Typedefs¶

typedef uint32_t time_t

Introduction to the Time functions¶

This file declares the time functions implemented in avr-libc.

The implementation aspires to conform with ISO/IEC 9899 (C90). However, due to limitations of the target processor and the nature of its development environment, a practical implementation must of necessity deviate from the standard.

Section 7.23.2.1 clock() The type clock_t, the macro CLOCKS_PER_SEC, and the function clock() are not implemented. We consider these items belong to operating system code, or to application code when no operating system is present.

Section 7.23.2.3 mktime() The standard specifies that mktime() should return (time_t) -1, if the time cannot be represented. This implementation always returns a 'best effort' representation.

Section 7.23.2.4 time() The standard specifies that time() should return (time_t) -1, if the time is not available. Since the application must initialize the time system, this functionality is not implemented.

Section 7.23.2.2, difftime() Due to the lack of a 64 bit double, the function difftime() returns a long integer. In most cases this change will be invisible to the user, handled automatically by the compiler.

Section 7.23.1.4 struct tm Per the standard, struct tm->tm_isdst is greater than zero when Daylight Saving time is in effect. This implementation further specifies that, when positive, the value of tm_isdst represents the amount time is advanced during Daylight Saving time.

Section 7.23.3.5 strftime() Only the 'C' locale is supported, therefore the modifiers 'E' and 'O' are ignored. The 'Z' conversion is also ignored, due to the lack of time zone name.

In addition to the above departures from the standard, there are some behaviors which are different from what is often expected, though allowed under the standard.

There is no 'platform standard' method to obtain the current time, time zone, or daylight savings 'rules' in the AVR environment. Therefore the application must initialize the time system with this information. The functions set_zone(), set_dst(), and set_system_time() are provided for initialization. Once initialized, system time is maintained by calling the function system_tick() at one second intervals.

Though not specified in the standard, it is often expected that time_t is a signed integer representing an offset in seconds from Midnight Jan 1 1970... i.e. 'Unix time'. This implementation uses an unsigned 32 bit integer offset from Midnight Jan 1 2000. The use of this 'epoch' helps to simplify the conversion functions, while the 32 bit value allows time to be properly represented until Tue Feb 7 06:28:15 2136 UTC. The macros UNIX_OFFSET and NTP_OFFSET are defined to assist in converting to and from Unix and NTP time stamps.

Unlike desktop counterparts, it is impractical to implement or maintain the 'zoneinfo' database. Therefore no attempt is made to account for time zone, daylight saving, or leap seconds in past dates. All calculations are made according to the currently configured time zone and daylight saving 'rule'.

In addition to C standard functions, re-entrant versions of ctime(), asctime(), gmtime() and localtime() are provided which, in addition to being re-entrant, have the property of claiming less permanent storage in RAM. An additional time conversion, isotime() and its re-entrant version, uses far less storage than either ctime() or asctime().

Along with the usual smattering of utility functions, such as is_leap_year(), this library includes a set of functions related the sun and moon, as well as sidereal time functions.

typedef uint32_t time_t¶

time_t represents seconds elapsed from Midnight, Jan 1 2000 UTC (the Y2K 'epoch'). Its range allows this implementation to represent time up to Tue Feb 7 06:28:15 2136 UTC.