pjlib/include/pj/except.h

/* $Id$
 *
 */

#ifndef __PJ_EXCEPTION_H__
#define __PJ_EXCEPTION_H__

/**
 * @file except.h
 * @brief Exception Handling in C.
 */

#include <pj/types.h>
#include <pj/compat/setjmp.h>


PJ_BEGIN_DECL


/**
 * @defgroup PJ_EXCEPT Exception Handling
 * @ingroup PJ_MISC
 * @{
 *
 * \section pj_except_sample_sec Quick Example
 *
 * For the impatient, take a look at some examples:
 *  - @ref page_pjlib_samples_except_c
 *  - @ref page_pjlib_exception_test
 *
 * \section pj_except_except Exception Handling
 *
 * This module provides exception handling syntactically similar to C++ in
 * C language. The underlying mechanism use setjmp() and longjmp(), and since
 * these constructs are ANSI standard, the mechanism here should be available
 * on most platforms/compilers which are ANSI compliant.
 *
 * If ANSI libc is not available, then setjmp()/longjmp() implementation will
 * be provided. See <pj/compat/setjmp.h> for compatibility.
 *
 * The exception handling mechanism is completely thread safe, so the exception
 * thrown by one thread will not interfere with other thread.
 *
 * CAVEATS:
 *  - unlike C++ exception, the scheme here won't call destructors of local
 *    objects if exception is thrown. Care must be taken when a function
 *    hold some resorce such as pool or mutex etc.
 *  - You CAN NOT make nested exception in one single function without using
 *    a nested PJ_USE_EXCEPTION.
 *  - Exceptions will always be caught by the first handle (unlike C++ where
 *    exception is only caught if the type matches.
 *
 * The exception handling constructs are similar to C++. The blocks will be
 * constructed similar to the following sample:
 *
 * \verbatim
   #define NO_MEMORY     1
   #define SYNTAX_ERROR  2
  
   int main()
   {
      PJ_USE_EXCEPTION;  // declare local exception stack.
  
      PJ_TRY {
        ...// do something..
      }
      PJ_CATCH(NO_MEMORY) {
        ... // handle exception 1
      }
      PJ_CATCH(SYNTAX_ERROR) {
        ... // handle exception 2
      }
      PJ_DEFAULT {
        ... // handle other exceptions.
      }
      PJ_END;
   }
   \endverbatim
 *
 * The above sample uses hard coded exception ID. It is @b strongly
 * recommended that applications request a unique exception ID instead
 * of hard coded value like above.
 *
 * \section pj_except_reg Exception ID Allocation
 *
 * To ensure that exception ID (number) are used consistently and to
 * prevent ID collisions in an application, it is strongly suggested that 
 * applications allocate an exception ID for each possible exception
 * type. As a bonus of this process, the application can identify
 * the name of the exception when the particular exception is thrown.
 *
 * Exception ID management are performed with the following APIs:
 *  - #pj_exception_id_alloc().
 *  - #pj_exception_id_free().
 *  - #pj_exception_id_name().
 *
 *
 * PJLIB itself automatically allocates one exception id, i.e.
 * #PJ_NO_MEMORY_EXCEPTION which is declared in <pj/pool.h>. This exception
 * ID is raised by default pool policy when it fails to allocate memory.
 *
 * \section PJ_EX_KEYWORDS Keywords
 *
 * \subsection PJ_THROW PJ_THROW(expression)
 * Throw an exception. The expression thrown is an integer as the result of
 * the \a expression. This keyword can be specified anywhere within the 
 * program.
 *
 * \subsection PJ_USE_EXCEPTION PJ_USE_EXCEPTION
 * Specify this in the variable definition section of the function block 
 * (or any blocks) to specify that the block has \a PJ_TRY/PJ_CATCH exception 
 * block. 
 * Actually, this is just a macro to declare local variable which is used to
 * push the exception state to the exception stack.
 *
 * \subsection PJ_TRY PJ_TRY
 * The \a PJ_TRY keyword is typically followed by a block. If an exception is
 * thrown in this block, then the execution will resume to the \a PJ_CATCH 
 * handler.
 *
 * \subsection PJ_CATCH PJ_CATCH(expression)
 * The \a PJ_CATCH is normally followed by a block. This block will be executed
 * if the exception being thrown is equal to the expression specified in the
 * \a PJ_CATCH.
 *
 * \subsection PJ_DEFAULT PJ_DEFAULT
 * The \a PJ_DEFAULT keyword is normally followed by a block. This block will
 * be executed if the exception being thrown doesn't match any of the \a
 * PJ_CATCH specification. The \a PJ_DEFAULT block \b MUST be placed as the
 * last block of the handlers.
 *
 * \subsection PJ_END PJ_END
 * Specify this keyword to mark the end of \a PJ_TRY / \a PJ_CATCH blocks.
 *
 * \subsection PJ_GET_EXCEPTION PJ_GET_EXCEPTION(void)
 * Get the last exception thrown. This macro is normally called inside the
 * \a PJ_CATCH or \a PJ_DEFAULT block, altough it can be used anywhere where
 * the \a PJ_USE_EXCEPTION definition is in scope.
 *
 * 
 * \section pj_except_examples_sec Examples
 *
 * For some examples on how to use the exception construct, please see:
 *  - @ref page_pjlib_samples_except_c
 *  - @ref page_pjlib_exception_test
 */

