249 lines
10 KiB
C
249 lines
10 KiB
C
/*===
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cexcept.h 2.0.1 (2008-Jul-19-Sat)
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http://www.nicemice.net/cexcept/
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Adam M. Costello
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http://www.nicemice.net/amc/
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An interface for exception-handling in ANSI C (C89 and subsequent ISO
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standards), developed jointly with Cosmin Truta.
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Copyright (c) 2000-2008 Adam M. Costello and Cosmin Truta.
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This software may be modified only if its author and version
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information is updated accurately, and may be redistributed
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only if accompanied by this unaltered notice. Subject to those
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restrictions, permission is granted to anyone to do anything
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with this software. The copyright holders make no guarantees
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regarding this software, and are not responsible for any damage
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resulting from its use.
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The cexcept interface is not compatible with and cannot interact
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with system exceptions (like division by zero or memory segmentation
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violation), compiler-generated exceptions (like C++ exceptions), or
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other exception-handling interfaces.
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When using this interface across multiple .c files, do not include
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this header file directly. Instead, create a wrapper header file that
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includes this header file and then invokes the define_exception_type
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macro (see below). The .c files should then include that header file.
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The interface consists of one type, one well-known name, and six macros.
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define_exception_type(type_name);
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This macro is used like an external declaration. It specifies
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the type of object that gets copied from the exception thrower to
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the exception catcher. The type_name can be any type that can be
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assigned to, that is, a non-constant arithmetic type, struct, union,
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or pointer. Examples:
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define_exception_type(int);
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enum exception { out_of_memory, bad_arguments, disk_full };
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define_exception_type(enum exception);
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struct exception { int code; const char *msg; };
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define_exception_type(struct exception);
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Because throwing an exception causes the object to be copied (not
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just once, but twice), programmers may wish to consider size when
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choosing the exception type.
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struct exception_context;
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This type may be used after the define_exception_type() macro has
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been invoked. A struct exception_context must be known to both
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the thrower and the catcher. It is expected that there be one
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context for each thread that uses exceptions. It would certainly
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be dangerous for multiple threads to access the same context.
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One thread can use multiple contexts, but that is likely to be
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confusing and not typically useful. The application can allocate
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this structure in any way it pleases--automatic, static, or dynamic.
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The application programmer should pretend not to know the structure
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members, which are subject to change.
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struct exception_context *the_exception_context;
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The Try/Catch and Throw statements (described below) implicitly
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refer to a context, using the name the_exception_context. It is
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the application's responsibility to make sure that this name yields
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the address of a mutable (non-constant) struct exception_context
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wherever those statements are used. Subject to that constraint, the
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application may declare a variable of this name anywhere it likes
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(inside a function, in a parameter list, or externally), and may
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use whatever storage class specifiers (static, extern, etc) or type
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qualifiers (const, volatile, etc) it likes. Examples:
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static struct exception_context
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* const the_exception_context = &foo;
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{ struct exception_context *the_exception_context = bar; ... }
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int blah(struct exception_context *the_exception_context, ...);
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extern struct exception_context the_exception_context[1];
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The last example illustrates a trick that avoids creating a pointer
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object separate from the structure object.
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The name could even be a macro, for example:
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struct exception_context ec_array[numthreads];
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#define the_exception_context (ec_array + thread_id)
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Be aware that the_exception_context is used several times by the
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Try/Catch/Throw macros, so it shouldn't be expensive or have side
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effects. The expansion must be a drop-in replacement for an
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identifier, so it's safest to put parentheses around it.
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void init_exception_context(struct exception_context *ec);
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For context structures allocated statically (by an external
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definition or using the "static" keyword), the implicit
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initialization to all zeros is sufficient, but contexts allocated
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by other means must be initialized using this macro before they
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are used by a Try/Catch statement. It does no harm to initialize
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a context more than once (by using this macro on a statically
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allocated context, or using this macro twice on the same context),
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but a context must not be re-initialized after it has been used by a
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Try/Catch statement.
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Try statement
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Catch (expression) statement
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The Try/Catch/Throw macros are capitalized in order to avoid
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confusion with the C++ keywords, which have subtly different
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semantics.
