsp??%rbp
40046c:       48 83 ec 10             sub    $0x10??%rsp
int a = 42;
400470:       c7 45 fc 2a 00 00 00    movl   $0x2a??0xfffffffffffffffc(%rbp)
a = g_func(a);
400477:       8b 7d fc                mov    0xfffffffffffffffc(%rbp)??%edi
40047a:       e8 0d 00 00 00          callq  40048c <g_func>
40047f:       89 45 fc                mov    %eax??0xfffffffffffffffc(%rbp)
return 0;
400482:       b8 00 00 00 00          mov    $0x0??%eax
}
400487:       c9                      leaveq
400488:       c3                      retq
400489:       90                      nop
40048a:       90                      nop
40048b:       90                      nop
000000000040048c <g_func>:
int g_share = 1;
int g_func(int a)
{
40048c:       55                      push   %rbp
40048d:       48 89 e5                mov    %rsp??%rbp
400490:       89 7d fc                mov    %edi??0xfffffffffffffffc(%rbp)
g_share += a;
400493:       8b 45 fc                mov    0xfffffffffffffffc(%rbp)??%eax
400496:       01 05 dc 03 10 00       add    %eax??1049564(%rip)        # 500878 <g_share>
return a * 2;
40049c:       8b 45 fc                mov    0xfffffffffffffffc(%rbp)??%eax
40049f:       01 c0                   add    %eax??%eax
}
4004a1:       c9                      leaveq
4004a2:       c3                      retq
// skip some of the ouput
????????????λ??????????巽?????浽???????????￡??????????
????????????????λ
???????????????????????????????????λ???????????????????????????????????????Щ???????λ????????????????????????Linux ?? ELF ??????????????????????????λ????????????????????????????????????????λ???????????????????????λ????μ?????????λ????????????????????????????????????????С?
int g_share = 1;
int g_func(int a)
{
g_share += a;
return a * 2;
}
int g_func2()
{
int a = 2;
int b = g_func(3);
return a + b;
}
// compile on 32bit linux OS
-bash-3.00$ gcc -c a.c main.c
-bash-3.00$ gcc -shared -o liba.so a.o
-bash-3.00$ gcc -o am main.o -L. -la
-bash-3.00$ objdump -S liba.so
// skip some of the output
000004f4 <g_func>:
int g_share = 1;
int g_func(int a)
{
4f4: 55                    push   %ebp
4f5: 89 e5                 mov    %esp??%ebp
g_share += a;
4f7: 8b 45 08              mov    0x8(%ebp)??%eax
4fa: 01 05 00 00 00 00     add    %eax??0x0
return a * 2;
500: 8b 45 08              mov    0x8(%ebp)??%eax
503: d1 e0                 shl    %eax
}
505: c9                    leave
506: c3                    ret
00000507 <g_func2>:
int g_func2()
{
507: 55                    push   %ebp
508: 89 e5                 mov    %esp??%ebp
50a: 83 ec 08              sub    $0x8??%esp
int a = 2;
50d: c7 45 fc 02 00 00 00  movl   $0x2??0xfffffffc(%ebp)
int b = g_func(3);
514: 6a 03                 push   $0x3
516: e8 fc ff ff ff        call   517 <g_func2+0x10>
51b: 83 c4 04              add    $0x4??%esp
51e: 89 45 f8              mov    %eax??0xfffffff8(%ebp)
return a + b;
521: 8b 45 f8              mov    0xfffffff8(%ebp)??%eax
524: 03 45 fc              add    0xfffffffc(%ebp)??%eax
}
527: c9                    leave
// skip some of the output
??????????е?? 4fa ?? 516 ????????????????? g_share ?? g_func ???????????????????????????????????Щ?????????????????????????????????λ??????????????????????????????????????λ???????????????????м???????????????
??????????????????????????ν??м???????????λ???????????????????????????е???????????????????????????????λ??????????????????????????????????????????????????????????????????????????????????????????÷??????????????????????????????????????????ж?÷???????y??????λ?????÷??????????????????????????????????
????ELF ??????????е???????ò???????ν??"????”(lazy binding)?? ??????е???????????ε??÷????????????????????????????????????????????????????????????????????????Щ??????????滻?????????????
??????????????????????????λ????????????????????????????????????????????????????????????????????????????????????????????????λ?????????????????????????????????????????????????????????????? copy ??????????????λ??????????????????в????????????????????????????????????