/**
 * Allocate a unique exception id.
 * Applications don't have to allocate a unique exception ID before using
 * the exception construct. However, by doing so it ensures that there is
 * no collisions of exception ID.
 *
 * As a bonus, when exception number is acquired through this function,
 * the library can assign name to the exception (only if 
 * PJ_HAS_EXCEPTION_NAMES is enabled (default is yes)) and find out the
 * exception name when it catches an exception.
 *
 * @param name      Name to be associated with the exception ID.
 * @param id        Pointer to receive the ID.
 *
 * @return          PJ_SUCCESS on success or PJ_ETOOMANY if the library 
 *                  is running out out ids.
 */
PJ_DECL(pj_status_t) pj_exception_id_alloc(const char *name,
                                           pj_exception_id_t *id);

/**
 * Free an exception id.
 *
 * @param id        The exception ID.
 *
 * @return          PJ_SUCCESS or the appropriate error code.
 */
PJ_DECL(pj_status_t) pj_exception_id_free(pj_exception_id_t id);

/**
 * Retrieve name associated with the exception id.
 *
 * @param id        The exception ID.
 *
 * @return          The name associated with the specified ID.
 */
PJ_DECL(const char*) pj_exception_id_name(pj_exception_id_t id);

/** @} */

/**
 * This structure (which should be invisible to user) manages the TRY handler
 * stack.
 */
struct pj_exception_state_t
{
    struct pj_exception_state_t *prev;  /**< Previous state in the list. */
    pj_jmp_buf state;                   /**< jmp_buf.                    */
};

/**
 * Throw exception.
 * @param id    Exception Id.
 */
PJ_DECL_NO_RETURN(void) 
pj_throw_exception_(pj_exception_id_t id) PJ_ATTR_NORETURN;

/**
 * Push exception handler.
 */
PJ_DECL(void) pj_push_exception_handler_(struct pj_exception_state_t *rec);

/**
 * Pop exception handler.
 */
PJ_DECL(void) pj_pop_exception_handler_(void);

/**
 * Declare that the function will use exception.
 * @hideinitializer
 */
#define PJ_USE_EXCEPTION    struct pj_exception_state_t pj_x_except__; int pj_x_code__

/**
 * Start exception specification block.
 * @hideinitializer
 */
#define PJ_TRY		    if (1) { \
				pj_push_exception_handler_(&pj_x_except__); \
				pj_x_code__ = pj_setjmp(pj_x_except__.state); \
				if (pj_x_code__ == 0)
/**
 * Catch the specified exception Id.
 * @param id    The exception number to catch.
 * @hideinitializer
 */
#define PJ_CATCH(id)	    else if (pj_x_code__ == (id))

/**
 * Catch any exception number.
 * @hideinitializer
 */
#define PJ_DEFAULT	    else

/**
 * End of exception specification block.
 * @hideinitializer
 */
#define PJ_END			pj_pop_exception_handler_(); \
			    } else {}

/**
 * Throw exception.
 * @param exception_id  The exception number.
 * @hideinitializer
 */
#define PJ_THROW(exception_id)	pj_throw_exception_(exception_id)

/**
 * Get current exception.
 * @return      Current exception code.
 * @hideinitializer
 */
#define PJ_GET_EXCEPTION()	(pj_x_code__)

PJ_END_DECL



#endif	/* __PJ_EXCEPTION_H__ */