对 strftime() 的输出结果感到困惑

Question

仔细查看time.h几次后，我写了以下func：

void output_date ( int day, int month, int year ) {
    char buffer[64] = "";
    struct tm *e_time = calloc( (size_t) 1, sizeof(struct tm) );

    e_time->tm_year = year - 1900;
    e_time->tm_mon = month - 1;
    e_time->tm_mday = day;
    e_time->tm_hour = 0;
    e_time->tm_min = 0;
    e_time->tm_sec = 0;
    e_time->tm_isdst = -1;

    /* strftime ( buffer, 64, (char *)0, e_time ); */
    strftime ( buffer, 64, "%a %b %e %H:%M:%S %Z (%z) %Y", e_time );

    printf ( "%s\n", buffer );

    free(e_time);
    e_time = NULL;

}

然后我用一系列可能的输入调用了这个函数，但什么也没看到 像这样的奇怪输出：

Sun Jul  8 00:00:00  () 2013
Sun Jul  9 00:00:00  () 2013
Sun Jul 10 00:00:00  () 2013
Sun Jul 11 00:00:00  () 2013
Sun Jul 12 00:00:00  () 2013

当我将格式字符串测试为 strftime 时，我看到了很好的结果：

$ date -u "+%a %b %e %H:%M:%S %Z (%z) %Y"
Sat Oct 12 00:40:05 GMT (+0000) 2013

我什至通过调试器单步执行并看到了相同的结果 奇怪的结果：

stopped in main at line 27 in file "flight.c"
   27                       output_date ( day+1, month+1, year );
(dbx) print day+1, month+1, year
day+1 = 1
month+1 = 1
year = 1977

(dbx) step                      
stopped in output_date at line 43 in file "flight.c"
   43       char buffer[64] = "";
(dbx) step
stopped in output_date at line 44 in file "flight.c"
   44       struct tm *e_time = calloc( (size_t) 1, sizeof(struct tm) );
(dbx) step
stopped in output_date at line 46 in file "flight.c"
   46       e_time->tm_year = year - 1900;
(dbx) print e_time
e_time = 0x100101640
(dbx) step        
stopped in output_date at line 47 in file "flight.c"
   47       e_time->tm_mon = month - 1;
(dbx) step
stopped in output_date at line 48 in file "flight.c"
   48       e_time->tm_mday = day;
(dbx) step
stopped in output_date at line 49 in file "flight.c"
   49       e_time->tm_hour = 0;
(dbx) step
stopped in output_date at line 50 in file "flight.c"
   50       e_time->tm_min = 0;
(dbx) step
stopped in output_date at line 51 in file "flight.c"
   51       e_time->tm_sec = 0;
(dbx) step
stopped in output_date at line 52 in file "flight.c"
   52       e_time->tm_isdst = -1;
(dbx) step
stopped in output_date at line 55 in file "flight.c"
   55       strftime ( buffer, 64, "%a %b %e %H:%M:%S %Z (%z) %Y", e_time );
(dbx) print *e_time
*e_time = {
    tm_sec   = 0
    tm_min   = 0
    tm_hour  = 0
    tm_mday  = 1
    tm_mon   = 0
    tm_year  = 77
    tm_wday  = 0
    tm_yday  = 0
    tm_isdst = -1
}
(dbx) step        
stopped in output_date at line 57 in file "flight.c"
   57       printf ( "%s\n", buffer );
(dbx) print buffer
buffer = "Sun Jan  1 00:00:00  () 1977"
(dbx) quit    

事实上，我所得到的只是一周中的某一天是星期天和正确的月份 正确的日期和年份。似乎没有什么是正确的。

我是否遗漏了一些明显的东西？

Answer 1

您为字段tm_isdst 提供了-1 的值，这意味着夏令时不可用，如果夏令时有效，则将其更改为1，否则将其更改为0。

正如@Paul Griffiths 指出的那样，在这里调用mktime(e_time) 是更好的选择，mktime 函数将填充分解的e_time，因为它还将修复tm_wday 和tm_day 之类的文件.对于tm_isdst的字段，遵循如下规则：

tm_isdst 的正值或零值导致mktime 函数最初分别假定夏令时在指定时间内有效或无效。负值会导致它尝试确定夏令时是否在指定时间内有效。

Answer 2

您应该在填充结构之后调用mktime(e_time)，但在调用strftime() 之前。您还有一些其他结构成员当前尚未完成。传递具有未初始化值的结构（或初始化为不正确的值，在这里，由于您的calloc() 调用）通常是一件坏事，mktime() 会修改您传递的结构来为您填充它们。

像这样：

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

void output_date ( int day, int month, int year ) {
    char buffer[64] = "";
    struct tm *e_time = calloc( (size_t) 1, sizeof(struct tm) );

    e_time->tm_year = year - 1900;
    e_time->tm_mon = month - 1;
    e_time->tm_mday = day;
    e_time->tm_hour = 0;
    e_time->tm_min = 0;
    e_time->tm_sec = 0;
    e_time->tm_isdst = -1;

    if ( mktime(e_time) < 0 ) {
        fprintf(stderr, "Error getting calendar time.\n");
        exit(EXIT_FAILURE);
    }

    int n = strftime ( buffer, 64, "%a %b %e %H:%M:%S %Z (%z) %Y", e_time );
    printf("Return from strftime() is %d\n", n);

    printf ( "%s\n", buffer );

    free(e_time);
    e_time = NULL;

}

int main(void) {
    output_date(12, 6, 2013);
    return 0;
}

产量：

paul@local:~/src/c/scratch$ ./st
Return from strftime() is 36
Wed Jun 12 00:00:00 EDT (-0400) 2013
paul@local:~/src/c/scratch$