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From: Corinna Vinschen <corinna@sourceware.org> To: newlib-cvs@sourceware.org Subject: [newlib-cygwin] ldtoa: fix dropping too many digits from output Date: Thu, 4 Nov 2021 12:35:25 +0000 (GMT) [thread overview] Message-ID: <20211104123525.9309D385802E@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=4d90e5335914551862831de3e02f6c102b78435b commit 4d90e5335914551862831de3e02f6c102b78435b Author: Corinna Vinschen <corinna@vinschen.de> Date: Thu Nov 4 11:30:44 2021 +0100 ldtoa: fix dropping too many digits from output ldtoa cuts the number of digits it returns based on a computation of number of supported bits (144) divide by log10(2). Not only is the integer approximation of log10(2) ~= 8/27 missing a digit here, it also fails to take really small double and long double values into account. Allow for the full potential precision of long double values. At the same time, change the local string array allocation to request only as much bytes as necessary to support the caller-requested number of digits, to keep the stack size low on small targets. In the long run a better fix would be to switch to gdtoa, as the BSD variants, as well as Mingw64 do. Signed-off-by: Corinna Vinschen <corinna@vinschen.de> Diff: --- newlib/libc/stdlib/ldtoa.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/newlib/libc/stdlib/ldtoa.c b/newlib/libc/stdlib/ldtoa.c index 1cc97151a..7da61457b 100644 --- a/newlib/libc/stdlib/ldtoa.c +++ b/newlib/libc/stdlib/ldtoa.c @@ -34,9 +34,9 @@ void _IO_ldtostr (long double *, char *, int, int, char); #define NBITS ((NI-4)*16) /* Maximum number of decimal digits in ASCII conversion - * = NBITS*log10(2) + * Take full possible size of output into account */ -#define NDEC (NBITS*8/27) +#define NDEC 1023 /* The exponent of 1.0 */ #define EXONE (0x3fff) @@ -2794,7 +2794,6 @@ _ldtoa_r (struct _reent *ptr, long double d, int mode, int ndigits, LDPARMS rnd; LDPARMS *ldp = &rnd; char *outstr; - char outbuf[NDEC + MAX_EXP_DIGITS + 10]; union uconv du; du.d = d; @@ -2841,6 +2840,8 @@ _ldtoa_r (struct _reent *ptr, long double d, int mode, int ndigits, if (ndigits > NDEC) ndigits = NDEC; + char outbuf[ndigits + MAX_EXP_DIGITS + 10]; + etoasc (e, outbuf, ndigits, mode, ldp); s = outbuf; if (eisinf (e) || eisnan (e)) @@ -3111,6 +3112,8 @@ tnzro: else { emovi (y, w); + /* Note that this loop does not access the incoming string array, + * which may be shorter than NDEC + 1 bytes! */ for (i = 0; i < NDEC + 1; i++) { if ((w[NI - 1] & 0x7) != 0)
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