source: mainline/uspace/lib/c/generic/time.c@ 56b308e

/*
 * Copyright (c) 2006 Ondrej Palkovsky
 * Copyright (c) 2011 Petr Koupy
 * Copyright (c) 2011 Jiri Zarevucky
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 * - Redistributions in binary form must reproduce the above copyright
 *   notice, this list of conditions and the following disclaimer in the
 *   documentation and/or other materials provided with the distribution.
 * - The name of the author may not be used to endorse or promote products
 *   derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/** @addtogroup libc
 * @{
 */
/** @file
 */

#include <sys/time.h>
#include <time.h>
#include <bool.h>
#include <libarch/barrier.h>
#include <macros.h>
#include <errno.h>
#include <sysinfo.h>
#include <as.h>
#include <ddi.h>
#include <libc.h>
#include <stdint.h>
#include <stdio.h>
#include <ctype.h>

#define ASCTIME_BUF_LEN 26

/** Pointer to kernel shared variables with time */
struct {
        volatile sysarg_t seconds1;
        volatile sysarg_t useconds;
        volatile sysarg_t seconds2;
} *ktime = NULL;

/* Helper functions ***********************************************************/

#define HOURS_PER_DAY (24)
#define MINS_PER_HOUR (60)
#define SECS_PER_MIN (60)
#define MINS_PER_DAY (MINS_PER_HOUR * HOURS_PER_DAY)
#define SECS_PER_HOUR (SECS_PER_MIN * MINS_PER_HOUR)
#define SECS_PER_DAY (SECS_PER_HOUR * HOURS_PER_DAY)

/**
 * Checks whether the year is a leap year.
 *
 * @param year Year since 1900 (e.g. for 1970, the value is 70).
 * @return true if year is a leap year, false otherwise
 */
static bool _is_leap_year(time_t year)
{
        year += 1900;

        if (year % 400 == 0)
                return true;
        if (year % 100 == 0)
                return false;
        if (year % 4 == 0)
                return true;
        return false;
}

/**
 * Returns how many days there are in the given month of the given year.
 * Note that year is only taken into account if month is February.
 *
 * @param year Year since 1900 (can be negative).
 * @param mon Month of the year. 0 for January, 11 for December.
 * @return Number of days in the specified month.
 */
static int _days_in_month(time_t year, time_t mon)
{
        assert(mon >= 0 && mon <= 11);

        static int month_days[] =
                { 31, 0, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

        if (mon == 1) {
                year += 1900;
                /* february */
                return _is_leap_year(year) ? 29 : 28;
        } else {
                return month_days[mon];
        }
}

/**
 * For specified year, month and day of month, returns which day of that year
 * it is.
 *
 * For example, given date 2011-01-03, the corresponding expression is:
 *     _day_of_year(111, 0, 3) == 2
 *
 * @param year Year (year 1900 = 0, can be negative).
 * @param mon Month (January = 0).
 * @param mday Day of month (First day is 1).
 * @return Day of year (First day is 0).
 */
static int _day_of_year(time_t year, time_t mon, time_t mday)
{
        static int mdays[] =
            { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };
        static int leap_mdays[] =
            { 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335 };

        return (_is_leap_year(year) ? leap_mdays[mon] : mdays[mon]) + mday - 1;
}

/**
 * Integer division that rounds to negative infinity.
 * Used by some functions in this file.
 *
 * @param op1 Dividend.
 * @param op2 Divisor.
 * @return Rounded quotient.
 */
static time_t _floor_div(time_t op1, time_t op2)
{
        if (op1 >= 0 || op1 % op2 == 0) {
                return op1 / op2;
        } else {
                return op1 / op2 - 1;
        }
}

/**
 * Modulo that rounds to negative infinity.
 * Used by some functions in this file.
 *
 * @param op1 Dividend.
 * @param op2 Divisor.
 * @return Remainder.
 */
static time_t _floor_mod(time_t op1, time_t op2)
{
        int div = _floor_div(op1, op2);

        /* (a / b) * b + a % b == a */
        /* thus, a % b == a - (a / b) * b */

        int result = op1 - div * op2;
        
