ldexp.c

操作系统SunOS 4.1.3版本的源码

#ifndef lint
static	char sccsid[] = "@(#)ldexp.c 1.1 92/07/30 SMI"; /* from S5R2 2.7 */
#endif

/*LINTLIBRARY*/
/*
 *	double ldexp (value, exp)
 *		double value;
 *		int exp;
 *
 *	Ldexp returns value * 2**exp, if that result is in range.
 *	If underflow occurs, it returns zero.  If overflow occurs,
 *	it returns a value of appropriate sign and largest single-
 *	precision magnitude.  In case of underflow or overflow,
 *	the external int "errno" is set to ERANGE.  Note that errno is
 *	not modified if no error occurs, so if you intend to test it
 *	after you use ldexp, you had better set it to something
 *	other than ERANGE first (zero is a reasonable value to use).
 */

#include <values.h>
#include <errno.h>
/* Largest signed long int power of 2 */
#define MAXSHIFT	(BITSPERBYTE * sizeof(long) - 2)

extern double frexp();

double
ldexp(value, exp)
register double value;
register int exp;
{
	int old_exp;

	if (exp == 0 || value == 0.0) /* nothing to do for zero */
		return (value);
#if	!(pdp11 || u3b5)	/* pdp11 "cc" can't handle cast of
				   double to void on pdp11 or 3b5 */
	(void)
#endif
	frexp(value, &old_exp);
	if (exp > 0) {
		if (exp + old_exp > MAXBEXP) { /* overflow */
			errno = ERANGE;
			return (value < 0 ? -1.0e999 : 1.0e999);
		}
		for ( ; exp > MAXSHIFT; exp -= MAXSHIFT)
			value *= (1L << MAXSHIFT);
		return (value * (1L << exp));
	}
	if (exp + old_exp < MINBEXP) { /* underflow */
		errno = ERANGE;
		return (0.0);
	}
	for ( ; exp < -MAXSHIFT; exp += MAXSHIFT)
		value *= 1.0/(1L << MAXSHIFT); /* mult faster than div */
	return (value / (1L << -exp));
}