为什么 strptime c 函数会改变结构？

Question

函数的不可理解行为 strptime():

#define _XOPEN_SOURCE
#include <stdio.h>
#include <time.h>

double getPeriod(char * dateStart, char * dateStop) {
    struct tm tmStart, tmStop;
    time_t timeStampStart, timeStampStop;

    strptime(dateStart, "%Y-%m-%d %H:%M:%S", &tmStart);
    strptime(dateStop, "%Y-%m-%d %H:%M:%S", &tmStop);

    timeStampStart = mktime(&tmStart);
    timeStampStop = mktime(&tmStop);

    printf("%d\t%d\n", tmStart.tm_hour, tmStop.tm_hour);
}

int main()
{
    getPeriod("2016-12-05 18:14:35", "2016-12-05 18:18:34");
    return 0;
}

输出：

17  18

为什么会这样？

编译器 gcc (GCC) 6.2.1 OSLinux

Answer 1

tmStart和tmStop没有初始化，所以传给mktime的时候有些字段会没有初始化。因此，该行为在技术上是未定义的。

来自 strptime 手册页（注意前两句）：

In principle, this function does not initialize tm but only stores the values specified. This means that tm should be initialized before the call. Details differ a bit between different UNIX systems. The glibc implementation does not touch those fields which are not explicitly specified, except that it recomputes the tm_wday and tm_yday field if any of the year, month, or day elements changed.

为什么 strptime c 函数会改变结构？

Why strptime c-function changes the structure?

c

linux

gcc

strptime

time.h