1+ /**
2+ * A program to output the contents of constant_pool
3+ * by parsing java .class files as that's made in javap tool.
4+ *
5+ * This code is written as an assignment to understand
6+ * the structure of the class files.
7+ *
8+ * Author: Aleksandr Ushaev
9+ * Created: 14.02.2020
10+ *
11+ */
12+ 13+ #include "class_reader.h"
14+ 15+ /**
16+ * Attempts to open a file, on success checks
17+ * if the file is a valid .class file.
18+ *
19+ * @param path of the file to open
20+ * @return file if it is valid .class file
21+ */
22+ FILE * open_class_file (char * path )
23+ {
24+ FILE * file = fopen (path , "rb" );
25+ 26+ if (!file )
27+ {
28+ perror ("Error " );
29+ exit (EXIT_FAILURE );
30+ }
31+ 32+ if (!is_class_file (file ))
33+ {
34+ fprintf (stderr , "This file is not a .class file!\n" );
35+ exit (0 );
36+ }
37+ 38+ return file ;
39+ }
40+ 41+ /**
42+ * Checks the file for a magic value 0xcafebabe
43+ *
44+ * @param file to check
45+ * @return true if file contatins magic value
46+ */
47+ bool is_class_file (FILE * file )
48+ {
49+ uint32_t magic_number ;
50+ size_t read_cnt = fread (& magic_number , sizeof (uint32_t ), 1 , file );
51+ magic_number = be32toh (magic_number );
52+ return read_cnt == 1 && magic_number == MAGIC_NUMBER ;
53+ }
54+ 55+ /**
56+ * Reads minor, major versions, constant pool size
57+ * and constant pool from a file
58+ *
59+ * @param file to read from
60+ * @return class struct filled with collected data
61+ */
62+ class * parse_class_file (FILE * class_file )
63+ {
64+ class * cls = (class * )malloc (sizeof (class ));
65+ 66+ // Read header
67+ parse_u2 (class_file , & cls -> minor_version );
68+ parse_u2 (class_file , & cls -> major_version );
69+ parse_u2 (class_file , & cls -> constant_pool_count );
70+ 71+ parse_constant_pool (class_file , cls );
72+ fclose (class_file );
73+ return cls ;
74+ }
75+ 76+ /**
77+ * Reads an "unsigned two-byte quantity" from a file
78+ *
79+ * @param file to read from
80+ * @param variable to write
81+ */
82+ void parse_u2 (FILE * class_file , uint16_t * cls )
83+ {
84+ fread (cls , sizeof (uint16_t ), 1 , class_file );
85+ (* cls ) = be16toh ((* cls ));
86+ }
87+ 88+ /**
89+ * Parse symbolic information from the "constant_pool" table
90+ * from a file
91+ *
92+ * @param file to read from
93+ * @param class struct to write
94+ */
95+ void parse_constant_pool (FILE * class_file , class * cls )
96+ {
97+ const uint16_t total_constants_count = cls -> constant_pool_count - 1 ;
98+ cls -> constant_pool = malloc (total_constants_count * sizeof (constant_info ));
99+ uint8_t tag ;
100+ 101+ CONSTANT_Class_info class_info ;
102+ CONSTANT_Ref_info ref_info ;
103+ CONSTANT_String_info string_info ;
104+ CONSTANT_IntFloat_info int_float_info ;
105+ CONSTANT_LongDouble_info long_double_info ;
106+ CONSTANT_Utf8_info utf_info ;
107+ CONSTANT_MethodHandle_info method_handle_info ;
108+ CONSTANT_MethodType_info method_type_info ;
109+ CONSTANT_InvokeDynamic_info invoke_info ;
110+ 111+ for (int i = 1 ; i <= total_constants_count ; i ++ )
112+ {
113+ constant_info * cur_constant_info = cls -> constant_pool + (i - 1 );
114+ fread (& tag , sizeof (uint8_t ), 1 , class_file );
115+ 116+ switch (tag )
117+ {
118+ case CONSTANT_Class :
119+ fread (& class_info .name_index , sizeof (class_info .name_index ), 1 , class_file );
120+ class_info .tag = tag ;
121+ class_info .name_index = be16toh (class_info .name_index );
122+ cur_constant_info -> class_i = class_info ;
123+ break ;
124+ 125+ case CONSTANT_Fieldref :
126+ case CONSTANT_InterfaceMethodref :
127+ case CONSTANT_Methodref :
128+ case CONSTANT_NameAndType :
