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<HTML><HEAD><TITLE><time.h></TITLE></HEAD><BODY><H1><A NAME="<time.h>"><CODE><time.h></CODE></A></H1><HR><P>Include the standard header <B><CODE><time.h></CODE></B> to declare severalfunctions that help you manipulate times. The diagramsummarizes the functions and the object types that they convertbetween.</P><P><IMG SRC="time.gif"></P><P>The functions share two static-duration objects that hold valuescomputed by the functions:</P><UL><LI>a <B><A NAME="time string">time string</A></B>of type array of <I>char</I><LI>a <B><A NAME="time structure">time structure</A></B>of type <CODE>struct tm</CODE></UL><P>A call to one of these functions can alter the value that wasstored earlier in a static-duration object by another of these functions.</P><PRE> /* MACROS */#define <A HREF="#CLOCKS_PER_SEC"><B>CLOCKS_PER_SEC</B></A> <I><integer constant expression > 0></I>#define <A HREF="#NULL"><B>NULL</B></A> <I><either 0, 0L, or (void *)0></I> <B>[0 in C++]</B> /* TYPES */typedef <I>a-type</I> <A HREF="#clock_t"><B>clock_t</B></A>;typedef <I>ui-type</I> <A HREF="#size_t"><B>size_t</B></A>;typedef <I>a-type</I> <A HREF="#time_t"><B>time_t</B></A>;struct <A HREF="#tm"><B>tm</B></A>; /* FUNCTIONS */char *<A HREF="#asctime"><B>asctime</B></A>(const struct tm *tptr);clock_t <A HREF="#clock"><B>clock</B></A>(void);char *<A HREF="#ctime"><B>ctime</B></A>(const time_t *tod);double <A HREF="#difftime"><B>difftime</B></A>(time_t t1, time_t t0);struct tm *<A HREF="#gmtime"><B>gmtime</B></A>(const time_t *tod);struct tm *<A HREF="#localtime"><B>localtime</B></A>(const time_t *tod);time_t <A HREF="#mktime"><B>mktime</B></A>(struct tm *tptr);size_t <A HREF="#strftime"><B>strftime</B></A>(char *s, size_t n, const char *format, const struct tm *tptr);time_t <A HREF="#time"><B>time</B></A>(time_t *tod);</PRE><H2><A NAME="asctime"><CODE>asctime</CODE></A></H2><PRE>char *<B>asctime</B>(const struct tm *tptr);</PRE><P>The function stores in the static-duration time string a 26-characterEnglish-language representation of the time encoded in <CODE>*tptr</CODE>.It returns the address of the static-duration<A HREF="#time string">time string</A>. The textrepresentation takes the form:</P><PRE>Sun Dec 2 06:55:15 1979\n\0</PRE><H2><A NAME="clock"><CODE>clock</CODE></A></H2><PRE>clock_t <B>clock</B>(void);</PRE><P>The function returns the number of clock ticks of elapsed processortime, counting from a time related to<A HREF="lib_over.html#program startup">program startup</A>, or it returns-1 if the target environment cannot measure elapsed processortime.</P><H2><A NAME="CLOCKS_PER_SEC"><CODE>CLOCKS_PER_SEC</CODE></A></H2><PRE>#define <B>CLOCKS_PER_SEC</B> <I><integer constant expression > 0></I></PRE><P>The macro yields the number of clock ticks,returned by <CODE>clock</CODE>, in one second.</P><H2><A NAME="clock_t"><CODE>clock_t</CODE></A></H2><PRE>typedef <I>a-type</I> <B>clock_t</B>;</PRE><P>The type is the arithmetic type <CODE><I>a-type</I></CODE>of an object that you declare to hold the value returned by<A HREF="#clock"><CODE>clock</CODE></A>, representingelapsed processor time.</P><H2><A NAME="ctime"><CODE>ctime</CODE></A></H2><PRE>char *<B>ctime</B>(const time_t *tod);</PRE><P>The function converts the calendar time in <CODE>*tod</CODE> to atext representation of the local time in the static-duration<A HREF="#time string">time string</A>.It returns the address of that string. It is equivalent to<CODE><A HREF="#asctime">asctime</A>(localtime(tod))</CODE>.</P><H2><A NAME="difftime"><CODE>difftime</CODE></A></H2><PRE>double <B>difftime</B>(time_t t1, time_t t0);</PRE><P>The function returns the difference <CODE>t1 - t0</CODE>, in seconds,between the calendar time <CODE>t0</CODE>and the calendar time <CODE>t1</CODE>.</P><H2><A NAME="gmtime"><CODE>gmtime</CODE></A></H2><PRE>struct tm *<B>gmtime</B>(const time_t *tod);</PRE><P>The function stores in the static-duration<A HREF="#time structure">time structure</A> anencoding of the calendar time in <CODE>*tod</CODE>, expressed as<B><A NAME="Coordinated Universal Time">Coordinated Universal Time</A></B>, or UTC [sic].(UTC was formerly Greenwich Mean Time, or GMT).It returns the address of that structure, or[added with <A HREF="lib_over.html#C99">C99</A>] a null pointer ifit cannot generate the encoding.