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1.1 root 1: /*
2: * Copyright (c) 1980 Regents of the University of California.
3: * All rights reserved. The Berkeley software License Agreement
4: * specifies the terms and conditions for redistribution.
5: *
6: * @(#)a.out.h 5.1 (Berkeley) 5/30/85
7: *
8: * The structure this file describes was originally taken from the above file
9: * and the above copyright has been carried over to this file.
10: */
11:
12: #ifndef _MACHO_RELOC_H_
13: #define _MACHO_RELOC_H_
14:
15: /*
16: * Format of a relocation entry of a Mach-O file. Modified from the 4.3BSD
17: * format. The modifications from the original format were changing the value
18: * of the r_symbolnum field for "local" (r_extern == 0) relocation entries.
19: * This modification is required to support symbols in an arbitrary number of
20: * sections not just the three sections (text, data and bss) in a 4.3BSD file.
21: * Also the last 4 bits have had the r_type tag added to them.
22: */
23: struct relocation_info {
24: long r_address; /* offset in the section to what is being
25: relocated */
26: unsigned int r_symbolnum:24, /* symbol index if r_extern == 1 or section
27: ordinal if r_extern == 0 */
28: r_pcrel:1, /* was relocated pc relative already */
29: r_length:2, /* 0=byte, 1=word, 2=long */
30: r_extern:1, /* does not include value of sym referenced */
31: r_type:4; /* if not 0, machine specific relocation type */
32: };
33: #define R_ABS 0 /* absolute relocation type for Mach-O files */
34:
35: /*
36: * The r_address is not really the address as it's name indicates but an offset.
37: * In 4.3BSD a.out objects this offset is from the start of the "segment" for
38: * which relocation entry is for (text or data). For Mach-O object files it is
39: * also an offset but from the start of the "section" for which the relocation
40: * entry is for.
41: *
42: * In 4.3BSD a.out objects if r_extern is zero then r_symbolnum is an ordinal
43: * for the segment the symbol being relocated is in. These ordinals are the
44: * symbol types N_TEXT, N_DATA, N_BSS or N_ABS. In Mach-O object files these
45: * ordinals refer to the sections in the object file in the order their section
46: * structures appear in the headers of the object file they are in. The first
47: * section has the ordinal 1, the second 2, and so on. This means that the
48: * same ordinal in two different object files could refer to two different
49: * sections. And further could have still different ordinals when combined
50: * by the link-editor. The value R_ABS is used for relocation entries for
51: * absolute symbols which need no further relocation.
52: */
53:
54: /*
55: * For RISC machines some of the references are split across two instructions
56: * and the instruction does not contain the complete value of the reference.
57: * In these cases a second, or paired relocation entry, follows each of these
58: * relocation entries, using a PAIR r_type, which contains the other part of the
59: * reference not contained in the instruction. This other part is stored in the
60: * pair's r_address field. The exact number of bits of the other part of the
61: * reference store in the r_address field is dependent on the particular
62: * relocation type for the particular architecture.
63: */
64:
65: /*
66: * To make scattered loading by the link editor work correctly "local"
67: * relocation entries can't be used when the item to be relocated is the value
68: * of a symbol plus an offset (where the resulting expresion is outside the
69: * block the link editor is moving, a blocks are divided at symbol addresses).
70: * In this case. where the item is a symbol value plus offset, the link editor
71: * needs to know more than just the section the symbol was defined. What is
72: * needed is the actual value of the symbol without the offset so it can do the
73: * relocation correctly based on where the value of the symbol got relocated to
74: * not the value of the expression (with the offset added to the symbol value).
75: * So for the NeXT 2.0 release no "local" relocation entries are ever used when
76: * there is a non-zero offset added to a symbol. The "external" and "local"
77: * relocation entries remain unchanged.
78: *
79: * The implemention is quite messy given the compatibility with the existing
80: * relocation entry format. The ASSUMPTION is that a section will never be
81: * bigger than 2**24 - 1 (0x00ffffff or 16,777,215) bytes. This assumption
82: * allows the r_address (which is really an offset) to fit in 24 bits and high
83: * bit of the r_address field in the relocation_info structure to indicate
84: * it is really a scattered_relocation_info structure. Since these are only
85: * used in places where "local" relocation entries are used and not where
86: * "external" relocation entries are used the r_extern field has been removed.
87: *
88: * For scattered loading to work on a RISC machine where some of the references
89: * are split across two instructions the link editor needs to be assured that
90: * each reference has a unique 32 bit reference (that more than one reference is
91: * NOT sharing the same high 16 bits for example) so it move each referenced
92: * item independent of each other. Some compilers guarantees this but the
93: * compilers don't so scattered loading can be done on those that do guarantee
94: * this.
95: */
96: #if defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__)
97: /*
98: * The reason for the ifdef's of __BIG_ENDIAN__ and __LITTLE_ENDIAN__ are that
99: * when stattered relocation entries were added the mistake of using a mask
100: * against a structure that is made up of bit fields was used. To make this
101: * design work this structure must be laid out in memory the same way so the
102: * mask can be applied can check the same bit each time (r_scattered).
103: */
104: #endif /* defined(__BIG_ENDIAN__) || defined(__LITTLE_ENDIAN__) */
105: #define R_SCATTERED 0x80000000 /* mask to be applied to the r_address field
106: of a relocation_info structure to tell that
107: is is really a scattered_relocation_info
108: stucture */
109: struct scattered_relocation_info {
110: #ifdef __BIG_ENDIAN__
111: unsigned int r_scattered:1, /* 1=scattered, 0=non-scattered (see above) */
112: r_pcrel:1, /* was relocated pc relative already */
113: r_length:2, /* 0=byte, 1=word, 2=long */
114: r_type:4, /* if not 0, machine specific relocation type */
115: r_address:24; /* offset in the section to what is being
116: relocated */
117: long r_value; /* the value the item to be relocated is
118: refering to (without any offset added) */
119: #endif /* __BIG_ENDIAN__ */
120: #ifdef __LITTLE_ENDIAN__
121: unsigned int
122: r_address:24, /* offset in the section to what is being
123: relocated */
124: r_type:4, /* if not 0, machine specific relocation type */
125: r_length:2, /* 0=byte, 1=word, 2=long */
126: r_pcrel:1, /* was relocated pc relative already */
127: r_scattered:1; /* 1=scattered, 0=non-scattered (see above) */
128: long r_value; /* the value the item to be relocated is
129: refering to (without any offset added) */
130: #endif /* __LITTLE_ENDIAN__ */
131: };
132:
133: /*
134: * Relocation types used in a generic implementation. Relocation entries for
135: * nornal things use the generic relocation as discribed above and their r_type
136: * is GENERIC_RELOC_VANILLA (a value of zero).
137: *
138: * The other type of generic relocation, GENERIC_RELOC_SECTDIFF, is to support
139: * the difference of two symbols defined in different sections. That is the
140: * expression "symbol1 - symbol2 + constant" is a relocatable expression when
141: * both symbols are defined in some section. For this type of relocation the
142: * both relocations entries are scattered relocation entries. The value of
143: * symbol1 is stored in the first relocation entry's r_value field and the
144: * value of symbol2 is stored in the pair's r_value field.
145: */
146: enum reloc_type_generic
147: {
148: GENERIC_RELOC_VANILLA, /* generic relocation as discribed above */
149: GENERIC_RELOC_PAIR, /* Only follows a GENRIC_RELOC_SECTDIFF */
150: GENERIC_RELOC_SECTDIFF
151: };
152:
153: #endif /* _MACHO_RELOC_H_ */
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