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Guido van Rossum
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Python/strtod.c
166
Python/strtod.c
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/* This is not a proper strtod() implementation, but sufficient for Python.
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Python won't detect floating point constant overflow, though. */
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/* comp.sources.misc strtod(), as posted in comp.lang.tcl,
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with bugfix for "123000.0" and acceptance of space after 'e' sign nuked.
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extern int errno;
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************************************************************
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* YOU MUST EDIT THE MACHINE-DEPENDENT DEFINITIONS BELOW!!! *
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************************************************************
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*/
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extern int strlen();
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extern double atof();
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/* File : stdtod.c (Modified version of str2dbl.c)
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Author : Richard A. O'Keefe @ Quintus Computer Systems, Inc.
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Updated: Tuesday August 2nd, 1988
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Defines: double strtod (char *str, char**ptr)
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*/
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double
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strtod(p, pp)
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char *p;
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char **pp;
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{
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double res;
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/* This is an implementation of the strtod() function described in the
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System V manuals, with a different name to avoid linker problems.
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All that str2dbl() does itself is check that the argument is well-formed
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and is in range. It leaves the work of conversion to atof(), which is
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assumed to exist and deliver correct results (if they can be represented).
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if (pp)
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*pp = p + strlen(p);
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res = atof(p);
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errno = 0;
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return res;
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There are two reasons why this should be provided to the net:
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(a) some UNIX systems do not yet have strtod(), or do not have it
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available in the BSD "universe" (but they do have atof()).
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(b) some of the UNIX systems that *do* have it get it wrong.
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(some crash with large arguments, some assign the wrong *ptr value).
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There is a reason why *we* are providing it: we need a correct version
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of strtod(), and if we give this one away maybe someone will look for
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mistakes in it and fix them for us (:-).
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*/
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/* The following constants are machine-specific. MD{MIN,MAX}EXPT are
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integers and MD{MIN,MAX}FRAC are strings such that
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0.${MDMAXFRAC}e${MDMAXEXPT} is the largest representable double,
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0.${MDMINFRAC}e${MDMINEXPT} is the smallest representable +ve double
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MD{MIN,MAX}FRAC must not have any trailing zeros.
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The values here are for IEEE-754 64-bit floats.
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It is not perfectly clear to me whether an IEEE infinity should be
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returned for overflow, nor what a portable way of writing one is,
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so HUGE is just 0.MAXFRAC*10**MAXEXPT (this seems still to be the
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UNIX convention).
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I do know about <values.h>, but the whole point of this file is that
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we can't always trust that stuff to be there or to be correct.
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*/
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static int MDMINEXPT = {-323};
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static char MDMINFRAC[] = "494065645841246544";
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static double ZERO = 0.0;
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static int MDMAXEXPT = { 309};
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static char MDMAXFRAC[] = "17976931348623147";
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static double HUGE = 1.7976931348623147e308;
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extern double atof(); /* Only called when result known to be ok */
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#include <errno.h>
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extern int errno;
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double strtod(str, ptr)
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char *str;
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char **ptr;
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{
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int sign, scale, dotseen;
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int esign, expt;
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char *save;
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register char *sp, *dp;
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register int c;
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char *buforg, *buflim;
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char buffer[64]; /* 45-digit significand + */
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/* 13-digit exponent */
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sp = str;
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while (*sp == ' ') sp++;
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sign = 1;
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if (*sp == '-') sign -= 2, sp++;
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dotseen = 0, scale = 0;
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dp = buffer;
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*dp++ = '0'; *dp++ = '.';
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buforg = dp, buflim = buffer+48;
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for (save = sp; c = *sp; sp++)
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if (c == '.') {
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if (dotseen) break;
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dotseen++;
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} else
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if ((unsigned)(c-'0') > (unsigned)('9'-'0')) {
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break;
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} else
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if (c == '0') {
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if (dp != buforg) {
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/* This is not the first digit, so we want to keep it */
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if (dp < buflim) *dp++ = c;
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if (!dotseen) scale++;
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} else {
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/* No non-zero digits seen yet */
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/* If a . has been seen, scale must be adjusted */
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if (dotseen) scale--;
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}
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} else {
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/* This is a nonzero digit, so we want to keep it */
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if (dp < buflim) *dp++ = c;
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/* If it precedes a ., scale must be adjusted */
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if (!dotseen) scale++;
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}
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if (sp == save) {
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if (ptr) *ptr = str;
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errno = EDOM; /* what should this be? */
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return ZERO;
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}
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}
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while (dp > buforg && dp[-1] == '0') --dp;
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if (dp == buforg) *dp++ = '0';
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*dp = '\0';
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/* Now the contents of buffer are
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+--+--------+-+--------+
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|0.|fraction|\|leftover|
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+--+--------+-+--------+
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^dp points here
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where fraction begins with 0 iff it is "0", and has at most
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45 digits in it, and leftover is at least 16 characters.
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*/
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save = sp, expt = 0, esign = 1;
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do {
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c = *sp++;
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if (c != 'e' && c != 'E') break;
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c = *sp++;
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if (c == '-') esign -= 2, c = *sp++; else
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if (c == '+' /* || c == ' ' */ ) c = *sp++;
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if ((unsigned)(c-'0') > (unsigned)('9'-'0')) break;
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while (c == '0') c = *sp++;
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for (; (unsigned)(c-'0') <= (unsigned)('9'-'0'); c = *sp++)
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expt = expt*10 + c-'0';
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if (esign < 0) expt = -expt;
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save = sp-1;
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} while (0);
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if (ptr) *ptr = save;
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expt += scale;
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/* Now the number is sign*0.fraction*10**expt */
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errno = ERANGE;
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if (expt > MDMAXEXPT) {
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return HUGE*sign;
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} else
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if (expt == MDMAXEXPT) {
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if (strcmp(buforg, MDMAXFRAC) > 0) return HUGE*sign;
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} else
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if (expt < MDMINEXPT) {
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return ZERO*sign;
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} else
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if (expt == MDMINEXPT) {
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if (strcmp(buforg, MDMINFRAC) < 0) return ZERO*sign;
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}
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/* We have now established that the number can be */
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/* represented without overflow or underflow */
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(void) sprintf(dp, "E%d", expt);
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errno = 0;
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return atof(buffer)*sign;
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}
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