The <time.h> header file defines two macros, and declares four types and several functions for manipulating time and date information. Some functions process local time, which may differ from calendar time because of time zone.

Types

size_t

• An unsigned integral type of the result of the sizeof operator.

clock_t time_t

• Arithmetic types capable of representing times.

struct tm

• Holds the components of a calendar time, called the broken-down time. The structure contains the following members:

  int tm_sec; /* seconds after the minute -- [0,61] */ int tm_min; /* minutes after the hour -- [0,59] */ int tm_hour; /* hours since midnight -- [0,23] */ int tm_mday; /* day of the month -- [1,31] */ int tm_mon; /* months since January -- [0,11] */ int tm_year; /* years since 1900 */ int tm_wday; /* days since Sunday -- [0,6] */ int tm_yday; /* days since January 1 -- [0,365] */ int tm_isdst; /* Daylight Saving Time flag -- 0 if */ /* DST not in effect; positive if it is; */ /* negative if information is not available. */

Macros

NULL

• Expands to an implementation-defined null pointer constant.

CLOCKS_PER_SEC

• The number per second of the value returned by the clock function.

Time Conversion Functions

char *asctime(const struct tm *timeptr);

• Converts a broken-down time in the structure pointed to by timeptr into a 26-character string in the form of this example:

  Sat Sep 08 08:10:32 1990\n\0

  
  A pointer to the string is returned.

char *ctime(const time_t *timer);

• Converts the calendar time pointed to by timer to local time in a string of the form generated by the asctime function. A pointer to the string is returned. The ctime function is equivalent to the following:

  asctime(localtime(timer))

struct tm *gmtime(const time_t *timer);

• Converts the calendar time pointed to by timer into a broken-down time expressed as Coordinated Universal Time (UTC). The gmtime function returns a pointer to the broken-down time, or a null pointer if UTC is not available.

struct tm *localtime(const time_t *timer);

• Converts the calendar time pointed to by timer into a broken-down time expressed as local time. The localtime function returns a pointer to the broken-down time.

size_t strftime(char *s, size_t maxsize, const char *format, const struct tm *timeptr);

• Places characters into the array pointed to by s as controlled by the string pointed to by format. The format string consists of zero or more conversion specifiers and ordinary multibyte characters. All ordinary multibyte characters (including the terminating null character) are copied unchanged into the array. Each conversion specifier is replaced by the appropriate characters as shown in Table 9-2. The appropriate characters are determined by the LC_TIME category of the current locale and by the values contained in the structure pointed to by timeptr.

  Table 9-2 strftime Conversion Specifiers

  Specifier	Replaced by
  %a	The locale's abbreviated weekday name
  %A	The locale's full weekday name
  %b	The locale's abbreviated month name
  %B	The locale's full month name
  %c	The locale's appropriate date and time representation
  %d	The day of the month as a decimal number (01 -- 31)
  %H	The hour (24-hour clock) as a decimal number (00 -- 23)
  %I	The hour (12-hour clock) as a decimal number (01 -- 12)
  %j	The day of the year as a decimal number (001 -- 366)
  %m	The month as a decimal number (01 -- 12)
  %M	The minute as a decimal number (00 -- 59)
  %p	The locale's equivalent of the AM/PM designations associated with a 12-hour clock
  %S	The second as a decimal number (00 -- 61)
  %U	The week number of the year (the first Sunday as the first day of week 1) as a decimal number (00 -- 53)
  %w	The weekday as a decimal number (0 [Sunday] -- 6)
  %W	The week number of the year (the first Monday as the first day of week 1) as a decimal number (00 -- 53)
  %x	The locale's appropriate date representation
  %X	The locale's appropriate time representation
  %y	The year without century as a decimal number (00 -- 99)
  %Y	The year with century as a decimal number
  %Z	The time zone name or abbreviation, or by no characters if no time zone can be determined
  %%	%

  
  If the total number of resulting characters including the terminating null character is not more than maxsize, the strftime function returns the number of characters placed into the array pointed to by s, not including the terminating null character. Otherwise, 0 is returned, and the array contents are indeterminate.

Time Manipulation Functions

clock_t clock(void);

• Determines the processor time used. The clock function returns the processor time used by the program since the beginning of an event related to the program invocation. To determine the time in seconds, divide the return value by the value of the CLOCKS_PER_SEC macro. If the processor time is not available or cannot be represented, the value returned is (clock_t)-1. (To measure the time spent in a program, call the clock function at the start of the program and subtract the return value from that of subsequent calls.)

double difftime(time_t time1, time_t time0);

• Returns the difference between the two calendar times time1 and time0, expressed in seconds, as a double.

time_t mktime(struct tm *timeptr);

• Converts the broken-down time, expressed as local time, in the structure pointed to by timeptr into a calendar time value with the same encoding as that of the values returned by the time function (that is, a value of type time_t), which it returns. If the calendar time cannot be represented, the value (time_t)-1 is returned.
  The original values of the tm_wday and tm_yday time components are ignored, and the original values of the other components are not restricted to the ranges indicated in the previous discussion of struct_tm. Upon successful completion of the function, the values of the tm_wday and tm_yday components are set appropriately, and the other components are set to represent the specified calendar time, but with their values forced to the ranges indicated in the discussion of struct_tm. The final value of tm_wday is not set until tm_mon and tm_year are determined.

time_t time(time_t *timer);

• Returns the current calendar time. If the calendar time is not available, the value (time_t)-1 is returned.

