C date and time functions
C standard library (libc) |
---|
General topics |
Miscellaneous headers |
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.
History
The format string used in strftime
traces back to at least PWB/UNIX 1.0, released in 1977. Its date
system command includes various formatting options.[2][3] In 1989, the ANSI C standard is released including strftime
and other date and time functions.[4]
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
| |
strptime
|
converts a string with time information to a struct tm
| |
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[5]
| |
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 Unix 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.[6] 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(¤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
See also
References
- ^ ISO/IEC 9899:1999 specification (PDF). p. 351, § 7.32.2.
- ^ "PWB1 date system command - man page". www.tuhs.org.
- ^ "date.c sourcecode of PWB1". www.tuhs.org.
- ^ "Rationale for American National Standard for Information Systems - Programming Language - C - Date and Time". www.lysator.liu.se.
- ^ open-std.org - Committee Draft -- May 6, 2005 page 355
- ^ Markus Kuhn. "Modernized API for ISO C". cl.cam.ac.uk.