        /* Some paranoid checking to ensure I didn't make a mistake here. */
        assert(result >= 0);
        assert(result < op2);
        assert(div * op2 + result == op1);
        
        return result;
}

/**
 * Number of days since the Epoch.
 * Epoch is 1970-01-01, which is also equal to day 0.
 *
 * @param year Year (year 1900 = 0, may be negative).
 * @param mon Month (January = 0).
 * @param mday Day of month (first day = 1).
 * @return Number of days since the Epoch.
 */
static time_t _days_since_epoch(time_t year, time_t mon, time_t mday)
{
        return (year - 70) * 365 + _floor_div(year - 69, 4) -
            _floor_div(year - 1, 100) + _floor_div(year + 299, 400) +
            _day_of_year(year, mon, mday);
}

/**
 * Seconds since the Epoch. see also _days_since_epoch().
 * 
 * @param tm Normalized broken-down time.
 * @return Number of seconds since the epoch, not counting leap seconds.
 */
static time_t _secs_since_epoch(const struct tm *tm)
{
        return _days_since_epoch(tm->tm_year, tm->tm_mon, tm->tm_mday) *
            SECS_PER_DAY + tm->tm_hour * SECS_PER_HOUR +
            tm->tm_min * SECS_PER_MIN + tm->tm_sec;
}

/**
 * Which day of week the specified date is.
 * 
 * @param year Year (year 1900 = 0).
 * @param mon Month (January = 0).
 * @param mday Day of month (first = 1).
 * @return Day of week (Sunday = 0).
 */
static int _day_of_week(time_t year, time_t mon, time_t mday)
{
        /* 1970-01-01 is Thursday */
        return _floor_mod((_days_since_epoch(year, mon, mday) + 4), 7);
}

/**
 * Normalizes the broken-down time and optionally adds specified amount of
 * seconds.
 * 
 * @param tm Broken-down time to normalize.
 * @param sec_add Seconds to add.
 * @return 0 on success, -1 on overflow
 */
static int _normalize_time(struct tm *tm, time_t sec_add)
{
        // TODO: DST correction

        /* Set initial values. */
        time_t sec = tm->tm_sec + sec_add;
        time_t min = tm->tm_min;
        time_t hour = tm->tm_hour;
        time_t day = tm->tm_mday - 1;
        time_t mon = tm->tm_mon;
        time_t year = tm->tm_year;

        /* Adjust time. */
        min += _floor_div(sec, SECS_PER_MIN);
        sec = _floor_mod(sec, SECS_PER_MIN);
        hour += _floor_div(min, MINS_PER_HOUR);
        min = _floor_mod(min, MINS_PER_HOUR);
        day += _floor_div(hour, HOURS_PER_DAY);
        hour = _floor_mod(hour, HOURS_PER_DAY);

        /* Adjust month. */
        year += _floor_div(mon, 12);
        mon = _floor_mod(mon, 12);

        /* Now the difficult part - days of month. */
        
        /* First, deal with whole cycles of 400 years = 146097 days. */
        year += _floor_div(day, 146097) * 400;
        day = _floor_mod(day, 146097);
        
        /* Then, go in one year steps. */
        if (mon <= 1) {
                /* January and February. */
                while (day > 365) {
                        day -= _is_leap_year(year) ? 366 : 365;
                        year++;
                }
        } else {
                /* Rest of the year. */
                while (day > 365) {
                        day -= _is_leap_year(year + 1) ? 366 : 365;
                        year++;
                }
        }
        
        /* Finally, finish it off month per month. */
        while (day >= _days_in_month(year, mon)) {
                day -= _days_in_month(year, mon);
                mon++;
                if (mon >= 12) {
                        mon -= 12;
                        year++;
                }
        }
        
        /* Calculate the remaining two fields. */
        tm->tm_yday = _day_of_year(year, mon, day + 1);
        tm->tm_wday = _day_of_week(year, mon, day + 1);
        
