C date and time functions
The C date and time functions are a group of functions in the standard library of the C programming language implementing date and time manipulation operations.[1] They provide support for time acquisition, conversion between date formats, and formatted output to strings.
C standard library |
---|
General topics |
Miscellaneous headers |
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 | |
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 |
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 |
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(¤t_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
gollark: (in any case, it's probably less than the resource waste from Electron etc. by rather a lot)
gollark: I do vaguely feel this way about encryption and whatever - if people were trustworthy and nice™, we could save some amount of system resources and key distribution hassle and whatever. As it turns out, though, they aren't, so it isn't very relevant, and even if everyone suddenly did stop being antagonistic, this is a ridiculously unstable state.
gollark: What of the GTech™ contrasocietous chambers™?
gollark: You don't get secure systems by saying "let's just trust Jeff here".
gollark: Well, the energy thing is separate, but this is good security design, yes.
See also
References
- ISO/IEC 9899:1999 specification (PDF). p. 351, § 7.32.2.
- open-std.org - Committee Draft -- May 6, 2005 page 355
External links
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