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```/* derived from /netlib/fdlibm */

/* @(#)e_log10.c 1.3 95/01/18 */
/*
* ====================================================
*
* Developed at SunSoft, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/

/* __ieee754_log10(x)
* Return the base 10 logarithm of x
*
* Method :
*	Let log10_2hi = leading 40 bits of log10(2) and
*	    log10_2lo = log10(2) - log10_2hi,
*	    ivln10   = 1/log(10) rounded.
*	Then
*		n = ilogb(x),
*		if(n<0)  n = n+1;
*		x = scalbn(x,-n);
*		log10(x) := n*log10_2hi + (n*log10_2lo + ivln10*log(x))
*
* Note 1:
*	To guarantee log10(10**n)=n, where 10**n is normal, the rounding
*	mode must set to Round-to-Nearest.
* Note 2:
*	[1/log(10)] rounded to 53 bits has error  .198   ulps;
*	log10 is monotonic at all binary break points.
*
* Special cases:
*	log10(x) is NaN with signal if x < 0;
*	log10(+INF) is +INF with no signal; log10(0) is -INF with signal;
*	log10(NaN) is that NaN with no signal;
*	log10(10**N) = N  for N=0,1,...,22.
*
* Constants:
* The hexadecimal values are the intended ones for the following constants.
* The decimal values may be used, provided that the compiler will convert
* from decimal to binary accurately enough to produce the hexadecimal values
* shown.
*/

#include "fdlibm.h"

static const double
two54      =  1.80143985094819840000e+16, /* 0x43500000, 0x00000000 */
ivln10     =  4.34294481903251816668e-01, /* 0x3FDBCB7B, 0x1526E50E */
log10_2hi  =  3.01029995663611771306e-01, /* 0x3FD34413, 0x509F6000 */
log10_2lo  =  3.69423907715893078616e-13; /* 0x3D59FEF3, 0x11F12B36 */

static double zero   =  0.0;

double __ieee754_log10(double x)
{
double y,z;
int i,k,hx;
unsigned lx;

hx = __HI(x);	/* high word of x */
lx = __LO(x);	/* low word of x */

k=0;
if (hx < 0x00100000) {                  /* x < 2**-1022  */
if (((hx&0x7fffffff)|lx)==0)
return -two54/zero;             /* log(+-0)=-inf */
if (hx<0) return (x-x)/zero;        /* log(-#) = NaN */
k -= 54; x *= two54; /* subnormal number, scale up x */
hx = __HI(x);                /* high word of x */
}
if (hx >= 0x7ff00000) return x+x;
k += (hx>>20)-1023;
i  = ((unsigned)k&0x80000000)>>31;
hx = (hx&0x000fffff)|((0x3ff-i)<<20);
y  = (double)(k+i);
__HI(x) = hx;
z  = y*log10_2lo + ivln10*__ieee754_log(x);
return  z+y*log10_2hi;
}
```