        /* And put the values back to the struct. */
        tm->tm_sec = (int) sec;
        tm->tm_min = (int) min;
        tm->tm_hour = (int) hour;
        tm->tm_mday = (int) day + 1;
        tm->tm_mon = (int) mon;
        
        /* Casts to work around libc brain-damage. */
        if (year > ((int)INT_MAX) || year < ((int)INT_MIN)) {
                tm->tm_year = (year < 0) ? ((int)INT_MIN) : ((int)INT_MAX);
                return -1;
        }
        
        tm->tm_year = (int) year;
        return 0;
}

/**
 * Which day the week-based year starts on, relative to the first calendar day.
 * E.g. if the year starts on December 31st, the return value is -1.
 *
 * @param Year since 1900.
 * @return Offset of week-based year relative to calendar year.
 */
static int _wbyear_offset(int year)
{
        int start_wday = _day_of_week(year, 0, 1);
        return _floor_mod(4 - start_wday, 7) - 3;
}

/**
 * Returns week-based year of the specified time.
 *
 * @param tm Normalized broken-down time.
 * @return Week-based year.
 */
static int _wbyear(const struct tm *tm)
{
        int day = tm->tm_yday - _wbyear_offset(tm->tm_year);
        if (day < 0) {
                /* Last week of previous year. */
                return tm->tm_year - 1;
        }
        if (day > 364 + _is_leap_year(tm->tm_year)) {
                /* First week of next year. */
                return tm->tm_year + 1;
        }
        /* All the other days are in the calendar year. */
        return tm->tm_year;
}

/**
 * Week number of the year, assuming weeks start on sunday.
 * The first Sunday of January is the first day of week 1;
 * days in the new year before this are in week 0.
 *
 * @param tm Normalized broken-down time.
 * @return The week number (0 - 53).
 */
static int _sun_week_number(const struct tm *tm)
{
        int first_day = (7 - _day_of_week(tm->tm_year, 0, 1)) % 7;
        return (tm->tm_yday - first_day + 7) / 7;
}

/**
 * Week number of the year, assuming weeks start on monday.
 * If the week containing January 1st has four or more days in the new year,
 * then it is considered week 1. Otherwise, it is the last week of the previous
 * year, and the next week is week 1. Both January 4th and the first Thursday
 * of January are always in week 1.
 *
 * @param tm Normalized broken-down time.
 * @return The week number (1 - 53).
 */
static int _iso_week_number(const struct tm *tm)
{
        int day = tm->tm_yday - _wbyear_offset(tm->tm_year);
        if (day < 0) {
                /* Last week of previous year. */
                return 53;
        }
        if (day > 364 + _is_leap_year(tm->tm_year)) {
                /* First week of next year. */
                return 1;
        }
        /* All the other days give correct answer. */
        return (day / 7 + 1);
}

/**
 * Week number of the year, assuming weeks start on monday.
 * The first Monday of January is the first day of week 1;
 * days in the new year before this are in week 0. 
 *
 * @param tm Normalized broken-down time.
 * @return The week number (0 - 53).
 */
static int _mon_week_number(const struct tm *tm)
{
        int first_day = (1 - _day_of_week(tm->tm_year, 0, 1)) % 7;
        return (tm->tm_yday - first_day + 7) / 7;
}

/******************************************************************************/


/** Add microseconds to given timeval.
 *
 * @param tv    Destination timeval.
 * @param usecs Number of microseconds to add.
 *
 */
void tv_add(struct timeval *tv, suseconds_t usecs)
{
        tv->tv_sec += usecs / 1000000;
        tv->tv_usec += usecs % 1000000;
        
        if (tv->tv_usec > 1000000) {
                tv->tv_sec++;
                tv->tv_usec -= 1000000;
        }
}

/** Subtract two timevals.
 *
 * @param tv1 First timeval.
 * @param tv2 Second timeval.
 *
 * @return Difference between tv1 and tv2 (tv1 - tv2) in
 *         microseconds.
 *
 */
suseconds_t tv_sub(struct timeval *tv1, struct timeval *tv2)
{
        return (tv1->tv_usec - tv2->tv_usec) +
            ((tv1->tv_sec - tv2->tv_sec) * 1000000);
}