</P><H2><A NAME="localtime"><CODE>localtime</CODE></A></H2><PRE>struct tm *<B>localtime</B>(const time_t *tod);</PRE><P>The function stores in the static-duration<A HREF="#time structure">time structure</A> anencoding of the calendar time in <CODE>*tod</CODE>, expressed as local time.It returns the address of that structure, or[added with <A HREF="lib_over.html#C99">C99</A>] a null pointer ifit cannot generate the encoding.</P><H2><A NAME="mktime"><CODE>mktime</CODE></A></H2><PRE>time_t <B>mktime</B>(struct tm *tptr);</PRE><P>The function alters the values stored in <CODE>*tptr</CODE> to representan equivalent encoded local time, but with the values of all memberswithin their normal ranges.It then determines the values <CODE>tptr->wday</CODE>and <CODE>tptr->yday</CODE> from the values of the other members.It returns the calendar time equivalent to the encoded time, or it returns avalue of -1 if the calendar time cannot be represented.</P><H2><A NAME="NULL"><CODE>NULL</CODE></A></H2><PRE>#define <B>NULL</B> <I><either 0, 0L, or (void *)0></I> <B>[0 in C++]</B></PRE><P>The macro yields a null pointer constant that is usable as an<A HREF="express.html#address constant expression">address constant expression</A>.</P><H2><A NAME="size_t"><CODE>size_t</CODE></A></H2><PRE>typedef <I>ui-type</I> <B>size_t</B>;</PRE><P>The type is the unsigned integer type <CODE><I>ui-type</I></CODE>of an object that you declare to store the result of the<A HREF="express.html#sizeof operator"><I>sizeof</I></A> operator.</P><H2><A NAME="strftime"><CODE>strftime</CODE></A></H2><PRE>size_t <B>strftime</B>(char *s, size_t n, const char *format, const struct tm *tptr);</PRE><P>The function generates formatted text, under the control ofthe format <CODE>format</CODE> and the values stored in the time structure<CODE>*tptr</CODE>. It stores each generated characterin successive locations of the array object of size<CODE>n</CODE> whose first element has the address <CODE>s</CODE>.The function then stores a null character in the next locationof the array. It returns <CODE>x</CODE>, the number of characters generated,if <CODE>x < n</CODE>; otherwise, it returns zero, and thevalues stored in the array are indeterminate.</P><P>For each multibyte character other than <CODE>%</CODE> in the format,the function stores that multibyte character in the array object.Each occurrence of <CODE>%</CODE> followed by an optional qualifier andanother character in the format is a<B><A NAME="conversion specifier">conversion specifier</A></B>.The optional qualifiers[added with <A HREF="lib_over.html#C99">C99</A>] are:</P><UL><LI><CODE>E</CODE>, to represent times in terms of a locale-specific<B><A NAME="era">era</A></B>(such as <CODE>1 BC</CODE> instead of <CODE>0000</CODE>).</LI><LI><CODE>O</CODE>, to represent numeric values with a set of locale-specific<B><A NAME="alternate digits">alternate digits</A></B>(such as <CODE>first</CODE> instead of <CODE>1</CODE>).</LI></UL><P>For each conversion specifier, thefunction stores a replacement character sequence.</P><P>The following table lists all conversion specifiersdefined for <CODE>strftime</CODE>.The fields used in <CODE>*tptr</CODE> follow.Example replacement character sequencesin parentheses follow each description. All examples are for the<A HREF="locale.html#C locale"><CODE>"C"</CODE></A> locale,which ignores any optional qualifier,using the date and time Sunday, 2 December 1979 at 06:55:15 AM EST.</P><P>For a Sunday week of the year, week 1 begins with the first Sunday onor after 1 January. For a Monday week of the year, week 1 begins with thefirst Monday on or after 1 January. An ISO 8601 week of the year is thesame as a Monday week of the year, with the following exceptions:</P><UL><LI>If 1 January is a Tuesday, Wednesday, or Thursday, the week number isone greater. Moreover, days back to and including the immediately precedingMonday in the preceding year are included in week 1 of the current year.</LI><LI>If 1 January is a Friday, Saturday, or Sunday, days up to but notincluding the immediately following Monday in the current year are includedin the last week (52 or 53) of the preceding year.