Question

在仔细查看time.h之后，我编写了以下函数：

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，只看到了奇怪的输出如此：

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

我甚至通过调试器单步执行并看到了同样的结果奇怪的结果：

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，我所得到的只是星期日和正确月份的一周中的一天用正确的日期和年份。其他似乎并不正确。

我错过了一些明显的东西吗？

Answer 1

您应该在填充结构后调用mktime(e_time)，但在调用strftime()之前。您有一些当前未完成的其他结构成员。传递具有未初始化值的结构（或者由于您的calloc()调用而初始化为不正确的值）通常是一件坏事，并且mktime()修改您传递的结构以填充它们

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;
}

的产率：

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$

Answer 2

您为字段tm_isdst提供了-1的值，这意味着夏令时不可用，如果夏令时生效，则将其更改为1，或{ {1}}如果不是。

正如@Paul Griffiths所指出的那样，调用0是更好的选择，mktime(e_time)函数将填充细分mktime，因为它会修复像{{1}这样的文件}和e_time也是如此。对于tm_wday字段，它遵循以下规则：

tm_day的正值或零值会导致tm_isdst函数最初假定夏令时分别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。

strftime（）的输出结果令人困惑

2 个答案: