Gemeint ist das vordefinierte Makro __TIME__ (jeweils 2 Unterstricche
vorn und hinten). Ist Teil des C-Standards und sollte in jedem
anständigen C-Buch beschrieben sein.
From ANSI/ISO 9899-1990, American National Standard for Programming
Languages - C
----
6.8.8 Predefined macro names
The following names shall be defined by the implementation:
__LINE__ The line number of the current source line (a decimal
constant).
__FILE__ The presumed name of the source file (a character string
literal).
__DATE__ The date of translation of the source file (a character
string literal of the form "Mmm dd yyyy", where the names of the months
are the same as those generated by the asctime function, and the first
character of dd is a space character if the value is less than 10). If
the date of translation is not available, an implementation-defined
valid date shall be supplied.
__TIME__ The time of translation of the source file (a character
string literal of the form "hh:mm:ss" as in the time generated by the
asctime function). If the time of translation is not available, an
implementation-defined valid time shall be supplied.
__STDC__ The decimal constant 1, intended to indicate a conforming
implementation.
The values of the predefined macros (except for __LINE__ and
__FILE__) remain constant throughout the translation unit.
None of these macro names, nor the identifier defined, shall be the
subject of a #define or a #undef preprocessing directive. All predefined
macro names shall begin with a leading underscore followed by an
uppercase letter or a second underscore.
----
Annex G (informative) Portability Issues
G.2 Undefined behavior
-- One of the following identifiers is the subject of a #define or
#undef preprocessing directive: defined, __LINE__, __FILE__,
__DATE__, __TIME__, or __STDC__ (6.8.8).
G.3 Implementation-defined behavior
G.3.13 Preprocessing directives
-- The definitions for __DATE__ and __TIME__ when respectively, the
date and time of translation are not available (6.8.8).
----
