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 - -

argument-expression-list: (§3.3.2)

assignment-expression argument-expression-list , assignment-expression

unary-expression: (§3.3.3)

postfix-expression ++ unary-expression - - unary-expression unary-operator cast-expression sizeof unary-expression sizeof ( type-name )

unary-operator: (§3.3.3) one of

& * + - ~ !

cast-expression: (§3.3.4)

unary-expression ( type-name ) cast-expression

multiplicative-expression: (§3.3.5)

cast-expression multiplicative-expression * cast-expression multiplicative-expression / cast-expression multiplicative-expression % cast-expression

additive-expression: (§3.3.6)

multiplicative-expression additive-expression + multiplicative-expression additive-expression - multiplicative-expression

shift-expression: (§3.3.7)

additive-expression shift-expression << additive-expression shift-expression >> additive-expression

relational-expression: (§3.3.8)

shift-expression relational-expression < shift-expression relational-expression > shift-expression relational-expression <= shift-expression relational-expression >= shift-expression

equality-expression: (§3.3.9)

relational-expression equality-expression :=,= relational-expression equality-expression != relational-expression

AND-expression: (§3.3.10)

equality-expression AND-expression & equality-expression

exclusive-OR-expression: (§3.3.11)

AND-expression exclusive-OR-expression ^ AND-expression

inclusive-OR-expression: (§3.3.12)

exclusive-OR-expression inclusive-OR-expression | exclusive-OR-expression

logical-AND-expression: (§3.3.13)

inclusive-OR-expression logical-AND-expression && inclusive-OR-expression

logical-OR-expression: (§3.3.14)

logical-AND-expression logical-OR-expression || logical-AND-expression

conditional-expression: (§3.3.15)

logical-OR-expression logical-OR-expression ? expression : conditional-expression

assignment-expression: (§3.3.16)

conditional-expression unary-expression assignment-operator assignment-expression

assignment-operator: (§3.3.16) one of

= *= /= %= += -= <<= >>= &= ^= |=

expression: (§3.3.17)

assignment-expression expression , assignment-expression

constant-expression: (§3.4)

conditional-expression

A.1.2.2 Declarations

declaration: (§3.5)

declaration-specifiers init-declarator-listopt ;

declaration-specifiers: (§3.5)

storage-class-specifier declaration-specifiersopt type-specifier declaration-specifiersopt type-qualifier declaration-specifiersopt

init-declarator-list: (§3.5)

init-declarator init-declarator-list , init-declarator

init-declarator: (§3.5)

declarator declarator = initializer

storage-class-specifier: (§3.5.1)

typedef extern static auto register

type-specifier: (§3.5.2)

void char short int long float double signed unsigned

struct-or-union-specifier enum-specifier typedef-name

struct-or-union-specifier: (§3.5.2.1)

struct-or-union identifieropt { struct-declaration-list } struct-or-union identifier

struct-or-union: (§3.5.2.1)

struct union

struct-declaration-list: (§3.5.2.1)

struct-declaration struct-declaration-list struct-declaration

struct-declaration: (§3.5.2.1)

specifier-qualifier-list struct-declarator-list ;

specifier-qualifier-list: (§3.5.2.1)

type-specifier specifier-qualifier-listopt type-qualifier specifier-qualifier-listopt