a.out格式解析
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最近在看linux0.11代码，涉及到很多linux系统特性的东西，以下收集到a.out格式的文档。维基上的介绍也不够详细：http://en.wikipedia.org/wiki/A.out
来源：http://modman.unixdev.net/?sektion=5&page=a.out&manpath=SunOS-4.1.3
A.OUT(5) A.OUT(5)NAME a.out - assembler and link editor output formatSYNOPSIS #include <<a.out.h>> #include <<stab.h>> #include <<nlist.h>>AVAILABILITY Sun-2, Sun-3, and Sun-4 systems only. For Sun386i systems refer to coff(5).DESCRIPTION a.out is the output format of the assembler as(1) and the link editor ld(1). The link editor makes a.out executable files. A file in a.out format consists of: a header, the program text, program data, text and data relocation information, a symbol table, and a string table (in that order). In the header, the sizes of each section are given in bytes. The last three sections may be absent if the pro- gram was loaded with the -s option of ld or if the symbols and reloca- tion have been removed by strip(1). The machine type in the header indicates the type of hardware on which the object code can be executed. Sun-2 code runs on Sun-3 systems, but not vice versa. Program files predating release 3.0 are recognized by a machine type of `0'. Sun-4 code may not be run on Sun-2 or Sun-3, nor vice versa. Header The header consists of a exec structure. The exec structure has the form: struct exec { unsigned char a_dynamic:1;/* has a __DYNAMIC */ unsigned char a_toolversion:7;/* version of toolset used to create this file */ unsigned char a_machtype;/* machine type */ unsigned short a_magic;/* magic number */ unsigned long a_text;/* size of text segment */ unsigned long a_data;/* size of initialized data */ unsigned long a_bss;/* size of uninitialized data */ unsigned long a_syms;/* size of symbol table */ unsigned long a_entry;/* entry point */ unsigned long a_trsize;/* size of text relocation */ unsigned long a_drsize;/* size of data relocation */ }; The members of the structure are: a_dynamic 1 if the a.out file is dynamically linked or is a shared object, 0 otherwise. a_toolversion The version number of the toolset (as, ld, etc.) used to create the file. a_machtype One of the following: 0 pre-3.0 executable image M_68010 executable image using only MC68010 instruc- tions that can run on Sun-2 or Sun-3 sys- tems. M_68020 executable image using MC68020 instructions that can run only on Sun-3 systems. M_SPARC executable image using SPARC instructions that can run only on Sun-4 systems. a_magic One of the following: OMAGIC An text executable image which is not to be write-protected, so the data segment is immediately contiguous with the text seg- ment. NMAGIC A write-protected text executable image. The data segment begins at the first segment boundary following the text segment, and the text segment is not writable by the program. When the image is started with execve(2V), the entire text and data segments will be read into memory. ZMAGIC A page-aligned text executable image. the data segment begins at the first segment boundary following the text segment, and the text segment is not writable by the program. The text and data sizes are both multiples of the page size, and the pages of the file will be brought into the running image as needed, and not pre-loaded as with the other formats. This is the default format pro- duced by ld(1). The macro N_BADMAG takes an exec structure as an argu- ment; it evaluates to 1 if the a_magic field of that structure is invalid, and evaluates to 0 if it is valid. a_text The size of the text segment, in bytes. a_data The size of the initialized portion of the data segment, in bytes. a_bss The size of the "uninitialized" portion of the data seg- ment, in bytes. This portion is actually initialized to zero. The zeroes are not stored in the a.out file; the data in this portion of the data segment is zeroed out when it is loaded. a_syms The size of the symbol table, in bytes. a_entry The virtual address of the entry point of the program; when the image is started with execve, the first instruction executed in the image is at this address. a_trsize The size of the relocation information for the text seg- ment. a_drsize The size of the relocation information for the data seg- ment. The macros N_TXTADDR, N_DATADDR, and N_BSSADDR give the memory addresses at which the text, data, and bss segments, respectively, will be loaded. In the ZMAGIC format, the size of the header is included in the size of the text section; in other formats, it is not. When an a.out file is executed, three logical segments are set up: the text segment, the data segment (with uninitialized data, which starts off as all 0, following initialized data), and a stack. For the ZMAGIC format, the header is loaded with the text segment; for other formats it is not. Program execution begins at the address given by the value of the a_entry field. The stack starts at the highest possible location in the memory image, and grows downwards. The stack is automatically extended as required. The data segment is extended as requested by brk(2) or sbrk. Text and Data Segments The text segment begins at the start of the file for ZMAGIC format, or just after the header for the other formats. The N_TXTOFF macro returns this absolute file position when given an exec structure as argument. The data segment is contiguous with the text and immediately followed by the text relocation and then the data relocation informa- tion. The N_DATOFF macro returns the absolute file position of the beginning of the data segment when given an exec structure as argument. Relocation The relocation information appears after the text and data segments. The N_TRELOFF macro returns the absolute file position of the reloca- tion information for the text segment, when given an exec structure as argument. The N_DRELOFF macro