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Luzifer
Asked:2020-05-26 21:30:29 +0000 UTC2020-05-26 21:30:29 +0000 UTC

如何读取 64 位值？

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我需要将有关 ELF 的信息放入标题结构中，其中结构的字段是 64 位值。当我尝试以通常的方式阅读它们时，我在所有领域都得到 0。您能告诉我如何使下面的代码正常工作吗？

  1. 用于读取 ELF 字节的 hexdump 文件

    00000000  7f 45 4c 46 02 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
00000010  01 00 3e 00 01 00 00 00  00 00 00 00 00 00 00 00  |..>.............|
00000020  00 00 00 00 00 00 00 00  40 00 00 00 00 00 00 00  |........@.......|
00000030  00 00 00 00 40 00 00 00  00 00 40 00 05 00 02 00  |....@.....@.....|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000080  01 00 00 00 01 00 00 00  06 00 00 00 00 00 00 00  |................|
00000090  00 00 00 00 00 00 00 00  80 01 00 00 00 00 00 00  |................|
000000a0  0c 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000000b0  10 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000000c0  07 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
000000d0  00 00 00 00 00 00 00 00  90 01 00 00 00 00 00 00  |................|
000000e0  21 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |!...............|
000000f0  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000100  11 00 00 00 02 00 00 00  00 00 00 00 00 00 00 00  |................|
00000110  00 00 00 00 00 00 00 00  c0 01 00 00 00 00 00 00  |................|
00000120  60 00 00 00 00 00 00 00  04 00 00 00 03 00 00 00  |`...............|
00000130  04 00 00 00 00 00 00 00  18 00 00 00 00 00 00 00  |................|
00000140  19 00 00 00 03 00 00 00  00 00 00 00 00 00 00 00  |................|
00000150  00 00 00 00 00 00 00 00  20 02 00 00 00 00 00 00  |........ .......|
00000160  0e 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000170  01 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000180  b8 3c 00 00 00 bf 00 00  00 00 0f 05 00 00 00 00  |.<..............|
00000190  00 2e 74 65 78 74 00 2e  73 68 73 74 72 74 61 62  |..text..shstrtab|
000001a0  00 2e 73 79 6d 74 61 62  00 2e 73 74 72 74 61 62  |..symtab..strtab|
000001b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001d0  00 00 00 00 00 00 00 00  01 00 00 00 04 00 f1 ff  |................|
000001e0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
000001f0  00 00 00 00 03 00 01 00  00 00 00 00 00 00 00 00  |................|
00000200  00 00 00 00 00 00 00 00  07 00 00 00 10 00 01 00  |................|
00000210  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000220  00 65 78 31 2e 73 00 5f  73 74 61 72 74 00 00 00  |.ex1.s._start...|

  2. 将ELF读入结构的源代码

    #include <iostream>
#include <fstream>
#include <stdint.h>

/* Type for a 16-bit quantity.  */
typedef uint16_t Elf64_Half;

/* Types for signed and unsigned 32-bit quantities.  */
typedef uint32_t Elf64_Word;

/* Type of addresses.  */
typedef uint64_t Elf64_Addr;

/* Type of file offsets.  */
typedef uint64_t Elf64_Off;

/* The ELF file header.  This appears at the start of every ELF file.  */

#define EI_NIDENT (16)

typedef struct
{
  unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
  Elf64_Half      e_type;                   /* Object file type */
  Elf64_Half      e_machine;              /* Architecture */
  Elf64_Word      e_version;            /* Object file version */
  Elf64_Addr      e_entry;            /* Entry point virtual address */
  Elf64_Off     e_phoff;              /* Program header table file offset */
  Elf64_Off     e_shoff;              /* Section header table file offset */
  Elf64_Word      e_flags;            /* Processor-specific flags */
  Elf64_Half    e_ehsize;             /* ELF header size in bytes */
  Elf64_Half    e_phentsize;            /* Program header table entry size */
  Elf64_Half    e_phnum;                /* Program header table entry count */
  Elf64_Half      e_shentsize;          /* Section header table entry size */
  Elf64_Half    e_shnum;                /* Section header table entry count */
  Elf64_Half      e_shstrndx;           /* Section header string table index */
} Elf64_Ehdr;

int main(int argc, char const *argv[])
{
    Elf64_Ehdr ehdr = {};

    std::ifstream ifs;

    ifs.open("ex1.o", std::ios::binary|std::ios::in);

    for (int i = 0; i < EI_NIDENT; i++)
    {
      ifs >> ehdr.e_ident[i];
    }

    ifs >> ehdr.e_type;
    ifs >> ehdr.e_machine;
    ifs >> ehdr.e_version;
    ifs >> ehdr.e_entry;
    ifs >> ehdr.e_phoff;
    ifs >> ehdr.e_shoff;
    ifs >> ehdr.e_flags;
    ifs >> ehdr.e_ehsize;
    ifs >> ehdr.e_phentsize;
    ifs >> ehdr.e_phnum;
    ifs >> ehdr.e_shentsize;
    ifs >> ehdr.e_shnum;
    ifs >> ehdr.e_shstrndx;

    return 0;
}

  3. 结果：除 e_ident 之外的所有字段都包含 0

c++

1 个回答

  1. Best Answer

    尝试而不是所有这些阅读每个字段依次阅读

    ifs.read((char*)&ehdr,sizeof(ehdr))

    幸运的是，你有一个显式的 POD（plain old data，C 风格的plain old data）。

    这很有趣 - 那些具有非 POD 结构的人 - 尝试将一块写入内存，而那些拥有纯 POD 的人 - 相反，开始不必要（而且不正确）按字段读取......

    • 1

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