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1.1 root 1: .TH A.OUT 5 "25 February 1983"
2: .UC 4
3: .SH NAME
4: a.out \- assembler and link editor output
5: .SH SYNOPSIS
6: .B #include <a.out.h>
7: .SH DESCRIPTION
8: .I A.out
9: is the output file of the assembler
10: .IR as (1)
11: and 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: Layout information as given in the include file for the Tahoe is:
18: .nf
19: .ta \w'#define 'u +\w'unsigned 'u +\w'a_dirsize 'u +4n
20: .PP
21: /*
22: .ti +\w'/'u
23: * Header prepended to each a.out file.
24: .ti +\w'/'u
25: */
26: struct exec {
27: long a_magic; /* magic number */
28: unsigned a_text; /* size of text segment */
29: unsigned a_data; /* size of initialized data */
30: unsigned a_bss; /* size of uninitialized data */
31: unsigned a_syms; /* size of symbol table */
32: unsigned a_entry; /* entry point */
33: unsigned a_trsize; /* size of text relocation */
34: unsigned a_drsize; /* size of data relocation */
35: };
36:
37: #define OMAGIC 0407 /* old impure format */
38: #define NMAGIC 0410 /* read-only text */
39: #define ZMAGIC 0413 /* demand load format */
40:
41: /*
42: .ti +\w'/'u
43: * Macros which take exec structures as arguments and tell whether
44: .ti +\w'/'u
45: * the file has a reasonable magic number or offsets to text\||\|symbols\||\|strings.
46: .ti +\w'/'u
47: */
48: #define N_BADMAG(x) \e
49: (((x).a_magic)!=OMAGIC && ((x).a_magic)!=NMAGIC && ((x).a_magic)!=ZMAGIC)
50:
51: #define N_TXTOFF(x) \e
52: ((x).a_magic==ZMAGIC ? 1024 : sizeof (struct exec))
53: #define N_SYMOFF(x) \e
54: (N_TXTOFF(x) + (x).a_text+(x).a_data + (x).a_trsize+(x).a_drsize)
55: #define N_STROFF(x) \e
56: (N_SYMOFF(x) + (x).a_syms)
57: .DT
58: .fi
59: .PP
60: The file has five sections:
61: a header, the program text and data,
62: relocation information, a symbol table and a string table (in that order).
63: The last three may be omitted
64: if the program was loaded
65: with the `\-s' option
66: of
67: .I ld
68: or if the symbols and relocation have been
69: removed by
70: .IR strip (1).
71: .PP
72: In the header the sizes of each section are given in bytes.
73: The size of the header is not included in any of the other sizes.
74: .PP
75: When an
76: .I a.out
77: file is executed, three logical segments are
78: set up: the text segment, the data segment
79: (with uninitialized data, which starts off as all 0, following
80: initialized),
81: and a stack.
82: The text segment begins at 0
83: in the core image; the header is not loaded.
84: If the magic number in the header is OMAGIC (0407),
85: it indicates that the text
86: segment is not to be write-protected and shared,
87: so the data segment is immediately contiguous
88: with the text segment.
89: This is the oldest kind of executable program and is rarely used.
90: If the magic number is NMAGIC (0410) or ZMAGIC (0413),
91: the data segment begins at the first 0 mod 1024 byte
92: boundary following the text segment,
93: and the text segment is not writable by the program;
94: if other processes are executing the same file,
95: they will share the text segment.
96: For ZMAGIC format, the text segment begins at a 0 mod 1024 byte boundary
97: in the
98: .I a.out
99: file, the remaining bytes after the header in the first block are
100: reserved and should be zero.
101: In this case the text and data sizes must both be multiples of 1024 bytes,
102: and the pages of the file will be brought into the running image as needed,
103: and not pre-loaded as with the other formats. This is especially suitable
104: for very large programs and is the default format produced by
105: .IR ld (1).
106: .PP
107: The stack will occupy the highest possible locations
108: in the core image: growing downwards from
109: .lg 0
110: 0xbffff000.
111: .lg 1
112: The stack is automatically extended as required.
113: The data segment is only extended as requested by
114: .IR brk (2).
115: .PP
116: After the header in the file follow the text, data, text relocation
117: data relocation, symbol table and string table in that order.
118: The text begins at the byte 1024 in the file for ZMAGIC format or just
119: after the header for the other formats. The N_TXTOFF macro returns
120: this absolute file position when given the name of an exec structure
121: as argument. The data segment is contiguous with the text and immediately
122: followed by the text relocation and then the data relocation information.
