#include <sys/time.h>
int gettimeofday(struct timeval *tv, struct timezone *tz);
          Returns 0 on success, or –1 on error
参数tv是指向如下数据结构的一个指针：
struct timeval {
    __kernel_time_t        tv_sec;        /* seconds */
    __kernel_suseconds_t    tv_usec;    /* microseconds */
};

#include <time.h>
time_t time(time_t *timep);
        Returns number of seconds since the Epoch,or (time_t) –1 on error

#include <time.h>
char *ctime(const time_t *timep)

#include<time.h>
struct tm *localtime(const time_t * timep);
struct tm *gmtime(const time_t *timep);

/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2015.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU General Public License as published by the   *
* Free Software Foundation, either version 3 or (at your option) any      *
* later version. This program is distributed without any warranty.  See   *
* the file COPYING.gpl-v3 for details.                                    *
\*************************************************************************/

/* Listing 10-1 */

/* calendar_time.c

   Demonstrate the use of functions for working with calendar time.

   This program retrieves the current time and displays it in various forms.
*/
#include <locale.h>
#include <time.h>
#include <sys/time.h>
#include "tlpi_hdr.h"

#define SECONDS_IN_TROPICAL_YEAR (365.24219 * 24 * 60 * 60)

int
main(int argc, char *argv[])
{
    time_t t;
    struct tm *gmp, *locp;
    struct tm gm, loc;
    struct timeval tv;

    /* Retrieve time, convert and display it in various forms */

    t = time(NULL);
    printf("Seconds since the Epoch (1 Jan 1970): %ld", (long) t);
    printf(" (about %6.3f years)\n", t / SECONDS_IN_TROPICAL_YEAR);

    if (gettimeofday(&tv, NULL) == -1)
        errExit("gettimeofday");
    printf("  gettimeofday() returned %ld secs, %ld microsecs\n",
            (long) tv.tv_sec, (long) tv.tv_usec);

    gmp = gmtime(&t);
    if (gmp == NULL)
        errExit("gmtime");

    gm = *gmp;          /* Save local copy, since *gmp may be modified
                           by asctime() or gmtime() */
    printf("Broken down by gmtime():\n");
    printf("  year=%d mon=%d mday=%d hour=%d min=%d sec=%d ", gm.tm_year,
            gm.tm_mon, gm.tm_mday, gm.tm_hour, gm.tm_min, gm.tm_sec);
    printf("wday=%d yday=%d isdst=%d\n", gm.tm_wday, gm.tm_yday, gm.tm_isdst);

    /* The TZ environment variable will affect localtime().
       Try, for example:

                TZ=Pacific/Auckland calendar_time
    */

    locp = localtime(&t);
    if (locp == NULL)
        errExit("localtime");

    loc = *locp;        /* Save local copy */

    printf("Broken down by localtime():\n");
    printf("  year=%d mon=%d mday=%d hour=%d min=%d sec=%d ",
            loc.tm_year, loc.tm_mon, loc.tm_mday,
            loc.tm_hour, loc.tm_min, loc.tm_sec);
    printf("wday=%d yday=%d isdst=%d\n\n",
            loc.tm_wday, loc.tm_yday, loc.tm_isdst);

    printf("asctime() formats the gmtime() value as: %s", asctime(&gm));
    printf("ctime() formats the time() value as:     %s", ctime(&t));

    printf("mktime() of gmtime() value:    %ld secs\n", (long) mktime(&gm));
    printf("mktime() of localtime() value: %ld secs\n", (long) mktime(&loc));

    exit(EXIT_SUCCESS);
}

/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2015.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU Lesser General Public License as published   *
* by the Free Software Foundation, either version 3 or (at your option)   *
* any later version. This program is distributed without any warranty.    *
* See the files COPYING.lgpl-v3 and COPYING.gpl-v3 for details.           *
\*************************************************************************/

/* Listing 10-2 */

/* curr_time.c

   Implement our currTime() function.
*/
#include <time.h>
#include "curr_time.h"          /* Declares function defined here */

#define BUF_SIZE 1000

/* Return a string containing the current time formatted according to
   the specification in 'format' (see strftime(3) for specifiers).
   If 'format' is NULL, we use "%c" as a specifier (which gives the'
   date and time as for ctime(3), but without the trailing newline).
   Returns NULL on error. */

char *
currTime(const char *format)
{
    static char buf[BUF_SIZE];  /* Nonreentrant */
    time_t t;
    size_t s;
    struct tm *tm;

    t = time(NULL);
    tm = localtime(&t);
    if (tm == NULL)
        return NULL;

    s = strftime(buf, BUF_SIZE, (format != NULL) ? format : "%c", tm);

    return (s == 0) ? NULL : buf;
}

/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2015.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU General Public License as published by the   *
* Free Software Foundation, either version 3 or (at your option) any      *
* later version. This program is distributed without any warranty.  See   *
* the file COPYING.gpl-v3 for details.                                    *
\*************************************************************************/

/* Listing 10-3 */

/* strtime.c

   Demonstrate the use of strptime() and strftime().

   Calls strptime() using the given "format" to process the "input-date+time".
   The conversion is then reversed by calling strftime() with the given
   "out-format" (or a default format if this argument is omitted).
*/
#if ! defined(__sun)
#ifndef _XOPEN_SOURCE
#define _XOPEN_SOURCE
#endif
#endif
#include <time.h>
#include <locale.h>
#include "tlpi_hdr.h"

#define SBUF_SIZE 1000

int
main(int argc, char *argv[])
{
    struct tm tm;
    char sbuf[SBUF_SIZE];
    char *ofmt;

    if (argc < 3 || strcmp(argv[1], "--help") == 0)
        usageErr("%s input-date-time in-format [out-format]\n", argv[0]);

    if (setlocale(LC_ALL, "") == NULL)
        errExit("setlocale");   /* Use locale settings in conversions */

    memset(&tm, 0, sizeof(struct tm));          /* Initialize 'tm' */
    if (strptime(argv[1], argv[2], &tm) == NULL)
        fatal("strptime");

    tm.tm_isdst = -1;           /* Not set by strptime(); tells mktime()
                                   to determine if DST is in effect */
    printf("calendar time (seconds since Epoch): %ld\n", (long) mktime(&tm));

    ofmt = (argc > 3) ? argv[3] : "%H:%M:%S %A, %d %B %Y %Z";
    if (strftime(sbuf, SBUF_SIZE, ofmt, &tm) == 0)
        fatal("strftime returned 0");
    printf("strftime() yields: %s\n", sbuf);

    exit(EXIT_SUCCESS);
}