The null pointer macro (NULL) is generally used to signify that a pointer does not point to a object.

The macro constant expression CLOCKS_PER_SEC specifies the relation between a clock tick and a second (clock ticks per second).

If you divide a count of clock ticks by this expression you get the number of seconds.

The CLK_TCK is an obsolete alias of this macro.

For a usage example take a look at the clock page.

The time structure tm contains a calendar date and time which are broken down into its components.
Structure containing a calendar date and time broken down into its components.

The structure contains nine members of type integer: More »

The time type (time_t) is capable of representing times and it also support arithmetical operations.

The time function will return this type. It is also used by some other functions of the time.h header file.

It represents the number of seconds elapsed since 00:00 hours, Jan 1, 1970 UTC. Also called EPOCH time (Unix time stamp). It is widely implemented in the C libraries across ALL platforms.

The unsigned integral type (size_t) corresponds to the integral data type returned by the language operator sizeof. It is used in (among others) in the time.h header file as an unsigned integral type.

In the header file time.h it is used by the function strftime as the type of its parameter maxsize. It is also used as the return type of this function. It is used to express counts of characters.

The clock type represent clock ticks counts and can be used for arithmetical operations.

The clock type clock_t is returned by the clock function of time.h header file.
It represents the number of clock ticks since the program was started.

Usage of strftime():

size_t strftime ( char * ptr_dest, size_t maxsize, const char * format, const struct tm * ptr_time );

Parameters:

ptr_dest is a pointer to the destination array where the resulting C string is copied too.

The maxsize contains the maximum number of characters to be copied to ptr_dest. More »

Usage of localtime():

struct tm * localtime ( const time_t * ptr_time );

Parameters:

The pointer ptr_time is a pointer to a time_t object that contains a calendar time.

Return Value:

The function localtime() returns a pointer to a tm structure. The tm structure contains the time information.

Explanation:

More »

Usage of gmtime():

struct tm * gmtime ( const time_t * ptr_time);

Parameters:

The pointer ptr_time is a pointer to a time_t object that contains a calendar time.
Calendar time is the number of seconds that have elapsed since EPOCH, which is 00:00:00, January 1, 1970 Universal Coordinate Time (UTC)

Return Value:

More »

Usage of ctime():

char * ctime ( const time_t * ptr_time );

Parameters:

Pointer ptr_time to a time_t object that contains a calendar time.

Return Value:

The function returns a C string containing the date and time information.
More »