所谓的代码段、数据段

全栈程序员-站长 • 2021年8月20日下午10:00 • 未分类 • 阅读 59

一般面试C++，都会很无聊的问到虚拟函数的实现机制，接着就会问到vtable放在哪里 – 实现细节中的细节，我个人觉得这个除了卖弄没啥大意义，但面试嘛，一般就这吊样。

#include <iostream>

using namespace std;

// .data - read-write data
int rwdata = 100;

// .rodata - read-only data
const char* rodata = "hello, world";

// .bss - non-initialized data
int bssdata;

// .text 
void text_code()
{
        cout << "text_code" << endl;
}

class VTableTest
{
public:
        virtual void virtualfunc()
        {
                cout << "virtualfunc" << endl;
        }
};

int main()
{
        cout << ".data: " << &rwdata << endl;
        cout << ".rodata: " << reinterpret_cast<const void*>(rodata) << endl;
        cout << ".bss: " << &bssdata << endl;

        cout << ".text-normal-function: " << reinterpret_cast<void*>(text_code) << endl;

        VTableTest* pV = new VTableTest();
        long* pVlong = reinterpret_cast<long*>(pV);
        void* vptr = reinterpret_cast<void*>(*pVlong);
        cout << ".rodata-vtable: " << vptr << endl;
}

编译运行：

$ segment
.data: 0x804a030
.rodata: 0x80489d0
.bss: 0x804a114
.text-normal-function: 0x80486e4
.rodata-vtable: 0x8048a40

查询各个段的地址范围：

$ readelf -S segment
There are 29 section headers, starting at offset 0x115c:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .interp           PROGBITS        08048154 000154 000013 00   A  0   0  1
  [ 2] .note.ABI-tag     NOTE            08048168 000168 000020 00   A  0   0  4
  [ 3] .note.gnu.build-i NOTE            08048188 000188 000024 00   A  0   0  4
  [ 4] .gnu.hash         GNU_HASH        080481ac 0001ac 000034 04   A  5   0  4
  [ 5] .dynsym           DYNSYM          080481e0 0001e0 0000f0 10   A  6   1  4
  [ 6] .dynstr           STRTAB          080482d0 0002d0 00018e 00   A  0   0  1
  [ 7] .gnu.version      VERSYM          0804845e 00045e 00001e 02   A  5   0  2
  [ 8] .gnu.version_r    VERNEED         0804847c 00047c 000060 00   A  6   2  4
  [ 9] .rel.dyn          REL             080484dc 0004dc 000018 08   A  5   0  4
  [10] .rel.plt          REL             080484f4 0004f4 000050 08   A  5  12  4
  [11] .init             PROGBITS        08048544 000544 00002e 00  AX  0   0  4
  [12] .plt              PROGBITS        08048580 000580 0000b0 04  AX  0   0 16
  [13] .text             PROGBITS        08048630 000630 00037c 00  AX  0   0 16
  [14] .fini             PROGBITS        080489ac 0009ac 00001a 00  AX  0   0  4
  [15] .rodata           PROGBITS        080489c8 0009c8 000094 00   A  0   0  8
  [16] .eh_frame_hdr     PROGBITS        08048a5c 000a5c 00005c 00   A  0   0  4
  [17] .eh_frame         PROGBITS        08048ab8 000ab8 000168 00   A  0   0  4
  [18] .init_array       INIT_ARRAY      08049ef8 000ef8 000004 00  WA  0   0  4
  [19] .ctors            PROGBITS        08049efc 000efc 000008 00  WA  0   0  4
  [20] .dtors            PROGBITS        08049f04 000f04 000008 00  WA  0   0  4
  [21] .jcr              PROGBITS        08049f0c 000f0c 000004 00  WA  0   0  4
  [22] .dynamic          DYNAMIC         08049f10 000f10 0000e0 08  WA  6   0  4
  [23] .got              PROGBITS        08049ff0 000ff0 000004 04  WA  0   0  4
  [24] .got.plt          PROGBITS        08049ff4 000ff4 000034 04  WA  0   0  4
  [25] .data             PROGBITS        0804a028 001028 000010 00  WA  0   0  4
  [26] .bss              NOBITS          0804a040 001038 0000dc 00  WA  0   0 32
  [27] .comment          PROGBITS        00000000 001038 00002a 01  MS  0   0  1
  [28] .shstrtab         STRTAB          00000000 001062 0000f8 00      0   0  1

值得注意的是，vtable是作为readonly的data被放在.rodata段，而不是大家所认为的.text段（代码段）

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