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| C standard library (libc) |
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| 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.
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]
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.
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:
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