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7 | .. |
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13 | .. |
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14 | .TH sym 3 "8 May 1999" mLib |
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15 | .SH NAME |
16 | sym \- symbol table manager |
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17 | .\" @sym_create |
18 | .\" @sym_destroy |
19 | .\" @sym_find |
20 | .\" @sym_remove |
21 | .\" @sym_mkiter |
22 | .\" @sym_next |
23 | .\" |
24 | .\" @SYM_NAME |
25 | .\" |
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26 | .SH SYNOPSIS |
27 | .nf |
28 | .B "#include <mLib/sym.h>" |
29 | |
30 | .BI "void sym_create(sym_table *" t ); |
31 | .BI "void sym_destroy(sym_table *" t ); |
32 | |
33 | .BI "char *SYM_NAME(void *" p ); |
34 | .BI "void *sym_find(sym_table *" t , |
35 | .BI " const char *" n ", long " l , |
36 | .BI " size_t " sz ", unsigned *" f ); |
37 | .BI "void sym_remove(sym_table *" t ", void *" b ); |
38 | |
39 | .BI "void sym_mkiter(sym_iter *" i ", sym_table *" t ); |
40 | .BI "void *sym_next(sym_iter *" i ); |
41 | .fi |
42 | .SH "HOW IT WORKS" |
43 | The |
44 | .B sym |
45 | functions implement a data structure often described as a dictionary, a |
46 | finite map, an associative array, or a symbol table. It associates |
47 | .I values |
48 | with |
49 | .I keys |
50 | such that the value corresponding to a given key can be found quickly. |
51 | Additionally, all stored associations can be enumerated. |
52 | .PP |
53 | The interface provides an |
54 | .I intrusive |
55 | symbol table. The data objects stored in the table must include a small |
56 | header used by the symbol table manager. This reduces the amount of |
57 | pointer fiddling that needs to be done, and in practice doesn't seem to |
58 | be much of a problem. It's also fairly easy to construct a |
59 | non-intrusive interface if you really want one. |
60 | .PP |
61 | There are three main data structures involved in the interface: |
62 | .TP |
63 | .B sym_table |
64 | Keeps track of the information associated with a particular table. |
65 | .TP |
66 | .B sym_base |
67 | The header which must be attached to the front of all the value |
68 | objects. |
69 | .TP |
70 | .B sym_iter |
71 | An iterator object, used for enumerating all of the associations stored |
72 | in a symbol table. |
73 | .PP |
74 | All of the above data structures should be considered |
75 | .IR opaque : |
76 | don't try looking inside. Representations have changed in the past, and |
77 | they may change again in the future. |
78 | .SH "CREATION AND DESTRUCTION" |
79 | The |
80 | .B sym_table |
81 | object itself needs to be allocated by the caller. It is initialized by |
82 | passing it to the function |
83 | .BR sym_create . |
84 | After initialization, the table contains no entries. |
85 | .PP |
86 | Initializing a symbol table involves allocating some memory. If this |
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87 | allocation fails, an |
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88 | .B EXC_NOMEM |
89 | exception is raised. |
90 | .PP |
91 | When a symbol table is no longer needed, the memory occupied by the |
92 | values and other maintenance structures can be reclaimed by calling |
93 | .BR sym_destroy . |
94 | Any bits of user data attached to the symbol table values should have |
95 | previously been destroyed. |
96 | .SH "ADDING, SEARCHING AND REMOVING" |
97 | Most of the actual work is done by the function |
98 | .BR sym_find . |
99 | It does both lookup and creation, depending on its arguments. To do its |
100 | job, it needs to know the following bits of information: |
101 | .TP |
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102 | .BI "sym_table *" t |
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103 | A pointer to a symbol table to manipulate. |
104 | .TP |
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105 | .BI "const char *" n |
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106 | The address of the |
107 | .I key |
108 | to look up or create. Usually this will be a simple text string, |
109 | although it can actually be any arbitrary binary data. |
110 | .TP |
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111 | .BI "long " l |
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112 | The length of the key. If this is \-1, |
113 | .B sym_find |
114 | assumes that the key is a null-terminated string, and calculates its |
115 | length itself. |
116 | .TP |