123: The symbol table follows all this; its position is computed by the
124: N_SYMOFF macro. Finally, the string table immediately follows the
125: symbol table at a position which can be gotten easily using N_STROFF.
126: The first 4 bytes of the string table are not used for string storage,
127: but rather contain the size of the string table; this size INCLUDES
128: the 4 bytes, the minimum string table size is thus 4.
129: .PP
130: The layout of a symbol table entry and the principal flag values
131: that distinguish symbol types are given in the include file as follows:
132: .PP
133: .nf
134: .ta \w'#define 'u +\w'char'u-1u +\w'unsigned 'u+1u +\w'*n_name 'u
135: /*
136: .ti +\w'/'u
137: * Format of a symbol table entry.
138: .ti +\w'/'u
139: */
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, i.e. N_TEXT etc; see below */
146: char n_other;
147: short n_desc; /* see <stab.h> */
148: unsigned n_value; /* value of this symbol (or offset) */
149: };
150: #define n_hash n_desc /* used internally by ld */
151:
152: /*
153: .ti +\w'/'u
154: * Simple values for n_type.
155: .ti +\w'/'u
156: */
157: #define N_UNDF 0x0 /* undefined */
158: #define N_ABS 0x2 /* absolute */
159: #define N_TEXT 0x4 /* text */
160: #define N_DATA 0x6 /* data */
161: #define N_BSS 0x8 /* bss */
162: #define N_COMM 0x12 /* common (internal to ld) */
163: #define N_FN 0x1f /* file name symbol */
164:
165: #define N_EXT 01 /* external bit, or'ed in */
166: #define N_TYPE 0x1e /* mask for all the type bits */
167:
168: /*
169: .ti +\w'/'u
170: * Other permanent symbol table entries have some of the N_STAB bits set.
171: .ti +\w'/'u
172: * These are given in <stab.h>
173: .ti +\w'/'u
174: */
175: #define N_STAB 0xe0 /* if any of these bits set, don't discard */
176:
177: /*
178: .ti +\w'/'u
179: * Format for namelist values.
180: .ti +\w'/'u
181: */
182: #define N_FORMAT "%08x"
183: .fi
184: .DT
185: .PP
186: In the
187: .I a.out
188: file a symbol's n_un.n_strx field gives an index into the
189: string table. A n_strx value of 0 indicates that no name is associated
190: with a particular symbol table entry. The field n_un.n_name can be used
191: to refer to the symbol name only if the program sets this up using
192: n_strx and appropriate data from the string table.
193: .PP
194: If a symbol's type is undefined external,
195: and the value field is non-zero,
196: the symbol is interpreted by the loader
197: .I ld
198: as
199: the name of a common region
200: whose size is indicated by the value of the
201: symbol.
202: .PP
203: The value of a byte in the text or data which is not
204: a portion of a reference to an undefined external symbol
205: is exactly that value which will appear in memory
206: when the file is executed.
207: If a byte in the text or data
208: involves a reference to an undefined external symbol,
209: as indicated by the relocation information,
210: then the value stored in the file
211: is an offset from the associated external symbol.
212: When the file is processed by the
213: link editor and the external symbol becomes
214: defined, the value of the symbol will
215: be added to the bytes in the file.
216: .PP
217: If relocation
218: information is present, it amounts to eight bytes per
219: relocatable datum as in the following structure:
220: .PP
221: .nf
222: .ta \w'#define 'u +\w'unsigned 'u +\w'r_symbolnum:24, 'u +4n
223: /*
224: .ti +\w'/'u
225: * Format of a relocation datum.
226: .ti +\w'/'u
227: */
228: struct relocation_info {
229: int r_address; /* address which is relocated */
230: unsigned r_symbolnum:24, /* local symbol ordinal */
231: r_pcrel:1, /* was relocated pc relative already */
232: r_length:2, /* 0=byte, 1=word, 2=long */
233: r_extern:1, /* does not include value of sym referenced */
234: :4; /* nothing, yet */
235: };
236: .fi
237: .DT
238: .PP
239: There is no relocation information if a_trsize+a_drsize==0.
240: If r_extern is 0, then r_symbolnum is actually a n_type for the relocation
241: (i.e. N_TEXT meaning relative to segment text origin.)
242: .fi
243: .SH "SEE ALSO"
244: adb(1), as(1), ld(1), nm(1), dbx(1), stab(5), strip(1)
245: .SH BUGS
246: Not having the size of the string table in the header is a loss, but
247: expanding the header size would have meant stripped executable file
248: incompatibility, and we couldn't hack this just now.
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