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.
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(¤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.
- ^ open-std.org - Committee Draft -- May 6, 2005 page 355
- ^ Markus Kuhn. "Modernized API for ISO C". cl.cam.ac.uk.