time.c

ReactOS是一些高手根据Windows XP的内核编写出的类XP。内核实现机理和API函数调用几乎相同。甚至可以兼容XP的程序。喜欢研究系统内核的人可以看一看。

/*
 * COPYRIGHT:   See COPYING in the top level directory
 * PROJECT:     ReactOS system libraries
 * FILE:        lib/msvcrt/time/time.c
 * PURPOSE:     Get system time
 * PROGRAMER:   Ariadne
 * UPDATE HISTORY:
 *              28/12/98: Created
 */
/*
 * DOS file system functions
 *
 * Copyright 1993 Erik Bos
 * Copyright 1996 Alexandre Julliard
 */

#include <precomp.h>

VOID STDCALL GetSystemTimeAsFileTime(LPFILETIME lpSystemTimeAsFileTime);

/*
 * @implemented
 */
time_t time(time_t* t)
{
	FILETIME  SystemTime;
	DWORD Remainder;
	time_t tt;
	GetSystemTimeAsFileTime(&SystemTime);
	tt = FileTimeToUnixTime(&SystemTime,&Remainder);
	if (t)
		*t = tt;
	return tt;
}

/***********************************************************************
 *           DOSFS_UnixTimeToFileTime
 *
 * Convert a Unix time to FILETIME format.
 * The FILETIME structure is a 64-bit value representing the number of
 * 100-nanosecond intervals since January 1, 1601, 0:00.
 * 'remainder' is the nonnegative number of 100-ns intervals
 * corresponding to the time fraction smaller than 1 second that
 * couldn't be stored in the time_t value.
 */
void UnixTimeToFileTime( time_t unix_time, FILETIME *filetime,
                               DWORD remainder )
{
    /* NOTES:

       CONSTANTS:
       The time difference between 1 January 1601, 00:00:00 and
       1 January 1970, 00:00:00 is 369 years, plus the leap years
       from 1604 to 1968, excluding 1700, 1800, 1900.
       This makes (1968 - 1600) / 4 - 3 = 89 leap days, and a total
       of 134774 days.

       Any day in that period had 24 * 60 * 60 = 86400 seconds.

       The time difference is 134774 * 86400 * 10000000, which can be written
       116444736000000000
       27111902 * 2^32 + 3577643008
       413 * 2^48 + 45534 * 2^32 + 54590 * 2^16 + 32768

       If you find that these constants are buggy, please change them in all
       instances in both conversion functions.

       VERSIONS:
       There are two versions, one of them uses long long variables and
       is presumably faster but not ISO C. The other one uses standard C
       data types and operations but relies on the assumption that negative
       numbers are stored as 2's complement (-1 is 0xffff....). If this
       assumption is violated, dates before 1970 will not convert correctly.
       This should however work on any reasonable architecture where WINE
       will run.

       DETAILS:

       Take care not to remove the casts. I have tested these functions
       (in both versions) for a lot of numbers. I would be interested in
       results on other compilers than GCC.

       The operations have been designed to account for the possibility
       of 64-bit time_t in future UNICES. Even the versions without
       internal long long numbers will work if time_t only is 64 bit.
       A 32-bit shift, which was necessary for that operation, turned out
       not to work correctly in GCC, besides giving the warning. So I
       used a double 16-bit shift instead. Numbers are in the ISO version
       represented by three limbs, the most significant with 32 bit, the
       other two with 16 bit each.

       As the modulo-operator % is not well-defined for negative numbers,
       negative divisors have been avoided in DOSFS_FileTimeToUnixTime.

       There might be quicker ways to do this in C. Certainly so in
       assembler.

