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timetoa.c revision 1.1 
      1 /*	$NetBSD: timetoa.c,v 1.1 2013/12/27 23:30:48 christos Exp $	*/
      2 
      3 /*
      4  * timetoa.c -- time_t related string formatting
      5  *
      6  * Written by Juergen Perlinger (perlinger (at) 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  */
     49 const char *
     50 format_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 	const char *	fmt;
     63 	ldiv_t		qr;
     64 
     65 	DEBUG_REQUIRE(prec != 0);
     66 
     67 	LIB_GETBUF(cp);
     68 	secs_u = (u_time)secs;
     69 	fmt = "-%" UTIME_FORMAT ".%0*ld";
     70 
     71 	/* check if we need signed or unsigned mode */
     72 	notneg = (prec < 0);
     73 	prec_u = abs(prec);
     74 	/* fraclimit = (long)pow(10, prec_u); */
     75 	for (fraclimit = 10, u = 1; u < prec_u; u++) {
     76 		DEBUG_INSIST(fraclimit < fraclimit * 10);
     77 		fraclimit *= 10;
     78 	}
     79 
     80 	/*
     81 	 * Since conversion to string uses lots of divisions anyway,
     82 	 * there's no big extra penalty for normalisation. We do it for
     83 	 * consistency.
     84 	 */
     85 	if (frac < 0 || frac >= fraclimit) {
     86 		qr = ldiv(frac, fraclimit);
     87 		if (qr.rem < 0) {
     88 			qr.quot--;
     89 			qr.rem += fraclimit;
     90 		}
     91 		secs_u += (time_t)qr.quot;
     92 		frac = qr.rem;
     93 	}
     94 
     95 	/* Get the absolute value of the split representation time. */
     96 	notneg = notneg || ((time_t)secs_u >= 0);
     97 	if (notneg) {
     98 		fmt++;		/* skip '-' */
     99 	} else {
    100 		secs_u = ~secs_u;
    101 		if (0 == frac)
    102 			secs_u++;
    103 		else
    104 			frac = fraclimit - frac;
    105 	}
    106 
    107 	/* finally format the data and return the result */
    108 	snprintf(cp, LIB_BUFLENGTH, fmt, secs_u, prec_u, frac);
    109 
    110 	return cp;
    111 }
    112