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
[edit ]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
[edit ]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
[edit ]The following C source code prints the current time to the standard output stream.
#include<time.h> #include<stdlib.h> #include<stdio.h> intmain(void) { time_tcurrent_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
[edit ]References
[edit ]- ^ 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.
