1/*===
2cexcept.h 2.0.1 (2008-Jul-19-Sat)
3http://www.nicemice.net/cexcept/
4Adam M. Costello
5http://www.nicemice.net/amc/
6
7An interface for exception-handling in ANSI C (C89 and subsequent ISO
8standards), 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
19The cexcept interface is not compatible with and cannot interact
20with system exceptions (like division by zero or memory segmentation
21violation), compiler-generated exceptions (like C++ exceptions), or
22other exception-handling interfaces.
23
24When using this interface across multiple .c files, do not include
25this header file directly.  Instead, create a wrapper header file that
26includes this header file and then invokes the define_exception_type
27macro (see below).  The .c files should then include that header file.
28
29The interface consists of one type, one well-known name, and six macros.
30
31
32define_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
53struct 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
67struct 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
102void 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
115Try statement
116Catch (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
163Throw 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
176Try statement
177Catch_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
183Everything below this point is for the benefit of the compiler.  The
184application programmer should pretend not to know any of it, because it
185is 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) \
197struct 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 */
249