/** Decide if one timeval is greater than the other.
 *
 * @param t1 First timeval.
 * @param t2 Second timeval.
 *
 * @return True if tv1 is greater than tv2.
 * @return False otherwise.
 *
 */
int tv_gt(struct timeval *tv1, struct timeval *tv2)
{
        if (tv1->tv_sec > tv2->tv_sec)
                return true;
        
        if ((tv1->tv_sec == tv2->tv_sec) && (tv1->tv_usec > tv2->tv_usec))
                return true;
        
        return false;
}

/** Decide if one timeval is greater than or equal to the other.
 *
 * @param tv1 First timeval.
 * @param tv2 Second timeval.
 *
 * @return True if tv1 is greater than or equal to tv2.
 * @return False otherwise.
 *
 */
int tv_gteq(struct timeval *tv1, struct timeval *tv2)
{
        if (tv1->tv_sec > tv2->tv_sec)
                return true;
        
        if ((tv1->tv_sec == tv2->tv_sec) && (tv1->tv_usec >= tv2->tv_usec))
                return true;
        
        return false;
}

/** Get time of day
 *
 * The time variables are memory mapped (read-only) from kernel which
 * updates them periodically.
 *
 * As it is impossible to read 2 values atomically, we use a trick:
 * First we read the seconds, then we read the microseconds, then we
 * read the seconds again. If a second elapsed in the meantime, set
 * the microseconds to zero.
 *
 * This assures that the values returned by two subsequent calls
 * to gettimeofday() are monotonous.
 *
 */
int gettimeofday(struct timeval *tv, struct timezone *tz)
{
        if (ktime == NULL) {
                uintptr_t faddr;
                int rc = sysinfo_get_value("clock.faddr", &faddr);
                if (rc != EOK) {
                        errno = rc;
                        return -1;
                }
                
                void *addr;
                rc = physmem_map((void *) faddr, 1,
                    AS_AREA_READ | AS_AREA_CACHEABLE, &addr);
                if (rc != EOK) {
                        as_area_destroy(addr);
                        errno = rc;
                        return -1;
                }
                
                ktime = addr;
        }
        
        if (tz) {
                tz->tz_minuteswest = 0;
                tz->tz_dsttime = DST_NONE;
        }
        
        sysarg_t s2 = ktime->seconds2;
        
        read_barrier();
        tv->tv_usec = ktime->useconds;
        
        read_barrier();
        sysarg_t s1 = ktime->seconds1;
        
        if (s1 != s2) {
                tv->tv_sec = max(s1, s2);
                tv->tv_usec = 0;
        } else
                tv->tv_sec = s1;
        
        return 0;
}

time_t time(time_t *tloc)
{
        struct timeval tv;
        if (gettimeofday(&tv, NULL))
                return (time_t) -1;
        
        if (tloc)
                *tloc = tv.tv_sec;
        
        return tv.tv_sec;
}

/** Wait unconditionally for specified number of microseconds
 *
 */
int usleep(useconds_t usec)
{
        (void) __SYSCALL1(SYS_THREAD_USLEEP, usec);
        return 0;
}

void udelay(useconds_t time)
{
        (void) __SYSCALL1(SYS_THREAD_UDELAY, (sysarg_t) time);
}


/** Wait unconditionally for specified number of seconds
 *
 */
unsigned int sleep(unsigned int sec)
{
        /*
         * Sleep in 1000 second steps to support
         * full argument range
         */
        
        while (sec > 0) {
                unsigned int period = (sec > 1000) ? 1000 : sec;
                
                usleep(period * 1000000);
                sec -= period;
        }
        
        return 0;
}

/**
 * This function first normalizes the provided broken-down time
 * (moves all values to their proper bounds) and then tries to
 * calculate the appropriate time_t representation.
 *
 * @param tm Broken-down time.
 * @return time_t representation of the time, undefined value on overflow.
 */
time_t mktime(struct tm *tm)
{
        // TODO: take DST flag into account
        // TODO: detect overflow

