/* FPRESET.C: This program uses signal to set up a
* routine for handling floating-point errors.
*/
#include <stdio.h>
#include <signal.h>
#include <setjmp.h>
#include <stdlib.h>
#include <float.h>
#include <math.h>
#include <string.h>
 
jmp_buf mark; /* Address for long jump to jump to */
int fperr; /* Global error number */
 
void __cdecl fphandler( int sig, int num ); /* Prototypes */
void fpcheck( void );
 
void main( void )
{
	double n1, n2, r;
	int jmpret;
	/* Unmask all floating-point exceptions. */
	_control87( 0, _MCW_EM );
	/* Set up floating-point error handler. The compiler
	* will generate a warning because it expects
	* signal-handling functions to take only one argument.
	*/
	if( signal( SIGFPE, fphandler ) == SIG_ERR )
		
	{
		fprintf( stderr, "Couldn't set SIGFPE\n" );
		abort(); }
	
		/* Save stack environment for return in case of error. First
		* time through, jmpret is 0, so true conditional is executed.
		* If an error occurs, jmpret will be set to -1 and false
		* conditional will be executed.
	*/
	
	// 注意，下面这条语句的作用是，保存程序当前运行的状态
	jmpret = setjmp( mark );
	if( jmpret == 0 )
	{
		printf( "Test for invalid operation - " );
		printf( "enter two numbers: " );
		scanf( "%lf %lf", &n1, &n2 );
		
		// 注意，下面这条语句可能出现异常，
		// 如果从终端输入的第2个变量是0值的话
		r = n1 / n2;
		/* This won't be reached if error occurs. */
		printf( "\n\n%4.3g / %4.3g = %4.3g\n", n1, n2, r );
		
		r = n1 * n2;
		/* This won't be reached if error occurs. */
		printf( "\n\n%4.3g * %4.3g = %4.3g\n", n1, n2, r );
	}
	else
		fpcheck();
}
/* fphandler handles SIGFPE (floating-point error) interrupt. Note
* that this prototype accepts two arguments and that the
* prototype for signal in the run-time library expects a signal
* handler to have only one argument.
*
* The second argument in this signal handler allows processing of
* _FPE_INVALID, _FPE_OVERFLOW, _FPE_UNDERFLOW, and
* _FPE_ZERODIVIDE, all of which are Microsoft-specific symbols
* that augment the information provided by SIGFPE. The compiler
* will generate a warning, which is harmless and expected.
*/
void fphandler( int sig, int num )
{
/* Set global for outside check since we don't want
* to do I/O in the handler.
	*/
	fperr = num;
	/* Initialize floating-point package. */
	_fpreset();
	/* Restore calling environment and jump back to setjmp. Return
	* -1 so that setjmp will return false for conditional test.
	*/
	// 注意，下面这条语句的作用是，恢复先前setjmp所保存的程序状态
	longjmp( mark, -1 );
}
void fpcheck( void )
{
	char fpstr[30];
	switch( fperr )
	{
	case _FPE_INVALID:
		strcpy( fpstr, "Invalid number" );
		break;
	case _FPE_OVERFLOW:
		strcpy( fpstr, "Overflow" );
		
		break;
	case _FPE_UNDERFLOW:
		strcpy( fpstr, "Underflow" );
		break;
	case _FPE_ZERODIVIDE:
		strcpy( fpstr, "Divide by zero" );
		break;
	default:
		strcpy( fpstr, "Other floating point error" );
		break;
	}
	printf( "Error %d: %s\n", fperr, fpstr );
}