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2003-12-30 Paul Eggert <eggert@twinsun.com> 

* time/mktime.c (verify): New macro.
	(time_t_is_integer, twos_complement_arithmetic,
	right_shift_propagates_sign, base_year_is_a_multiple_of_100,
	C99_integer_division): Document these longstanding assumptions in the
	code, and verify them at compile-time.
This commit is contained in:
Roland McGrath 2004-01-02 22:16:49 +00:00
parent 56b168be5d
commit a28a0500c0
1 changed files with 14 additions and 1 deletions
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15
time/mktime.c
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@ -61,8 +61,19 @@
# define TIME_T_MAX TYPE_MAXIMUM (time_t)
#endif
#define TM_YEAR_BASE 1900
/* Verify a requirement at compile-time (unlike assert, which is runtime). */
#define verify(name, assertion) struct name { char a[(assertion) ? 1 : -1]; }
verify (time_t_is_integer, (time_t) 0.5 == 0);
verify (twos_complement_arithmetic, -1 == ~1 + 1);
verify (right_shift_propagates_sign, -1 >> 1 == -1);
/* The code also assumes that signed integer overflow silently wraps
around, but this assumption can't be stated without causing a
diagnostic on some hosts. */
#define EPOCH_YEAR 1970
#define TM_YEAR_BASE 1900
verify (base_year_is_a_multiple_of_100, TM_YEAR_BASE % 100 == 0);
#ifndef __isleap
/* Nonzero if YEAR is a leap year (every 4 years,
@ -109,6 +120,8 @@ ydhms_tm_diff (int year, int yday, int hour, int min, int sec,
return 1;
else
{
verify (C99_integer_division, -1 / 2 == 0);
/* Compute intervening leap days correctly even if year is negative.
Take care to avoid int overflow. time_t overflow is OK, since
only the low order bits of the correct time_t answer are needed.