This section summarizes the syntax of the C language, using the syntax of the ANSI C Standard. Syntactic categories are indicated with bold type, and literal words or characters are indicated with monospaced, nonitalicized type. A colon following a syntactic category introduces its definition. Alternative definitions are listed on separate lines, or are prefaced by the words "one of." An optional element is indicated by the subscript _opt. For example, the following line indicates an optional expression enclosed in braces:

{ expressionopt }

The section numbers shown in parentheses refer to the section of the American National Standard for Information Systems-Programming Language C (document number: X3.159-1989) that discusses that part of the language.

A.1.1 Lexical Grammar

A.1.1.1 Tokens

token: (§3.1)

keyword identifier constant string-literal operator punctuator

preprocessing-token: (§3.1)

header-name identifier pp-number character-constant string-literal operator punctuator each nonwhite-space character that cannot be one of the above

A.1.1.2 Keywords

keyword: (§3.1.1) one of

auto double int struct break else long switch case enum register typedef char extern return union const float short unsigned continue for signed void default goto sizeof volatile do if static while

A.1.1.3 Identifiers

identifier: (§3.1.2)

nondigit identifier nondigit identifier digit

nondigit: §3.1.2 one of

a b c d e f g h i j k l m n o p q r s t u v w x y z A B C D E F G H I J K L M N O P Q R S T U V W X Y Z _

digit: (§3.1.2) one of

0 1 2 3 4 5 6 7 8 9

A.1.1.4 Constants

constant: (§3.1.3)

floating-constant integer-constant enumeration-constant character-constant

floating-constant: (§3.1.3.1)

fractional-constant exponent-partopt floating-suffixopt digit-sequence exponent-part floating-suffixopt

fractional-constant: (§3.1.3.1)

digit-sequenceopt . digit-sequence digit-sequence .

exponent-part: (§3.1.3.1)

e signopt digit-sequence E signopt digit-sequence

sign: (§3.1.3.1) one of

+ --

digit-sequence: (§3.1.3.1)

digit digit-sequence digit

floating-suffix: (§3.1.3.1) one of

• f l F L

integer-constant: (§3.1.3.2)

decimal-constant integer-suffixopt octal-constant integer-suffixopt hexadecimal-constant integer-suffixopt

decimal-constant: (§3.1.3.2)

nonzero-digit decimal-constant digit

octal-constant: (§3.1.3.2)

0 octal-constant octal-digit

hexadecimal-constant: (§3.1.3.2)

0x hexadecimal-digit 0X hexadecimal-digit hexadecimal-constant hexadecimal-digit

nonzero-digit: (§3.1.3.2) one of

1 2 3 4 5 6 7 8 9

octal-digit: (§3.1.3.2) one of

0 1 2 3 4 5 6 7

hexadecimal-digit: (§3.1.3.2) one of

0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F

integer-suffix: (§3.1.3.2)

unsigned-suffix long-suffixopt long-suffix unsigned-suffixopt

unsigned-suffix: (§3.1.3.2) one of

• u U

long-suffix: (§3.1.3.2) one of

• l L

enumeration-constant: (§3.1.3.3)

identifier

character-constant: (§3.1.3.4)

' c-char-sequence' L' c-char-sequence'

c-char-sequence: (§3.1.3.4)

c-char c-char-sequence c-char

c-char: (§3.1.3.4)

any member of the source character set except the single-quote ('), backslash (\), or new-line character escape-sequence

escape-sequence: (§3.1.3.4)

simple-escape-sequence octal-escape-sequence hexadecimal-escape-sequence

simple-escape-sequence: (§3.1.3.4) one of

\' \" \? \\ \a \b \f \n \r \t \v

octal-escape-sequence: (§3.1.3.4)

\ octal-digit \ octal-digit octal-digit \ octal-digit octal-digit octal-digit

hexadecimal-escape-sequence:(§3.1.3.4)

\x hexadecimal-digit hexadecimal-escape-sequence hexadecimal-digit

A.1.1.5 String Literals

string-literal: (§3.1.4)

"s-char-sequenceopt" L"s-char-sequenceopt"

s-char-sequence: (§3.1.4)

s-char s-char-sequence s-char

s-char: (§3.1.4)

any member of the source character set except the double-quote ("), backslash (\), or new-line character escape-sequence

A.1.1.6 Operators

operator: (§3.1.5) one of

[ ] ( ) . -> ++ -- & * + - ~ ! sizeof / % << >> < > <= >= == != ^ | && || ? : = *= /= %= += -= <<= >>= &= ^= |= , # ##

A.1.1.7 Punctuators

punctuator: (§3.1.6) one of

[ ] ( ) { } * , : = ; ... #

A.1.1.8 Header Names

header-name: (§3.1.7)

<h-char-sequence> "q-char-sequence"

h-char-sequence: (§3.1.7)

h-char h-char-sequence h-char

h-char: (§3.1.7)

any member of the source character set except the new-line character and >

q-char-sequence: (§3.1.7)

q-char q-char-sequence q-char

q-char: (§3.1.7)

any member of the source character set except the new-line character and "

A.1.1.9 Preprocessing Numbers

pp-number: (§3.1.8)

digit . digit pp-number digit pp-number nondigit pp-number e sign pp-number E sign pp-number .

A.1.2 Phrase Structure Grammar

A.1.2.1 Expressions

primary-expression: (§3.3.1)

identifier constant string-literal ( expression )

postfix-expression: (§3.3.2)

primary-expression postfix-expression [ expression ] postfix-expression ( argument-expression-listopt ) postfix-expression . identifier postfix-expression -> identifier postfix-expression ++ postfix-expression - -