       Claus Fischer, [e-mail]
       */




    unsigned long a0;                  /* 16 bit, low    bits */
    unsigned long a1;                  /* 16 bit, medium bits */
    unsigned long a2;                  /* 32 bit, high   bits */

    /* Copy the unix time to a2/a1/a0 */
    a0 =  unix_time & 0xffff;
    a1 = (unix_time >> 16) & 0xffff;
    /* This is obsolete if unix_time is only 32 bits, but it does not hurt.
       Do not replace this by >> 32, it gives a compiler warning and it does
       not work. */
    a2 = (unix_time >= 0 ? (unix_time >> 16) >> 16 :
          ~((~unix_time >> 16) >> 16));

    /* Multiply a by 10000000 (a = a2/a1/a0)
       Split the factor into 10000 * 1000 which are both less than 0xffff. */
    a0 *= 10000;
    a1 = a1 * 10000 + (a0 >> 16);
    a2 = a2 * 10000 + (a1 >> 16);
    a0 &= 0xffff;
    a1 &= 0xffff;

    a0 *= 1000;
    a1 = a1 * 1000 + (a0 >> 16);
    a2 = a2 * 1000 + (a1 >> 16);
    a0 &= 0xffff;
    a1 &= 0xffff;

    /* Add the time difference and the remainder */
    a0 += 32768 + (remainder & 0xffff);
    a1 += 54590 + (remainder >> 16   ) + (a0 >> 16);
    a2 += 27111902                     + (a1 >> 16);
    a0 &= 0xffff;
    a1 &= 0xffff;

    /* Set filetime */
    filetime->dwLowDateTime  = (a1 << 16) + a0;
    filetime->dwHighDateTime = a2;
}


/***********************************************************************
 *           DOSFS_FileTimeToUnixTime
 *
 * Convert a FILETIME format to Unix time.
 * If not NULL, 'remainder' contains the fractional part of the filetime,
 * in the range of [0..9999999] (even if time_t is negative).
 */
time_t FileTimeToUnixTime( const FILETIME *filetime, DWORD *remainder )
{
    /* Read the comment in the function DOSFS_UnixTimeToFileTime. */

    unsigned long a0;                  /* 16 bit, low    bits */
    unsigned long a1;                  /* 16 bit, medium bits */
    unsigned long a2;                  /* 32 bit, high   bits */
    unsigned long r;                   /* remainder of division */
    unsigned int carry;         /* carry bit for subtraction */
    int negative;               /* whether a represents a negative value */

    /* Copy the time values to a2/a1/a0 */
    a2 =  (unsigned long)filetime->dwHighDateTime;
    a1 = ((unsigned long)filetime->dwLowDateTime ) >> 16;
    a0 = ((unsigned long)filetime->dwLowDateTime ) & 0xffff;

    /* Subtract the time difference */
    if (a0 >= 32768           ) a0 -=             32768        , carry = 0;
    else                        a0 += (1 << 16) - 32768        , carry = 1;

    if (a1 >= 54590    + carry) a1 -=             54590 + carry, carry = 0;
    else                        a1 += (1 << 16) - 54590 - carry, carry = 1;

    a2 -= 27111902 + carry;

    /* If a is negative, replace a by (-1-a) */
    negative = (a2 >= ((unsigned long)1) << 31);
    if (negative)
    {
        /* Set a to -a - 1 (a is a2/a1/a0) */
        a0 = 0xffff - a0;
        a1 = 0xffff - a1;
        a2 = ~a2;
    }

    /* Divide a by 10000000 (a = a2/a1/a0), put the rest into r.
       Split the divisor into 10000 * 1000 which are both less than 0xffff. */
    a1 += (a2 % 10000) << 16;
    a2 /=       10000;
    a0 += (a1 % 10000) << 16;
    a1 /=       10000;
    r   =  a0 % 10000;
    a0 /=       10000;

    a1 += (a2 % 1000) << 16;
    a2 /=       1000;
    a0 += (a1 % 1000) << 16;
    a1 /=       1000;
    r  += (a0 % 1000) * 10000;
    a0 /=       1000;

    /* If a was negative, replace a by (-1-a) and r by (9999999 - r) */
    if (negative)
    {
        /* Set a to -a - 1 (a is a2/a1/a0) */
        a0 = 0xffff - a0;
        a1 = 0xffff - a1;
        a2 = ~a2;

        r  = 9999999 - r;
    }

    if (remainder) *remainder = r;

    /* Do not replace this by << 32, it gives a compiler warning and it does
       not work. */
    return ((((time_t)a2) << 16) << 16) + (a1 << 16) + a0;

}