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1.1 root 1: .TH A.OUT 5
2: .CT 1 lib_obj
3: .SH NAME
4: a.out \- object file format
5: .SH SYNOPSIS
6: .B #include <a.out.h>
7: .SH DESCRIPTION
8: .I A.out
9: is the default name of the output file of the assembler
10: .IR as (1)
11: or the link editor
12: .IR ld (1).
13: Both programs make
14: .I a.out
15: executable if there were no
16: errors and no unresolved external references.
17: An object file has five sections:
18: a header, the program text and data,
19: relocation information, a symbol table and a string table (in that order).
20: The last three may be absent; see
21: .IR strip (1)
22: and option
23: .B -s
24: of
25: .IR ld (1).
26: The header format, given in
27: .BR <a.out.h> ,
28: is
29: .PP
30: .EX
31: .ta \w'#define 'u +\w'unsigned 'u +\w'a_dirsize 'u +4n
32: struct exec {
33: long a_magic; /* magic number */
34: unsigned a_text; /* size of text segment */
35: unsigned a_data; /* size of initialized data */
36: unsigned a_bss; /* size of uninitialized data */
37: unsigned a_syms; /* size of symbol table */
38: unsigned a_entry; /* entry point */
39: unsigned a_trsize; /* size of text relocation */
40: unsigned a_drsize; /* size of data relocation */
41: };
42: #define OMAGIC 0407 /* old impure format */
43: #define NMAGIC 0410 /* read-only text */
44: #define ZMAGIC 0413 /* demand load format */
45: .EE
46: .PP
47: Macros which take
48: .L exec
49: structures as arguments and tell whether
50: the file has a reasonable magic number or return offsets:
51: .EX
52: #define N_BADMAG(x) (((x).a_magic)!=OMAGIC && \e
53: ((x).a_magic)!=NMAGIC && ((x).a_magic)!=ZMAGIC)
54: #define N_TXTOFF(x) \e
55: ((x).a_magic==ZMAGIC ? 1024 : sizeof (struct exec))
56: #define N_SYMOFF(x) (N_TXTOFF(x) + (x).a_text+(x).a_data + \e
57: (x).a_trsize+(x).a_drsize)
58: #define N_STROFF(x) (N_SYMOFF(x) + (x).a_syms)
59: .EE
60: .DT
61: .PP
62: Sizes are expressed in bytes.
63: The size of the header is not included in any of the other sizes.
64: .PP
65: When an
66: .F a.out
67: file is executed, a memory image of three segments is
68: set up: the text segment, the data segment,
69: and a stack.
70: The text segment begins at virtual address 0.
71: Following the text segment is the data segment, in which
72: explicitly initialized data come first, then
73: other data, initialized to 0.
74: The stack occupies the highest possible locations
75: in the core image, automatically growing downwards from
76: .lg 0
77: .BR 0x7ffff400
78: .lg 1
79: as needed.
80: The data segment may be extended by
81: .IR brk (2).
82: .PP
83: If the magic number in the header is
84: .BR OMAGIC ,
85: the text
86: segment is neither write-protected nor shared and
87: the data segment is immediately contiguous
88: with the text segment.
89: This kind of executable program is rarely used.
90: If the magic number is
91: .BR NMAGIC
92: or
93: .BR ZMAGIC ,
94: the data segment is loaded at the first 0 mod 1024 address
95: following the text segment,
96: and the text segment is write-protected and shared among
97: all processes that are executing the same file.
98: .BR ZMAGIC
99: format, which
100: .IR ld (1)
101: produces by default, supports demand paging: the
102: text and data segments are multiples of
103: 1024 bytes long and begin at byte offsets of 0 mod 1024
104: in the
105: .F a.out
106: file.
107: .PP
108: Macros are provided to compute the absolute offset of various
109: parts of the file:
110: .TF N_TXTOFF
111: .TP
112: .B N_TXTOFF
113: Text segment.
114: .TP
115: .B N_SYMOFF
116: Symbol table.
117: .TP
118: .B N_STROFF
119: String table.
120: .PD
121: .PP
122: The offsets of the data segment, text relocation information,
123: and data relocation information are obtained by successively
124: adding to
125: .B N_TXTOFF
126: the size fields
127: .BR a_text ,
128: .BR a_data ,
129: and
130: .BR a_trsize .
131: The first 4 bytes of the string table contain it size, including
132: the 4 bytes.
133: .PP
134: The layout of a symbol table entry, as given in
135: .BR <a.out.h> ,
136: is
137: .PP
138: .EX
139: .ta \w'#define 'u +\w'char'u-1u +\w'unsigned 'u+1u +\w'*n_name 'u
140: struct nlist {
141: union {
142: char *n_name; /* for use when in-core */
143: long n_strx; /* index into file string table */
144: } n_un;
145: unsigned char n_type; /* type flag; see below */
146: char n_other;
147: short n_desc; /* see \fIstab\fR(5)\f5 */
148: unsigned n_value; /* value of this symbol (or struct offset) */
149: };
150: .EE
151: .PP
152: Basic values for
153: .BR n_type :
154: .PP
155: .EX
156: #define N_UNDF 0x0 /* undefined */
157: #define N_ABS 0x2 /* absolute */
158: #define N_TEXT 0x4 /* text */
159: #define N_DATA 0x6 /* data */
160: #define N_BSS 0x8 /* bss */
161: #define N_COMM 0x12 /* common (internal to ld) */
162: #define N_FN 0x1f /* file name symbol */
163: #define N_EXT 01 /* external bit, or'ed in */
164: #define N_TYPE 0x1e /* mask for all the type bits */
165: .EE
166: .PP
167: Other permanent symbol table entries have some
168: .B N_STAB
169: bits set.
170: These are given in
171: .LR <stab.h> :
172: .EX
173: #define N_STAB 0xe0 /* if any of these bits set, keep */
174: .EE
175: .DT
176: .PP
177: The field
178: .B n_un.n_strx
179: gives an index into the
180: string table; 0
181: indicates that no name is associated
182: with the entry.
183: The field
184: .B n_un.n_name
185: can be used
186: to refer to the symbol name only if the program sets this up using
187: .B n_strx
188: and appropriate data from the string table.
189: .PP
190: A symbol of type undefined external with nonzero value field
191: names a common region; the value specifies its size.
192: .PP
193: Relocation
194: information occupies eight bytes per
195: relocatable datum:
196: .PP
197: .EX
198: .ta \w'#define 'u +\w'unsigned 'u +\w'r_symbolnum:24, 'u +4n
199: struct relocation_info {
200: int r_address; /* address of datum to be relocated */
201: unsigned r_symbolnum:24, /* local symbol ordinal */
202: r_pcrel:1, /* is referenced relative to pc */
203: r_length:2, /* 0=byte, 1=word, 2=long */
204: r_extern:1, /* symbol value unknown */
205: :4; /* nothing, yet */
206: };
207: .EE
208: .DT
209: .PP
210: If
211: .B r_extern
212: is 1, the datum designated by
213: .B r_address
214: and
215: .B r_length
216: will be relocated by adding to it the value of the associated
217: external symbol.
218: If
219: .B r_extern
220: is 0,
221: .BR r_symbolnum
222: is encoded in the style of
223: .B n_type
224: and the value will be relocated by adding the relocated base of
225: the designated area (text, initialized data, or common data).
226: .SH "SEE ALSO"
227: .IR adb (1),
228: .IR as (1),
229: .IR ld (1),
230: .IR nm (1),
231: .IR stab (5),
232: .IR strip (1)
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