C date and time functions

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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.

Contents

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++).

IdentifierDescription
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_UTCtime 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
struct 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>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(&current_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

  1. ↑ ISO/IEC 9899:1999 specification (PDF). p. 351, § 7.32.2.
  2. ↑ "PWB1 date system command - man page". www.tuhs.org.
  3. ↑ "date.c sourcecode of PWB1". www.tuhs.org.
  4. ↑ "Rationale for American National Standard for Information Systems - Programming Language - C - Date and Time". www.lysator.liu.se.
  5. ↑ open-std.org - Committee Draft -- May 6, 2005 page 355
  6. ↑ Markus Kuhn. "Modernized API for ISO C". cl.cam.ac.uk.
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