returns the absolute file position of the relocation information for the data segment, when given an exec structure as argument. There is no relocation information if a_trsize+a_drsize==0. Relocation (Sun-2 and Sun-3 Systems) If a byte in the text or data involves a reference to an undefined external symbol, as indicated by the relocation information, then the value stored in the file is an offset from the associated external sym- bol. When the file is processed by the link editor and the external symbol becomes defined, the value of the symbol is added to the bytes in the file. If a byte involves a reference to a relative location, or relocatable segment, then the value stored in the file is an offset from the associated segment. If relocation information is present, it amounts to eight bytes per relocatable datum as in the following structure: struct reloc_info_68k { long r_address;/* address which is relocated */ unsigned intr_symbolnum:24,/* local symbol ordinal */ r_pcrel:1,/* was relocated pc relative already */ r_length:2,/* 0=byte, 1=word, 2=long */ r_extern:1,/* does not include value of sym referenced */ r_baserel:1,/* linkage table relative */ r_jmptable:1,/* pc-relative to jump table */ r_relative:1,/* relative relocation */ :1; }; If r_extern is 0, then r_symbolnum is actually an n_type for the relo- cation (for instance, N_TEXT meaning relative to segment text origin.) Relocation (Sun-4 System) If a byte in the text or data involves a reference to an undefined external symbol, as indicated by the relocation information, then the value stored in the file is ignored. Unlike the Sun-2 and Sun-3 system, the offset from the associated symbol is kept with the relocation record. When the file is processed by the link editor and the external symbol becomes defined, the value of the symbol is added to this off- set, and the sum is inserted into the bytes in the text or data seg- ment. If relocation information is present, it amounts to twelve bytes per relocatable datum as in the following structure: enum reloc_type { RELOC_8,RELOC_16,RELOC_32,/* simplest relocs */ RELOC_DISP8,RELOC_DISP16,RELOC_DISP32,/* Disp's (pc-rel) */ RELOC_WDISP30,RELOC_WDISP22,/* SR word disp's */ RELOC_HI22,RELOC_22,/* SR 22-bit relocs */ RELOC_13,RELOC_LO10,/* SR 13&&10-bit relocs */ RELOC_SFA_BASE,RELOC_SFA_OFF13,/* SR S.F.A. relocs */ RELOC_BASE10,RELOC_BASE13,RELOC_BASE22,/* base_relative pic */ RELOC_PC10,RELOC_PC22,/* special pc-rel pic*/ RELOC_JMP_TBL,/* jmp_tbl_rel in pic */ RELOC_SEGOFF16,/* ShLib offset-in-seg */ RELOC_GLOB_DAT,RELOC_JMP_SLOT,RELOC_RELATIVE,/* rtld relocs */ }; struct reloc_info_sparc/* used when header.a_machtype == M_SPARC */ { unsigned long intr_address;/* relocation addr (offset in segment) */ unsigned intr_index:24;/* segment index or symbol index */ unsigned intr_extern: 1;/* if F, r_index==SEG#; if T, SYM idx */ int: 2;/* <<unused>> */ enum reloc_typer_type: 5;/* type of relocation to perform */ long intr_addend;/* addend for relocation value */ }; If r_extern is 0, then r_index is actually a n_type for the relocation (for instance, N_TEXT meaning relative to segment text origin.) Symbol Table The N_SYMOFF macro returns the absolute file position of the symbol ta- ble when given an exec structure as argument. Within this symbol ta- ble, distinct symbols point to disjoint areas in the string table (even when two symbols have the same name). The string table immediately follows the symbol table; the N_STROFF macro returns the absolute file position of the string table when given an exec structure as argument. The first 4 bytes of the string table are not used for string storage, but rather contain the size of the string table. This size includes the 4 bytes; thus, the minimum string table size is 4. Layout information as given in the include file for the Sun system is shown below. The layout of a symbol table entry and the principal flag values that distinguish symbol types are given in the include file as follows: struct nlist { union { char*n_name;/* for use when in-memory */ longn_strx;/* index into file string table */ } n_un; unsigned charn_type;/* type flag, that is, N_TEXT etc; see below */ charn_other; shortn_desc;/* see <<stab.h>> */ unsignedn_value;/* value of this symbol (or adb offset) */ }; #definen_hashn_desc/* used internally by ld */ /* * Simple values for n_type. */ #defineN_UNDF0x0/* undefined */ #defineN_ABS0x2/* absolute */ #defineN_TEXT0x4/* text */ #defineN_DATA0x6/* data */ #defineN_BSS0x8/* bss */ #defineN_COMM0x12/* common (internal to ld) */ #defineN_FN0x1f/* file name symbol */ #defineN_EXT01/* external bit, or'ed in */ #defineN_TYPE0x1e/* mask for all the type bits */ /* * Other permanent symbol table entries have some of the N_STAB bits set. * These are given in <<stab.h>> */ #defineN_STAB0xe0/* if any of these bits set, don't discard */ In the a.out file a symbol's n_un.n_strx field gives an index into the string table. A n_strx value of 0 indicates that no name is associated with a particular symbol table entry. The field n_un.n_name can be used to refer to the symbol name only if the program sets this up using n_strx and appropriate data from the string table. Because of the union in the nlist declaration, it is impossible in C to statically initialize such a structure. If this must be done (as when using nlist(3V)) include the file <<nlist.h>>, rather than <<a.out.h>>. This contains the declaration without the union. If a symbol's type is undefined external, and the value field is non- zero, the symbol is interpreted by the loader ld as the name of a com- mon region whose size is indicated by the value of the symbol.SEE ALSO adb(1), as(1), cc(1V), dbx(1), ld(1), nm(1), strip(1), brk(2), nlist(3V), coff(5) 18 February 1988 A.OUT(5)
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