1/*	$NetBSD: timetoa.c,v 1.7 2020/05/25 20:47:24 christos Exp $	*/
2
3/*
4 * timetoa.c -- time_t related string formatting
5 *
6 * Written by Juergen Perlinger (perlinger@ntp.org) for the NTP project.
7 * The contents of 'html/copyright.html' apply.
8 *
9 * Printing a 'time_t' has a lot of portability pitfalls, due to it's
10 * opaque base type. The only requirement imposed by the standard is
11 * that it must be a numeric type. For all practical purposes it's a
12 * signed int, and 32 bits are common.
13 *
14 * Since the UN*X time epoch will cause a signed integer overflow for
15 * 32-bit signed int in the year 2038, implementations slowly move to
16 * 64bit base types for time_t, even in 32-bit environments.
17 *
18 * As the printf() family has no standardised type specifier for time_t,
19 * guessing the right output format specifier is a bit troublesome and
20 * best done with the help of the preprocessor and "config.h".
21 */
22
23#include "config.h"
24
25#include <math.h>
26#include <stdio.h>
27
28#include "timetoa.h"
29#include "ntp_assert.h"
30#include "lib_strbuf.h"
31
32/*
33 * Formatting to string needs at max 40 bytes (even with 64 bit time_t),
34 * so we check LIB_BUFLENGTH is big enough for our purpose.
35 */
36#if LIB_BUFLENGTH < 40
37# include "GRONK: LIB_BUFLENGTH is not sufficient"
38#endif
39
40/*
41 * general fractional timestamp formatting
42 *
43 * Many pieces of ntpd require a machine with two's complement
44 * representation of signed integers, so we don't go through the whole
45 * rigamarole of creating fully portable code here. But we have to stay
46 * away from signed integer overflow, as this might cause trouble even
47 * with two's complement representation.
48 */
49const char *
50format_time_fraction(
51	time_t	secs,
52	long	frac,
53	int	prec
54	)
55{
56	char *		cp;
57	u_int		prec_u;
58	u_time		secs_u;
59	u_int		u;
60	long		fraclimit;
61	int		notneg;	/* flag for non-negative value	*/
62	ldiv_t		qr;
63
64	DEBUG_REQUIRE(prec != 0);
65
66	LIB_GETBUF(cp);
67	secs_u = (u_time)secs;
68
69	/* check if we need signed or unsigned mode */
70	notneg = (prec < 0);
71	prec_u = abs(prec);
72	/* fraclimit = (long)pow(10, prec_u); */
73	for (fraclimit = 10, u = 1; u < prec_u; u++) {
74		DEBUG_INSIST(fraclimit < fraclimit * 10);
75		fraclimit *= 10;
76	}
77
78	/*
79	 * Since conversion to string uses lots of divisions anyway,
80	 * there's no big extra penalty for normalisation. We do it for
81	 * consistency.
82	 */
83	if (frac < 0 || frac >= fraclimit) {
84		qr = ldiv(frac, fraclimit);
85		if (qr.rem < 0) {
86			qr.quot--;
87			qr.rem += fraclimit;
88		}
89		secs_u += (time_t)qr.quot;
90		frac = qr.rem;
91	}
92
93	/* Get the absolute value of the split representation time. */
94	notneg = notneg || ((time_t)secs_u >= 0);
95	if (!notneg) {
96		secs_u = ~secs_u;
97		if (0 == frac)
98			secs_u++;
99		else
100			frac = fraclimit - frac;
101	}
102
103	/* finally format the data and return the result */
104	snprintf(cp, LIB_BUFLENGTH, "%s%" UTIME_FORMAT ".%0*ld",
105	    notneg? "" : "-", secs_u, prec_u, frac);
106
107	return cp;
108}
109