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117 | .BI "size_t " sz |
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118 | The size of the value block to allocate if the key could not be found. |
119 | If this is zero, no value is allocated, and a null pointer is returned |
120 | to indicate an unsuccessful lookup. |
121 | .TP |
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122 | .BI "unsigned *" f |
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123 | The address of a `found' flag to set. This is an output parameter. On |
124 | exit, |
125 | .B sym_find |
126 | will set the value of |
127 | .BI * f |
128 | to zero if the key could not be found, or nonzero if it was found. This |
129 | can be used to tell whether the value returned has been newly allocated, |
130 | or whether it was already in the table. |
131 | .PP |
132 | The macro |
133 | .B SYM_NAME |
134 | will return the key associated with a given value block. You must use |
135 | this macro rather than fiddling inside the |
136 | .B sym_base |
137 | structure. |
138 | .PP |
139 | A symbol can be removed from the table by calling |
140 | .BR sym_remove , |
141 | passing the symbol table itself, and the value block that needs |
142 | removing. |
143 | .SH ENUMERATION |
144 | Enumerating the values in a symbol table is fairly simple. Allocate a |
145 | .B sym_iter |
146 | object from somewhere. Attach it to a symbol table by calling |
147 | .BR sym_mkiter , |
148 | and passing in the addresses of the iterator and the symbol table. |
149 | Then, each call to |
150 | .B sym_next |
151 | will return a different value from the symbol table, until all of them |
152 | have been enumerated, at which point, |
153 | .B sym_next |
154 | returns a null pointer. |
155 | .PP |
156 | It's safe to remove the symbol you've just been returned by |
157 | .BR sym_next . |
158 | However, it's not safe to remove any other symbol. So don't do that. |
159 | .PP |
160 | When you've finished with an iterator, it's safe to just throw it away. |
161 | You don't need to call any functions beforehand. |
162 | .SH "USE IN PRACTICE" |
163 | In normal use, the keys are simple strings (usually identifiers from |
164 | some language), and the values are nontrivial structures providing |
165 | information about types and values. |
166 | .PP |
167 | In this case, you'd define something like the following structure for |
168 | your values: |
169 | .VS |
170 | typedef struct val { |
171 | sym_base _base; /* Symbol header */ |
172 | unsigned type; /* Type of this symbol */ |
173 | int dispoff; /* Which display variable is in */ |
174 | size_t frameoff; /* Offset of variable in frame */ |
175 | } val; |
176 | .VE |
177 | Given a pointer |
178 | .I v |
179 | to a |
180 | .BR val , |
181 | you can find the variable's name by calling |
182 | .BI SYM_NAME( v )\fR. |
183 | .PP |
184 | You can look up a name in the table by saying something like: |
185 | .VS |
186 | val *v = sym_find(t, name, -1, 0, 0); |
187 | if (!v) |
188 | error("unknown variable `%s'", name); |
189 | .VE |
190 | You can add in a new variable by saying something like |
191 | .VS |
192 | unsigned f; |
193 | val *v = sym_find(t, name, -1, sizeof(val), &f); |
194 | if (f) |
195 | error("variable `%s' already exists", name); |
196 | /* fill in v */ |
197 | .VE |
198 | You can examine all the variables in your symbol table by saying |
199 | something like: |
200 | .VS |
201 | sym_iter i; |
202 | val *v; |
203 | |
204 | for (sym_mkiter(&i, t); (v = sym_next(&i)) != 0; ) { |
205 | /* ... */ |
206 | } |
207 | .VE |
208 | That ought to be enough examples to be getting on with. |
209 | .SH CAVEATS |
210 | The symbol table manager requires the suballocator (see |
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211 | .BR sub (3) |
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212 | for details). You must ensure that |
213 | .B sub_init |
214 | has been called before using any symbol tables in your program. |
215 | .SH IMPLEMENTATION |
216 | The symbol table is an extensible hashtable, using a 32-bit CRC as the |
217 | hash function. The hash chains are kept very short (probably too short, |
218 | actually). Every time a symbol is found, its block is promoted to the |
219 | front of its bin chain so it gets found faster next time. |
220 | .SH SEE ALSO |
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221 | .BR sub (3), |
222 | .BR mLib (3). |
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223 | .SH AUTHOR |
224 | Mark Wooding, <mdw@nsict.org> |