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A Try/Catch statement has a syntax similar to an if/else statement,
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except that the parenthesized expression goes after the second
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keyword rather than the first. As with if/else, there are two
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clauses, each of which may be a simple statement ending with a
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semicolon or a brace-enclosed compound statement. But whereas
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the else clause is optional, the Catch clause is required. The
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expression must be a modifiable lvalue (something capable of being
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assigned to) of the same type (disregarding type qualifiers) that
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was passed to define_exception_type().
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If a Throw that uses the same exception context as the Try/Catch is
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executed within the Try clause (typically within a function called
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by the Try clause), and the exception is not caught by a nested
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Try/Catch statement, then a copy of the exception will be assigned
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to the expression, and control will jump to the Catch clause. If no
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such Throw is executed, then the assignment is not performed, and
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the Catch clause is not executed.
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The expression is not evaluated unless and until the exception is
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caught, which is significant if it has side effects, for example:
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Try foo();
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Catch (p[++i].e) { ... }
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IMPORTANT: Jumping into or out of a Try clause (for example via
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return, break, continue, goto, longjmp) is forbidden--the compiler
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will not complain, but bad things will happen at run-time. Jumping
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into or out of a Catch clause is okay, and so is jumping around
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inside a Try clause. In many cases where one is tempted to return
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from a Try clause, it will suffice to use Throw, and then return
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from the Catch clause. Another option is to set a flag variable and
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use goto to jump to the end of the Try clause, then check the flag
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after the Try/Catch statement.
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IMPORTANT: The values of any non-volatile automatic variables
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changed within the Try clause are undefined after an exception is
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caught. Therefore, variables modified inside the Try block whose
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values are needed later outside the Try block must either use static
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storage or be declared with the "volatile" type qualifier.
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Throw expression;
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A Throw statement is very much like a return statement, except that
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the expression is required. Whereas return jumps back to the place
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where the current function was called, Throw jumps back to the Catch
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clause of the innermost enclosing Try clause. The expression must
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be compatible with the type passed to define_exception_type(). The
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exception must be caught, otherwise the program may crash.
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Slight limitation: If the expression is a comma-expression, it must
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be enclosed in parentheses.
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Try statement
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Catch_anonymous statement
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When the value of the exception is not needed, a Try/Catch statement
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can use Catch_anonymous instead of Catch (expression).
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Everything below this point is for the benefit of the compiler. The
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application programmer should pretend not to know any of it, because it
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is subject to change.
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===*/
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#ifndef CEXCEPT_H
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#define CEXCEPT_H
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#include <setjmp.h>
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#define define_exception_type(etype) \
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struct exception_context { \
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jmp_buf *penv; \
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int caught; \
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volatile struct { etype etmp; } v; \
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}
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/* etmp must be volatile because the application might use automatic */
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/* storage for the_exception_context, and etmp is modified between */
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/* the calls to setjmp() and longjmp(). A wrapper struct is used to */
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/* avoid warnings about a duplicate volatile qualifier in case etype */
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/* already includes it. */
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#define init_exception_context(ec) ((void)((ec)->penv = 0))
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#define Try \
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{ \
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jmp_buf *exception__prev, exception__env; \
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exception__prev = the_exception_context->penv; \
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the_exception_context->penv = &exception__env; \
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if (setjmp(exception__env) == 0) { \
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do
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#define exception__catch(action) \
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while (the_exception_context->caught = 0, \
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the_exception_context->caught); \
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} \
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else { \
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the_exception_context->caught = 1; \
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} \
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the_exception_context->penv = exception__prev; \
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} \
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if (!the_exception_context->caught || action) { } \
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else
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#define Catch(e) exception__catch(((e) = the_exception_context->v.etmp, 0))
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#define Catch_anonymous exception__catch(0)
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/* Try ends with do, and Catch begins with while(0) and ends with */
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/* else, to ensure that Try/Catch syntax is similar to if/else */
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/* syntax. */
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/* */
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/* The 0 in while(0) is expressed as x=0,x in order to appease */
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/* compilers that warn about constant expressions inside while(). */
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/* Most compilers should still recognize that the condition is always */
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/* false and avoid generating code for it. */
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#define Throw \
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for (;; longjmp(*the_exception_context->penv, 1)) \
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the_exception_context->v.etmp =
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#endif /* CEXCEPT_H */
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