        _normalize_time(tm, 0);
        return _secs_since_epoch(tm);
}

/**
 * Convert time and date to a string, based on a specified format and
 * current locale.
 * 
 * @param s Buffer to write string to.
 * @param maxsize Size of the buffer.
 * @param format Format of the output.
 * @param tm Broken-down time to format.
 * @return Number of bytes written.
 */
size_t strftime(char *restrict s, size_t maxsize,
    const char *restrict format, const struct tm *restrict tm)
{
        assert(s != NULL);
        assert(format != NULL);
        assert(tm != NULL);

        // TODO: use locale
        static const char *wday_abbr[] = {
                "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
        };
        static const char *wday[] = {
                "Sunday", "Monday", "Tuesday", "Wednesday",
                "Thursday", "Friday", "Saturday"
        };
        static const char *mon_abbr[] = {
                "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
        };
        static const char *mon[] = {
                "January", "February", "March", "April", "May", "June", "July",
                "August", "September", "October", "November", "December"
        };
        
        if (maxsize < 1) {
                return 0;
        }
        
        char *ptr = s;
        size_t consumed;
        size_t remaining = maxsize;
        
        #define append(...) { \
                /* FIXME: this requires POSIX-correct snprintf */ \
                /*        otherwise it won't work with non-ascii chars */ \
                consumed = snprintf(ptr, remaining, __VA_ARGS__); \
                if (consumed >= remaining) { \
                        return 0; \
                } \
                ptr += consumed; \
                remaining -= consumed; \
        }
        
        #define recurse(fmt) { \
                consumed = strftime(ptr, remaining, fmt, tm); \
                if (consumed == 0) { \
                        return 0; \
                } \
                ptr += consumed; \
                remaining -= consumed; \
        }
        
        #define TO_12H(hour) (((hour) > 12) ? ((hour) - 12) : \
            (((hour) == 0) ? 12 : (hour)))
        
        while (*format != '\0') {
                if (*format != '%') {
                        append("%c", *format);
                        format++;
                        continue;
                }
                
                format++;
                if (*format == '0' || *format == '+') {
                        // TODO: padding
                        format++;
                }
                while (isdigit(*format)) {
                        // TODO: padding
                        format++;
                }
                if (*format == 'O' || *format == 'E') {
                        // TODO: locale's alternative format
                        format++;
                }
                
                switch (*format) {
                case 'a':
                        append("%s", wday_abbr[tm->tm_wday]); break;
                case 'A':
                        append("%s", wday[tm->tm_wday]); break;
                case 'b':
                        append("%s", mon_abbr[tm->tm_mon]); break;
                case 'B':
                        append("%s", mon[tm->tm_mon]); break;
                case 'c':
                        // TODO: locale-specific datetime format
                        recurse("%Y-%m-%d %H:%M:%S"); break;
                case 'C':
                        append("%02d", (1900 + tm->tm_year) / 100); break;
                case 'd':
                        append("%02d", tm->tm_mday); break;
                case 'D':
                        recurse("%m/%d/%y"); break;
                case 'e':
                        append("%2d", tm->tm_mday); break;
                case 'F':
                        recurse("%+4Y-%m-%d"); break;
                case 'g':
                        append("%02d", _wbyear(tm) % 100); break;
                case 'G':
                        append("%d", _wbyear(tm)); break;
                case 'h':
                        recurse("%b"); break;
                case 'H':
                        append("%02d", tm->tm_hour); break;
                case 'I':
                        append("%02d", TO_12H(tm->tm_hour)); break;
                case 'j':
                        append("%03d", tm->tm_yday); break;
                case 'k':
                        append("%2d", tm->tm_hour); break;
                case 'l':
                        append("%2d", TO_12H(tm->tm_hour)); break;
                case 'm':
                        append("%02d", tm->tm_mon); break;
                case 'M':
                        append("%02d", tm->tm_min); break;
                case 'n':
                        append("\n"); break;
                case 'p':
                        append("%s", tm->tm_hour < 12 ? "AM" : "PM"); break;
                case 'P':
                        append("%s", tm->tm_hour < 12 ? "am" : "PM"); break;
                case 'r':
                        recurse("%I:%M:%S %p"); break;
                case 'R':
                        recurse("%H:%M"); break;
                case 's':
                        append("%ld", _secs_since_epoch(tm)); break;
                case 'S':
                        append("%02d", tm->tm_sec); break;
                case 't':
                        append("\t"); break;
                case 'T':
                        recurse("%H:%M:%S"); break;
                case 'u':
                        append("%d", (tm->tm_wday == 0) ? 7 : tm->tm_wday);
                        break;
                case 'U':
                        append("%02d", _sun_week_number(tm)); break;
                case 'V':
                        append("%02d", _iso_week_number(tm)); break;
                case 'w':
                        append("%d", tm->tm_wday); break;
                case 'W':
                        append("%02d", _mon_week_number(tm)); break;
                case 'x':
                        // TODO: locale-specific date format
                        recurse("%Y-%m-%d"); break;
                case 'X':
                        // TODO: locale-specific time format
                        recurse("%H:%M:%S"); break;
                case 'y':
                        append("%02d", tm->tm_year % 100); break;
                case 'Y':
                        append("%d", 1900 + tm->tm_year); break;
                case 'z':
                        // TODO: timezone
                        break;
                case 'Z':
                        // TODO: timezone
                        break;
                case '%':
                        append("%%");
                        break;
                default:
                        /* Invalid specifier, print verbatim. */
                        while (*format != '%') {
                                format--;
                        }
                        append("%%");
                        break;
                }
                format++;
        }
        
