timetoa.c revision 290000 
1/*
2 * timetoa.c -- time_t related string formatting
3 *
4 * Written by Juergen Perlinger (perlinger@ntp.org) for the NTP project.
5 * The contents of 'html/copyright.html' apply.
6 *
7 * Printing a 'time_t' has a lot of portability pitfalls, due to it's
8 * opaque base type. The only requirement imposed by the standard is
9 * that it must be a numeric type. For all practical purposes it's a
10 * signed int, and 32 bits are common.
11 *
12 * Since the UN*X time epoch will cause a signed integer overflow for
13 * 32-bit signed int in the year 2038, implementations slowly move to
14 * 64bit base types for time_t, even in 32-bit environments.
15 *
16 * As the printf() family has no standardised type specifier for time_t,
17 * guessing the right output format specifier is a bit troublesome and
18 * best done with the help of the preprocessor and "config.h".
19 */
20
21#include "config.h"
22
23#include <math.h>
24#include <stdio.h>
25
26#include "timetoa.h"
27#include "ntp_assert.h"
28#include "lib_strbuf.h"
29
30/*
31 * Formatting to string needs at max 40 bytes (even with 64 bit time_t),
32 * so we check LIB_BUFLENGTH is big enough for our purpose.
33 */
34#if LIB_BUFLENGTH < 40
35# include "GRONK: LIB_BUFLENGTH is not sufficient"
36#endif
37
38/*
39 * general fractional timestamp formatting
40 *
41 * Many pieces of ntpd require a machine with two's complement
42 * representation of signed integers, so we don't go through the whole
43 * rigamarole of creating fully portable code here. But we have to stay
44 * away from signed integer overflow, as this might cause trouble even
45 * with two's complement representation.
46 */
47const char *
48format_time_fraction(
49	time_t	secs,
50	long	frac,
51	int	prec
52	)
53{
54	char *		cp;
55	u_int		prec_u;
56	u_time		secs_u;
57	u_int		u;
58	long		fraclimit;
59	int		notneg;	/* flag for non-negative value	*/
60	ldiv_t		qr;
61
62	DEBUG_REQUIRE(prec != 0);
63
64	LIB_GETBUF(cp);
65	secs_u = (u_time)secs;
66
67	/* check if we need signed or unsigned mode */
68	notneg = (prec < 0);
69	prec_u = abs(prec);
70	/* fraclimit = (long)pow(10, prec_u); */
71	for (fraclimit = 10, u = 1; u < prec_u; u++) {
72		DEBUG_INSIST(fraclimit < fraclimit * 10);
73		fraclimit *= 10;
74	}
75
76	/*
77	 * Since conversion to string uses lots of divisions anyway,
78	 * there's no big extra penalty for normalisation. We do it for
79	 * consistency.
80	 */
81	if (frac < 0 || frac >= fraclimit) {
82		qr = ldiv(frac, fraclimit);
83		if (qr.rem < 0) {
84			qr.quot--;
85			qr.rem += fraclimit;
86		}
87		secs_u += (time_t)qr.quot;
88		frac = qr.rem;
89	}
90
91	/* Get the absolute value of the split representation time. */
92	notneg = notneg || ((time_t)secs_u >= 0);
93	if (!notneg) {
94		secs_u = ~secs_u;
95		if (0 == frac)
96			secs_u++;
97		else
98			frac = fraclimit - frac;
99	}
100
101	/* finally format the data and return the result */
102	snprintf(cp, LIB_BUFLENGTH, "%s%" UTIME_FORMAT ".%0*ld",
103	    notneg? "" : "-", secs_u, prec_u, frac);
104
105	return cp;
106}
107