在 C 中打印时间(0)的格式说明符 [英] Format specifier to print time(0) in C

问题描述

我们可以声明一个变量来保存来自系统的当前时间:

We can declare a variable to hold the current time from the system using:

time_t now = time(0);

time(0) 也可用于生成随机值:

time(0) can also be use in generating random values:

#define SEED time(0);
srand((unsigned int )SEED);

我的问题是:time(0) 究竟是什么.默认情况下，long 类型是毫秒吗?如果要打印time(0)的值，可以用"%d"打印出来吗?(Sicne printf() 要求我们传入它需要的类型，但我怎么知道 time(0) 的类型是什么?).

My question is: What exactly is time(0). By default is it in milliseconds with type long? If I want to print out the value of time(0), can I use "%d" to print it out? (Sicne printf() requires us to pass in the type it requires, but how can I know what is the type of time(0) ?).

如果是 time_t 类型，我应该在 printf 中使用什么格式说明符?

If it is of type time_t, what format specifier should I use in printf?

推荐答案

time_t 由 C 标准，C11，7.27.1/3 定义为:

time_t is defined by C standard, C11, 7.27.1/3 as:

[...] 是能够表示时间的实数类型；

[...] which are real types capable of representing times;

这意味着它可以是 int、long、unsigned long、double 或任何其他实数类型.基本上这是一个实现的选择.同样，它也没有定义 time_t 返回什么单位，C11, 7.27.2.4/3:

That means it could be int, long, unsigned long, double or any other real type. Basically it's an implementation's choice. Similarly, it doesn't define what units time_t returns either, C11, 7.27.2.4/3:

时间函数返回实现的最佳近似值到当前日历时间.返回值 (time_t)(-1)如果日历时间不可用.如果计时器不为空指针，返回值也赋值给它指向的对象.

The time function returns the implementation’s best approximation to the current calendar time. The value (time_t)(-1) is returned if the calendar time is not available. If timer is not a null pointer, the return value is also assigned to the object it points to.

您必须阅读您的实现内容.我在 Linux 上的 glibc 实现说，time_t 返回的单位是秒.所以你可以将它转换为 uintmax_t 并打印:

You'll have to read what your implementation says. The glibc implementation I have on my Linux says, the unit returned by time_t is in seconds. So you could convert it to uintmax_t and print:

time_t tvalue = time(0);
printf("%ju", (uintmax_t)tvalue);

或者你可以使用 difftime() 返回一个double 作为两个 time_t 值之间的差异，这样您就不必担心 time_t 的底层类型.

Or you could use difftime() which returns a double as difference between two time_t values so that you don't have to worry about the underlying type of time_t.

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