C date and time functions - JohnHau/mis GitHub Wiki

Overview of functions The C date and time operations are defined in the time.h header file (ctime header in C++).

Identifier Description Time manipulation difftime computes the difference in seconds between two time_t values time returns the current time of the system as a time_t value, number of seconds, (which is usually time since an epoch, typically the Unix epoch). The value of the epoch is operating system dependent; 1900 and 1970 are often used. See RFC 868. clock returns a processor tick count associated with the process timespec_get (C11) returns a calendar time based on a time base Format conversions asctime converts a struct tm object to a textual representation (deprecated) ctime converts a time_t value to a textual representation strftime converts a struct tm object to custom textual representation wcsftime converts a struct tm object to custom wide string textual representation gmtime converts a time_t value to calendar time expressed as Coordinated Universal Time[2] localtime converts a time_t value to calendar time expressed as local time mktime converts calendar time to a time_t value. Constants CLOCKS_PER_SEC number of processor clock ticks per second TIME_UTC time base for UTC Types struct tm broken-down calendar time type: year, month, day, hour, minute, second time_t arithmetic time type (typically time since the epoch) clock_t process running time type timespec time with seconds and nanoseconds The timespec and related types were originally proposed by Markus Kuhn to provide a variety of time bases, but only TIME_UTC was accepted.[3] The functionalities were, however, added to C++ in 2020 in std::chrono.

Example The following C source code prints the current time to the standard output stream.

#include <time.h> #include <stdlib.h> #include <stdio.h>

int main(void) { time_t current_time; char* c_time_string;

/* Obtain current time. */
current_time = time(NULL);

if (current_time == ((time_t)-1))
{
    (void) fprintf(stderr, "Failure to obtain the current time.\n");
    exit(EXIT_FAILURE);
}

/* Convert to local time format. */
c_time_string = ctime(&current_time);

if (c_time_string == NULL)
{
    (void) fprintf(stderr, "Failure to convert the current time.\n");
    exit(EXIT_FAILURE);
}

/* Print to stdout. ctime() has already added a terminating newline character. */
(void) printf("Current time is %s", c_time_string);
exit(EXIT_SUCCESS);

} The output is:

Current time is Thu Sep 15 21:18:23 2016