1 /*===
2 cexcept.h 2.0.1 (2008-Jul-19-Sat)
3 http://www.nicemice.net/cexcept/
4 Adam M. Costello
5 http://www.nicemice.net/amc/
6
7 An interface for exception-handling in ANSI C (C89 and subsequent ISO
8 standards), developed jointly with Cosmin Truta.
9
10 Copyright (c) 2000-2008 Adam M. Costello and Cosmin Truta.
11 This software may be modified only if its author and version
12 information is updated accurately, and may be redistributed
13 only if accompanied by this unaltered notice. Subject to those
14 restrictions, permission is granted to anyone to do anything
15 with this software. The copyright holders make no guarantees
16 regarding this software, and are not responsible for any damage
17 resulting from its use.
18
19 The cexcept interface is not compatible with and cannot interact
20 with system exceptions (like division by zero or memory segmentation
21 violation), compiler-generated exceptions (like C++ exceptions), or
22 other exception-handling interfaces.
23
24 When using this interface across multiple .c files, do not include
25 this header file directly. Instead, create a wrapper header file that
26 includes this header file and then invokes the define_exception_type
27 macro (see below). The .c files should then include that header file.
28
29 The interface consists of one type, one well-known name, and six macros.
30
31
32 define_exception_type(type_name);
33
34 This macro is used like an external declaration. It specifies
35 the type of object that gets copied from the exception thrower to
36 the exception catcher. The type_name can be any type that can be
37 assigned to, that is, a non-constant arithmetic type, struct, union,
38 or pointer. Examples:
39
40 define_exception_type(int);
41
42 enum exception { out_of_memory, bad_arguments, disk_full };
43 define_exception_type(enum exception);
44
45 struct exception { int code; const char *msg; };
46 define_exception_type(struct exception);
47
48 Because throwing an exception causes the object to be copied (not
49 just once, but twice), programmers may wish to consider size when
50 choosing the exception type.
51
52
53 struct exception_context;
54
55 This type may be used after the define_exception_type() macro has
56 been invoked. A struct exception_context must be known to both
57 the thrower and the catcher. It is expected that there be one
58 context for each thread that uses exceptions. It would certainly
59 be dangerous for multiple threads to access the same context.
60 One thread can use multiple contexts, but that is likely to be
61 confusing and not typically useful. The application can allocate
62 this structure in any way it pleases--automatic, static, or dynamic.
63 The application programmer should pretend not to know the structure
64 members, which are subject to change.
65
66
67 struct exception_context *the_exception_context;
68
69 The Try/Catch and Throw statements (described below) implicitly
70 refer to a context, using the name the_exception_context. It is
71 the application's responsibility to make sure that this name yields
72 the address of a mutable (non-constant) struct exception_context
73 wherever those statements are used. Subject to that constraint, the
74 application may declare a variable of this name anywhere it likes
75 (inside a function, in a parameter list, or externally), and may
76 use whatever storage class specifiers (static, extern, etc) or type
77 qualifiers (const, volatile, etc) it likes. Examples:
78
79 static struct exception_context
80 * const the_exception_context = &foo;
81
82 { struct exception_context *the_exception_context = bar; ... }
83
84 int blah(struct exception_context *the_exception_context, ...);
85
86 extern struct exception_context the_exception_context[1];
87
88 The last example illustrates a trick that avoids creating a pointer
89 object separate from the structure object.
90
91 The name could even be a macro, for example:
92
93 struct exception_context ec_array[numthreads];
94 #define the_exception_context (ec_array + thread_id)
95
96 Be aware that the_exception_context is used several times by the
97 Try/Catch/Throw macros, so it shouldn't be expensive or have side
98 effects. The expansion must be a drop-in replacement for an
99 identifier, so it's safest to put parentheses around it.
100
101
102 void init_exception_context(struct exception_context *ec);
103
104 For context structures allocated statically (by an external
105 definition or using the "static" keyword), the implicit
106 initialization to all zeros is sufficient, but contexts allocated
107 by other means must be initialized using this macro before they
108 are used by a Try/Catch statement. It does no harm to initialize
109 a context more than once (by using this macro on a statically
110 allocated context, or using this macro twice on the same context),
111 but a context must not be re-initialized after it has been used by a
112 Try/Catch statement.
113
114
115 Try statement
116 Catch (expression) statement
117
118 The Try/Catch/Throw macros are capitalized in order to avoid
119 confusion with the C++ keywords, which have subtly different
120 semantics.
