New version from the net.

Python/strtod.c

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