</LI></UL><P>Conversion specifications marked with a<CODE>+</CODE> in the table below are alladded with <A HREF="lib_over.html#C99">C99</A>:</P><PRE><B>SPECIFIER FIELDS DESCRIPTION (EXAMPLE)</B> %a tm_wday <B>abbreviated weekday name (Sun)</B> %A tm_wday <B>full weekday name (Sunday)</B> %b tm_mon <B>abbreviated month name (Dec)</B> %B tm_mon <B>full month name (December)</B> %c [all] <B>date and time (Sun Dec 2 06:55:15 1979)</B> %Ec [all] + <B>era-specific date and time</B> %C tm_year + <B>year/100 (19)</B> %EC tm_mday + <B>era specific era name</B> tm_mon tm_year %d tm_mday <B>day of the month (02)</B> %D tm_mday + <B>month/day/year from 01/01/00 (12/02/79)</B> tm_mon tm_year %e tm_mday + <B>day of the month, leading space for zero ( 2)</B> %F tm_mday + <B>year-month-day (1979-12-02)</B> tm_mon tm_year %g tm_wday + <B>year for ISO 8601 week, from 00 (79)</B> tm_yday tm_year %G tm_wday + <B>year for ISO 8601 week, from 0000 (1979)</B> tm_yday tm_year %h tm_mon + <B>same as %b (Dec)</B> %H tm_hour <B>hour of the 24-hour day, from 00 (06)</B> %I tm_hour <B>hour of the 12-hour day, from 01 (06)</B> %j tm_yday <B>day of the year, from 001 (336)</B> %m tm_mon <B>month of the year, from 01 (12)</B> %M tm_min <B>minutes after the hour (55)</B> %n + <B>newline character \n</B> %p tm_hour <B>AM/PM indicator (AM)</B> %r tm_sec + <B>12-hour time, from 01:00:00 AM (06:55:15 AM)</B> tm_min tm_hour %Er tm_sec + <B>era-specific date and 12-hour time</B> tm_min tm_hour tm_mday tm_mon tm_year %R tm_min + <B>hour:minute, from 01:00 (06:55)</B> tm_hour %S tm_sec <B>seconds after the minute (15)</B> %t + <B>horizontal tab character \t</B> %T tm_sec + <B>24-hour time, from 00:00:00 (06:55:15)</B> tm_min tm_hour %u tm_wday + <B>ISO 8601 day of the week, to 7 for Sunday (7)</B> %U tm_wday <B>Sunday week of the year, from 00 (48)</B> tm_yday %V tm_wday + <B>ISO 8601 week of the year, from 01 (48)</B> tm_yday tm_year %w tm_wday <B>day of the week, from 0 for Sunday (0)</B> %W tm_wday <B>Monday week of the year, from 00 (48)</B> tm_yday %x [all] <B>date (02/12/79)</B> %Ex [all] + <B>era-specific date</B> %X [all] <B>time, from 00:00:00 (06:55:15)</B> %EX [all] + <B>era-specific time</B> %y tm_year <B>year of the century, from 00 (79)</B> %Ey tm_mday + <B>year of the era</B> tm_mon tm_year %Y tm_year <B>year (1979)</B> %EY tm_mday + <B>era-specific era name and year of the era</B> tm_mon tm_year %z tm_isdst + <B>time zone (hours east*100 + minutes), if any (-0500)</B> %Z tm_isdst <B>time zone name, if any (EST)</B> %% <B>percent character %</B></PRE><P>The current<A HREF="locale.html#locale category">locale category</A><A HREF="locale.html#LC_TIME"><CODE>LC_TIME</CODE></A> can affect thesereplacement character sequences.</P><H2><A NAME="time"><CODE>time</CODE></A></H2><PRE>time_t <B>time</B>(time_t *tod);</PRE><P>If <CODE>tod</CODE> is not a null pointer, the function stores thecurrent calendar time in <CODE>*tod</CODE>. The function returns the currentcalendar time, if the target environment can determine it; otherwise,it returns -1.</P><H2><A NAME="time_t"><CODE>time_t</CODE></A></H2><PRE>typedef <I>a-type</I> <B>time_t</B>;</PRE><P>The type is the arithmetic type <CODE><I>a-type</I></CODE>of an object that you declare to holdthe value returned by <CODE>time</CODE>.The value represents calendar time.</P><H2><A NAME="tm"><CODE>tm</CODE></A></H2><PRE>struct <B>tm</B> { int tm_sec; <B>seconds after the minute (from 0)</B> int tm_min; <B>minutes after the hour (from 0)</B> int tm_hour; <B>hour of the day (from 0)</B> int tm_mday; <B>day of the month (from 1)</B> int tm_mon; <B>month of the year (from 0)</B> int tm_year; <B>years since 1900 (from 0)</B> int tm_wday; <B>days since Sunday (from 0)</B> int tm_yday; <B>day of the year (from 0)</B> int tm_isdst; <B>Daylight Saving Time flag</B> };</PRE><P><CODE>struct tm</CODE> contains members that describe variousproperties of the calendar time. The members shown above can occurin any order, interspersed with additional members. The comment followingeach member briefly describes its meaning.</P><P>The member <CODE>tm_isdst</CODE> contains:</P><UL><LI>a positive value if<B><A NAME="Daylight Saving Time">Daylight Saving Time</A></B> is in effect<LI>zero if Daylight Saving Time is not in effect<LI>a negative value if the status of Daylight Saving Time is notknown (so the target environment should attempt to determine its status)</UL><HR><P>See also the<B><A HREF="index.html#Table of Contents">Table of Contents</A></B> and the<B><A HREF="_index.html">Index</A></B>.</P><P><I><A HREF="crit_pb.html">Copyright</A> © 1989-2002by P.J. Plauger and Jim Brodie. All rights reserved.</I></P><!--V4.01:1125--></BODY></HTML>⌨️ 快捷键说明
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