COFF - 中间文件格式解析

G Common Object File Format (COFF)

Overall structure 630

File header 632

Optional header 633

Section headers 634

Raw data sections 636

COFF relocation information 637

Line number information 639

Symbol table 641

Additional symbols 643

String table 643

This section describes the Common Object File Format,

本节描述了通用对象文件格式

COFF, used by the linker.

COFF文件, 提供给链接器连接成可执行文件的中间文件

Overall structure

整体的COFF文件结构体:

The COFF Object Format is used both for object files (.o extension) and executable files.

COFF目标格式既用于中间文件,也用于可执行文件

Some of the information is only present in object files,

一些信息只出现在对象文件中

other information is only present in the executable files.

其他的信息只出现在可执行文件中

Table G-1   COFF file components COFF文件组成

The following figure shows the COFF file structure:

下图显示的是COFF文件结构

File header 文件头

The file header contains general information about the object file

文件头包含目标文件的一般信息

and has the following structure from the file filehdr.h:

下面是来自filehdr.h文件的结构体

 1 struct filehdr {
 3     unsigned short  f_magic;    /* magic 魔术字 */
 5     unsigned short  f_nscns;    /* number of sections 区段个数*/
 7     long            f_timdat;   /* date stamp  时间戳*/
 9     long            f_symptr;   /* fileptr to symtab 符号表的文件偏移*/
11     long            f_nsyms;    /* symtab count 符号表个数*/
13     unsigned short  f_opthdr;   /* sizeof(optional hdr) 扩展头的大小*/
15     unsigned short  f_flags;    /* flags COFF文件属性*/
17 }; 

Table G-2   COFF header fields

Optional header

The optional header contains information about an executable file and has the following structure from the file aouthdr.h:

扩展头包含可执行文件的信息.下面是来自aouthdr.h头文件的结构体:

 1 typedef struct aouthdr {
 3     short   magic;              /* a.out magic */
 5     short   vstamp;             /* version stamp */
 7     long    tsize;              /* .text size */
 9     long    dsize;              /* .data size */
11     long    bsize;              /* .bss size */
13     long    entry;              /* entry point */
15     long    text_start;         /* fileptr to .text */
17     long    data_start;         /* fileptr to .data */
19 } AOUTHDR; 

Table G-3   COFF optional (executable) header fields

There is one section header for each section in the COFF file,

每个COFF文件的区段都有像下面一样的区段头

specified by the f_nscns field in the COFF File Header.

由COFF的文件头结构体中的 f_nscns字段指出它的文件偏移

Section headers have the following structure from the file scnhdr.h:

下面是来自scnhdr.h头文件的区段头结构体

 1 struct scnhdr {                     /* modified COFF*/
 3     char            s_name[8];      /* section name 区段名*/
 5     long            s_paddr;        /* physical address 物理地址*/
 7     long            s_vaddr;        /* virtual address 虚拟地址*/
 9     long            s_size;         /* size of section 区段的字节数*/
11     long            s_scnptr;       /* fileptr to raw data 指向原始数据的文件偏移*/
13     long            s_relptr;       /* fileptr to reloc 指向重定位表的文件偏移*/
15     long            s_lnnoptr;      /* fileptr to lineno 指向行号表的文件偏移*/
17     unsigned long short  s_nreloc;       /* reloc count 重定位表条目数*/
19     unsigned long short  s_nlnno;        /* line number count 行号表条目数*/
21     long            s_flags;        /* flags */
23 };

#define SCNHDR struct scnhdr

#define SCNHSZ sizeof(SCNHDR)

Table G-4   COFF section header fields

The following table shows the correspondence between the type-spec as defined on p.409 and the COFF section flags assigned to the output section.

Table G-5   type-spec - COFF section flag correspondence

Raw data sections 原始数据区段

The Raw Data Sections contain the actual raw data for each section.

原始数据区段包含每个区段的实际的原始数据

Table G-6   COFF section names

COFF relocation information

The Relocation Information segment contains information about unresolved references.

重定位段包含外部未分配地址的符号.