        #undef append
        #undef recurse
        
        return maxsize - remaining;
}

struct tm *gmtime(const time_t *timer)
{
        assert(timer != NULL);

        static struct tm result;

        /* Set result to epoch. */
        result.tm_sec = 0;
        result.tm_min = 0;
        result.tm_hour = 0;
        result.tm_mday = 1;
        result.tm_mon = 0;
        result.tm_year = 70; /* 1970 */

        if (_normalize_time(&result, *timer) == -1) {
                errno = EOVERFLOW;
                return NULL;
        }

        return &result;
}

/**
 * Converts broken-down time to a string in format
 * "Sun Jan 1 00:00:00 1970\n". (Obsolete)
 *
 * @param timeptr Broken-down time structure.
 * @return Pointer to a statically allocated string.
 */
char *asctime(const struct tm *timeptr)
{
        static char buf[ASCTIME_BUF_LEN];

        assert(timeptr != NULL);

        static const char *wday[] = {
                "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"
        };
        static const char *mon[] = {
                "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"
        };

        snprintf(buf, ASCTIME_BUF_LEN, "%s %s %2d %02d:%02d:%02d %d\n",
            wday[timeptr->tm_wday],
            mon[timeptr->tm_mon],
            timeptr->tm_mday, timeptr->tm_hour,
            timeptr->tm_min, timeptr->tm_sec,
            1900 + timeptr->tm_year);

        return buf;

}

/**
 * Converts a time value to a broken-down local time.
 *
 * @param timer Time to convert.
 * @return Normalized broken-down time in local timezone, NULL on overflow.
 */
struct tm *localtime(const time_t *timer)
{
        // TODO: deal with timezone
        // currently assumes system and all times are in GMT

        static struct tm result;

        /* Set result to epoch. */
        result.tm_sec = 0;
        result.tm_min = 0;
        result.tm_hour = 0;
        result.tm_mday = 1;
        result.tm_mon = 0;
        result.tm_year = 70; /* 1970 */

        if (_normalize_time(&result, *timer) == -1) {
                errno = EOVERFLOW;
                return NULL;
        }

        return &result;
}

/**
 * Equivalent to asctime(localtime(clock)).
 * 
 * @param timer Time to convert.
 * @return Pointer to a statically allocated string holding the date.
 */
char *ctime(const time_t *timer)
{
        struct tm *loctime = localtime(timer);
        if (loctime == NULL) {
                return NULL;
        }
        return asctime(loctime);
}

/** @}
 */