/* * File: exception.h * Last modified on Sun Jul 24 10:26:20 1994 by eroberts * ----------------------------------------------------- * The exception package provides a general exception * handling mechanism for use with C that is portable * across a variety of compilers and operating systems. */#ifndef _exception_h#define _exception_h/* * Overview: * -------- * The exception package makes it possible for clients to * specify a handler for an exceptional conditions in a * syntactically readable way.  As a client, your first step * is to declare an exception condition name by declaring * a variable of type exception, as in * *       exception MyException; * * Normal visibility rules apply, so that you should declare * the exception variable at the appropriate level.  For * example, if an exception is local to an implementation, * it should be declared statically within that module.  If * an exception condition is shared by many modules, the * exception variable should be declared in an interface * and exported to all clients that need it.  This package * defines and exports the exception ErrorException, which * is likely to be sufficient for many clients. * * The basic functionality of exceptions is that one piece * of code can "raise" an exception so that it can then be * "handled" by special code in a dynamically enclosing * section of the program.  Exceptions are raised by calling * the pseudo-function raise with the exception name, as in * *     raise(MyException); * * Exceptions are handled using the "try" statement * (actually implemented using macros), which has the form: * *     try { *        . . . statements in the body of the block . . . *     except(exception1) *        . . . statements to handle exception 1 . . . *     except(exception2) *        . . . statements to handle exception 2 . . . *     except(ANY) *        . . . statements to handle any exception . . . *     } endtry * * Any number of except clauses may appear (up to a * maximum defined by the constant MaxExceptionsPerScope), * and the ANY clause is optional. * * When the program encounters the "try" statement, the * statements in the body are executed.  If no exception * conditions are raised during that execution, either * in this block or by a function call nested inside * this block, control passes to the end of the "try" * statement when the last statement in the block is * executed.  If an exception is raised during the * dynamic execution of the block, control immediately * passes to the statements in the appropriate except * clause.  Only the statements in that clause are * executed; no break statement is required to exit * the block.  If no handler for the raised exception * appears anywhere in the control history, the program * exits with an error. * * Examples of use: * * 1.  Catching errors. * * The following code fragment traps calls to Error, so * that the program does not quit but instead returns * to the top-level read-and-execute loop. * *     while (TRUE) { *         try { *             printf("> "); *             cmd = ReadCommand(); *             ExecuteCommand(cmd); *         except(ErrorException) *             -- additional handling code, if any -- *         } endtry *     } * * If either ReadCommand or ExecuteCommand calls Error, * control will be passed back to the main loop, after * executing any additional handler code.  (The error * message is printed by the Error routine, so it is * often the case that no additional work is needed.) * * 2.  Handling control-C * * The following code extends the example above so that * typing ^C also returns to top-level. * *     #include <signal.h> * *     static exception ControlCException; *     static int errorCount = 0; *     static int ControlCHandler(); * *     main() *     { *         string cmd; * *         signal(SIGINT, ControlCHandler); *         while (TRUE) { *             try { *                 printf("> "); *                 cmd = ReadCommand(); *                 ExecuteCommand(cmd); *             except(ControlCException); *                 printf("^C\n"); *                 signal(SIGINT, ControlCHandler); *             except(ErrorException) *                 errorCount++; *             } endtry *         } *     } * *     static int ControlCHandler() *     { *         raise(ControlCException); *     } *//* * Actual interface specification * ------------------------------ * Most of the implementation of the exception mechanism is * actually done in the macros defined by this file. * Clients should ordinarily be able to read the description * above and ignore the detailed code below. */#include <setjmp.h>#include <string.h>#include "genlib.h"/* Define parameters and error status indicators */#define MaxExceptionsPerScope 10#define ETooManyExceptClauses 101#define EUnhandledException 102/* Codes to keep track of the state of the try handler */#define ES_Initialize 0#define ES_EvalBody 1#define ES_Exception 2/* * Type: exception * --------------- * Exceptions are specified by their address, so that the * actual structure does not matter.  Strings are used here * so that exporters of exceptions can store the exception * name for the use of debuggers and other tools. */typedef struct { string name; } exception;/* * Type: context_block * ------------------- * This structure is used internally to maintain a chain of * exception scopes on the control stack. */typedef struct ctx_block {    jmp_buf jmp;    int nx;    exception *array[MaxExceptionsPerScope];    exception *id;    void *value;    string name;    struct ctx_block *link;} context_block;/* Declare the built-in exceptions */extern exception ErrorException;extern exception ANY;/* Declare a global pointer to the context stack */extern context_block *exceptionStack;/* * Function: RaiseException * Usage: RaiseException(&e, name, value); * --------------------------------------- * This function is called by the raise macro and does the * work necessary to raise the exception.  See the exception.c file * for details.  Clients do not ordinarily call this directly. */void RaiseException(exception *e, string name, void *value);/* * Function: HandlerExists * Usage: if (HandlerExists(&e)) ... * --------------------------------- * Determines whether a handler exists for an exception in * the dynamically enclosing scope.  Intended only for use * by special clients, such as the Error package. */bool HandlerExists(exception *e);/* Define the pseudo-functions for raise and try */#define raise(e) RaiseException(&e, #e, NULL)#define try \      { \          jmp_buf _tmpbuf; \          context_block _ctx; \          volatile int _es; \          _es = ES_Initialize; \          _ctx.nx = 0; \          _ctx.link = exceptionStack; \          exceptionStack = (context_block *) &_ctx; \          if (setjmp(_tmpbuf) != 0) _es = ES_Exception; \          memcpy((void *) _ctx.jmp, (void *) _tmpbuf, sizeof(jmp_buf)); \          while (1) { \              if (_es == ES_EvalBody)#define except(e) \                  if (_es == ES_EvalBody) exceptionStack = _ctx.link; \                  break; \              } \              if (_es == ES_Initialize) { \                  if (_ctx.nx >= MaxExceptionsPerScope) \                      exit(ETooManyExceptClauses); \                  _ctx.array[_ctx.nx++] = &e; \              } else if (_ctx.id == &e || &e == &ANY) { \                  exceptionStack = _ctx.link;#define endtry \              if (_es != ES_Initialize) break; \              _es = ES_EvalBody; \          } \      }#define GetExceptionName() _ctx.name#define GetExceptionValue() _ctx.value#define GetCurrentException() _ctx.id#endif