Since compilers and assemblers do not know at what absolute memory address a symbol will be allocated,

当汇编器和编译器不知道怎么给一个符号分配绝对的内存地址时.

and since they are unaware of definitions of symbols in other files,

因为汇编器和编译器不知道该符号会在其他文件定义

every reference to such a symbol will create a relocation entry.

所有这样的符号引用将被创建一个重定位条目

The relocation entry will point to the address where the reference is being made,

这个重定位条目将会指向这个符号被引用的地址.

and to the symbol table entry that contains the symbol that is referenced.

所以,当一个符号是被引用的,它会被包含在符号表的条目中.

The linker will use this information to fill in the correct address after it has allocated addresses to all symbols.

链接器可以使用这些信息给所有符号分配地址后,纠正这些被引用符号的地址

When an offset is added to a symbol in the assembly source,

当添加一个汇编源码中的符号时

lwz     r3,(var+16)(r0)

move.l  var+16,d0

that offset is stored in the addressing mode,

这个偏移到的地方是没有寻址模式的

so that adding the real address of the symbol with the address field will yield a correct reference.

这样添加的符号的真正地址的字段将产生一个正确的参考。

The relocation segment does not exist in executable files.

重定位段在可执行文件中不存在

A relocation entry has the following structure from the file reloc.h:

 1 struct reloc {                  /* modified COFF */
 3     long            r_vaddr;    /* 引用的地址(文件偏移) */
 5     long            r_symndx;   /* 在符号表的索引(符号名) */
 7     unsigned short  r_type;     /* 重定位类型 */
 9     unsigned short  r_offset;   /* 高位的字是真实地址*/
11 };
12
13
14
15 #define RELOC   struct reloc
16
17 #define RELSZ   sizeof(RELOC)
18
19 #define RELSZ   10              /* sizeof(RELOC) */ 

Table G-7   COFF relocation entry fields

Table G-8   COFF relocation types

Line number information

The line number information segment contains the mapping from source line numbers to machine instruction addresses used by symbolic debuggers. This information is only available if the -g option is specified to the compiler.

Line number entries for a section form groups of pairs where the first pair in a group is a pointer to the function containing the source. After that, every source line that has generated any instruction has an entry specifying the line number relative to the beginning of the function, and the corresponding instruction address. Normally only the .text section has line number information. The following table demonstrates the layout of the line number entries:

A line number entry has the following structure from the file linenum.h:

 1 struct lineno {
 3     union {
 5         long        l_symndx;
 7         long        l_paddr;
 9     } l_addr;
11     unsigned long short  l_lnno;
13 };
14
15
16
17 #define LINENO      struct lineno
18
19 #define LINESZ      sizeof(LINENO)
20
21 #define LINESZ      6
22
23 Table G-9   COFF line number fields 

Symbol table

The symbol table is an array of entries containing information about the symbols referenced in the COFF file.

符号表是一个包含COFF文件的符号引用的一个数组.

A symbol table entry has the following structure from the file syms.h:

 1 struct syment {
 3     union {
 5         char        _n_name[8];
 7         struct {
 9             long    _n_zeroes;
11             long    _n_offset;
13         } _n_n;
15         char        *_n_nptr[2]
17     } _n;
19     long            n_value;
21     short           n_scnum;
25     unsigned short  n_type;
27     char            n_sclass;
29     char            n_numaux;
31     short           n_pad;
33 };
34
35
36
37 #define SYMENT      struct syment
38
39 #define SYMESZ c    20
40
41 #define SYMESZ      18
42
43 #define n_name      _n._n_name
44
45 #define n_nptr      _n._n_nptr[1]
46
47 #define n_zeroes    _n._n_n._n_zeroes
48
49 #define n_offset    _n._n_n._n_offset 

Table G-10   COFF symbol table fields

Any auxiliary entries to a symbol are stored immediately after the symbol in the table. They are mainly used for symbolic debugging (-g option) and are not discussed here.

Additional symbols

Wind River uses special COFF symbols as follows:

Table G-11   Special COFF Symbols

String table

The string table contains the null terminated names of symbols longer than eight characters. Those symbols point into the string table through an offset, n_offset. The first four bytes of the string table contain the size of the table and after that all strings are stored sequentially.

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