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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.

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++).

IdentifierDescription
Time
manipulation		difftimecomputes the difference in seconds between two time_t values
timereturns 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.
clockreturns a processor tick count associated with the process
timespec_get (C11)returns a calendar time based on a time base
Format
conversions		asctimeconverts a struct tm object to a textual representation (deprecated)
ctimeconverts a time_t value to a textual representation
strftimeconverts a struct tm object to custom textual representation
strptimeconverts a string with time information to a struct tm
wcsftimeconverts a struct tm object to custom wide string textual representation
gmtimeconverts a time_t value to calendar time expressed as Coordinated Universal Time[5]
localtimeconverts a time_t value to calendar time expressed as local time
mktimeconverts calendar time to a time_t value.
ConstantsCLOCKS_PER_SECnumber of processor clock ticks per second
TIME_UTCtime base for UTC
Typesstruct tmbroken-down calendar time type: year, month, day, hour, minute, second
time_tarithmetic time type (typically time since the Unix epoch)
clock_tprocess running time type
struct timespectime 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++ with the release of C++20 in std::chrono.

Example#edit

The following C source code prints the current time to the standard output stream.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void) {
    // Obtain current time.
    time_t current_time = time(NULL);

    if (current_time == (time_t)-1) {
        fprintf(stderr, "Failure to obtain the current time.\n");
        return EXIT_FAILURE;
    }

    // Convert to local time format. 
    char* time_string = ctime(&current_time);

    if (!time_string) {
        fprintf(stderr, "Failure to convert the current time.\n");
        return EXIT_FAILURE;
    }

    // Print to stdout. ctime() has already added a terminating newline character.
    printf("Current time is %s", time_string);
    return EXIT_SUCCESS;
}

The output is:

Current time is Thu Sep 15 21:18:23 2016

See also#edit

References#edit

  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.
Categories C standard libraryTime

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