121
122 A Try/Catch statement has a syntax similar to an if/else statement,
123 except that the parenthesized expression goes after the second
124 keyword rather than the first. As with if/else, there are two
125 clauses, each of which may be a simple statement ending with a
126 semicolon or a brace-enclosed compound statement. But whereas
127 the else clause is optional, the Catch clause is required. The
128 expression must be a modifiable lvalue (something capable of being
129 assigned to) of the same type (disregarding type qualifiers) that
130 was passed to define_exception_type().
131
132 If a Throw that uses the same exception context as the Try/Catch is
133 executed within the Try clause (typically within a function called
134 by the Try clause), and the exception is not caught by a nested
135 Try/Catch statement, then a copy of the exception will be assigned
136 to the expression, and control will jump to the Catch clause. If no
137 such Throw is executed, then the assignment is not performed, and
138 the Catch clause is not executed.
139
140 The expression is not evaluated unless and until the exception is
141 caught, which is significant if it has side effects, for example:
142
143 Try foo();
144 Catch (p[++i].e) { ... }
145
146 IMPORTANT: Jumping into or out of a Try clause (for example via
147 return, break, continue, goto, longjmp) is forbidden--the compiler
148 will not complain, but bad things will happen at run-time. Jumping
149 into or out of a Catch clause is okay, and so is jumping around
150 inside a Try clause. In many cases where one is tempted to return
151 from a Try clause, it will suffice to use Throw, and then return
152 from the Catch clause. Another option is to set a flag variable and
153 use goto to jump to the end of the Try clause, then check the flag
154 after the Try/Catch statement.
155
156 IMPORTANT: The values of any non-volatile automatic variables
157 changed within the Try clause are undefined after an exception is
158 caught. Therefore, variables modified inside the Try block whose
159 values are needed later outside the Try block must either use static
160 storage or be declared with the "volatile" type qualifier.
161
162
163 Throw expression;
164
165 A Throw statement is very much like a return statement, except that
166 the expression is required. Whereas return jumps back to the place
167 where the current function was called, Throw jumps back to the Catch
168 clause of the innermost enclosing Try clause. The expression must
169 be compatible with the type passed to define_exception_type(). The
170 exception must be caught, otherwise the program may crash.
171
172 Slight limitation: If the expression is a comma-expression, it must
173 be enclosed in parentheses.
174
175
176 Try statement
177 Catch_anonymous statement
178
179 When the value of the exception is not needed, a Try/Catch statement
180 can use Catch_anonymous instead of Catch (expression).
181
182
183 Everything below this point is for the benefit of the compiler. The
184 application programmer should pretend not to know any of it, because it
185 is subject to change.
186
187 ===*/
188
189
190 #ifndef CEXCEPT_H
191 #define CEXCEPT_H
192
193
194 #include <setjmp.h>
195
196 #define define_exception_type(etype) \
197 struct exception_context { \
198 jmp_buf *penv; \
199 int caught; \
200 volatile struct { etype etmp; } v; \
201 }
202
203 /* etmp must be volatile because the application might use automatic */
204 /* storage for the_exception_context, and etmp is modified between */
205 /* the calls to setjmp() and longjmp(). A wrapper struct is used to */
206 /* avoid warnings about a duplicate volatile qualifier in case etype */
207 /* already includes it. */
208
209 #define init_exception_context(ec) ((void)((ec)->penv = 0))
210
211 #define Try \
212 { \
213 jmp_buf *exception__prev, exception__env; \
214 exception__prev = the_exception_context->penv; \
215 the_exception_context->penv = &exception__env; \
216 if (setjmp(exception__env) == 0) { \
217 do
218
219 #define exception__catch(action) \
220 while (the_exception_context->caught = 0, \
221 the_exception_context->caught); \
222 } \
223 else { \
224 the_exception_context->caught = 1; \
225 } \
226 the_exception_context->penv = exception__prev; \
227 } \
228 if (!the_exception_context->caught || action) { } \
229 else
230
231 #define Catch(e) exception__catch(((e) = the_exception_context->v.etmp, 0))
232 #define Catch_anonymous exception__catch(0)
233
234 /* Try ends with do, and Catch begins with while(0) and ends with */
235 /* else, to ensure that Try/Catch syntax is similar to if/else */
236 /* syntax. */
237 /* */
238 /* The 0 in while(0) is expressed as x=0,x in order to appease */
239 /* compilers that warn about constant expressions inside while(). */
240 /* Most compilers should still recognize that the condition is always */
241 /* false and avoid generating code for it. */
242
243 #define Throw \
244 for (;; longjmp(*the_exception_context->penv, 1)) \
245 the_exception_context->v.etmp =
246
247
248 #endif /* CEXCEPT_H */