Update:
Edit your dtostrf.c to look like the following. It fixes LOTS of bugs in the current libraries.
----<SNIP>----
/*
dtostrf - Emulation for dtostrf function from avr-libc
Copyright (c) 2015 Arduino LLC. All rights reserved.
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*/
#include <stdio.h>
#if 0
char *dtostrf (double val, signed char width, unsigned char prec, char *sout) {
char fmt[20];
sprintf(fmt, "%%%d.%df", width, prec);
sprintf(sout, fmt, val);
return sout;
}
#else
#include <string.h>
#include <stdlib.h>
char *dtostrf(double val, int width, unsigned int prec, char *sout)
{
int decpt, sign, reqd, pad;
const char *s, *e;
char *p;
s = fcvt(val, prec, &decpt, &sign);
if (prec == 0 && decpt == 0) {
s = (*s < '5') ? "0" : "1";
reqd = 1;
} else {
reqd = strlen(s);
if (reqd > decpt) reqd++;
if (decpt == 0) reqd++;
}
if (sign) reqd++;
p = sout;
e = p + reqd;
pad = width - reqd;
if (pad > 0) {
e += pad;
while (pad-- > 0) *p++ = ' ';
}
if (sign) *p++ = '-';
if (decpt <= 0 && prec > 0) {
*p++ = '0';
*p++ = '.';
e++;
while ( decpt < 0 ) {
decpt++;
*p++ = '0';
}
}
while (p < e) {
*p++ = *s++;
if (p == e) break;
if (--decpt == 0) *p++ = '.';
}
if (width < 0) {
pad = (reqd + width) * -1;
while (pad-- > 0) *p++ = ' ';
}
*p = 0;
return sout;
}
#endif