129+ fread (& ref_info .class_index , sizeof (ref_info .class_index ), 1 , class_file );
130+ fread (& ref_info .name_and_type_index , sizeof (ref_info .name_and_type_index ), 1 , class_file );
131+ ref_info .tag = tag ;
132+ ref_info .class_index = be16toh (ref_info .class_index );
133+ ref_info .name_and_type_index = be16toh (ref_info .name_and_type_index );
134+ cur_constant_info -> ref_i = ref_info ;
135+ break ;
136+ 137+ case CONSTANT_String :
138+ fread (& string_info .string_index , sizeof (string_info .string_index ), 1 , class_file );
139+ string_info .tag = tag ;
140+ string_info .string_index = be16toh (string_info .string_index );
141+ cur_constant_info -> string_i = string_info ;
142+ break ;
143+ 144+ case CONSTANT_Integer :
145+ case CONSTANT_Float :
146+ fread (& int_float_info .bytes , sizeof (int_float_info .bytes ), 1 , class_file );
147+ int_float_info .tag = tag ;
148+ int_float_info .bytes = be32toh (int_float_info .bytes );
149+ cur_constant_info -> int_float_i = int_float_info ;
150+ break ;
151+ 152+ case CONSTANT_Long :
153+ case CONSTANT_Double :
154+ fread (& long_double_info .high_bytes , sizeof (long_double_info .high_bytes ), 1 , class_file );
155+ fread (& long_double_info .low_bytes , sizeof (long_double_info .low_bytes ), 1 , class_file );
156+ long_double_info .tag = tag ;
157+ long_double_info .high_bytes = be32toh (long_double_info .high_bytes );
158+ long_double_info .low_bytes = be32toh (long_double_info .low_bytes );
159+ cur_constant_info -> long_double_i = long_double_info ;
160+ i ++ ; // Takes two entries
161+ break ;
162+ 163+ case CONSTANT_Utf8 :
164+ fread (& utf_info .length , sizeof (utf_info .length ), 1 , class_file );
165+ utf_info .length = be16toh (utf_info .length );
166+ utf_info .bytes = malloc (sizeof (char ) * (utf_info .length + 1 ));
167+ fread (utf_info .bytes , sizeof (char ), utf_info .length , class_file );
168+ utf_info .bytes [utf_info .length ] = '0円' ;
169+ utf_info .tag = tag ;
170+ cur_constant_info -> utf_i = utf_info ;
171+ break ;
172+ 173+ case CONSTANT_MethodHandle :
174+ fread (& method_handle_info .reference_kind , sizeof (method_handle_info .reference_kind ), 1 , class_file );
175+ fread (& method_handle_info .reference_index , sizeof (method_handle_info .reference_index ), 1 , class_file );
176+ method_handle_info .reference_index = be16toh (method_handle_info .reference_index );
177+ method_handle_info .tag = tag ;
178+ cur_constant_info -> method_handle_i = method_handle_info ;
179+ break ;
180+ 181+ case CONSTANT_MethodType :
182+ fread (& method_type_info .descriptor_index , sizeof (method_type_info .descriptor_index ), 1 , class_file );
183+ method_type_info .descriptor_index = be16toh (method_type_info .descriptor_index );
184+ method_type_info .tag = tag ;
185+ cur_constant_info -> method_type_i = method_type_info ;
186+ break ;
187+ 188+ case CONSTANT_InvokeDynamic :
189+ fread (& invoke_info .bootstrap_method_attr_index , sizeof (invoke_info .bootstrap_method_attr_index ), 1 , class_file );
190+ fread (& invoke_info .name_and_type_index , sizeof (invoke_info .name_and_type_index ), 1 , class_file );
191+ invoke_info .bootstrap_method_attr_index = be16toh (invoke_info .bootstrap_method_attr_index );
192+ invoke_info .name_and_type_index = be16toh (invoke_info .name_and_type_index );
193+ invoke_info .tag = tag ;
194+ cur_constant_info -> invoke_dynamic_i = invoke_info ;
195+ break ;
196+ 197+ default :
198+ fprintf (stderr , "Don't know what to do with %d tag byte :(\n" , tag );
199+ cur_constant_info = NULL ;
200+ break ;
201+ }
202+ }
203+ }
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