I have 2 variables of type time_t - varEnd and varStart. Now in order to see the difference between them Either I can do
我有两个类型为time_t - varEnd和varStart的变量。为了看到它们之间的区别，我可以这样做
varEnd - varStart;
or
或
difftime(varEnd, varStart);
and both returns number of seconds.
两者都返回秒数。
Please let me know, if they have any difference? or which is the recommended one?
如果有什么不同，请告诉我。或者哪一个是推荐的?
2 个解决方案
#1
28
The language specifies that time_t is an arithmetic type capable of representing times. It doesn't require it to represent times in any particular way.
该语言指定time_t是一种能够表示时间的算术类型。它不要求它以任何特定的方式表示时间。
If time_t represents time as the number of seconds since some moment, the - operator will correctly compute the difference in seconds between two time_t values.
如果time_t表示时间为某个时刻之后的秒数，那么-操作符将正确地计算两个time_t值之间的秒数。
If it doesn't (say, if the granularity is one millisecond, or if the bits of a time_t are divided into groups representing years, months, days, etc.), then the - operator can yield meaningless results.
如果它没有(例如，如果粒度是1毫秒，或者如果时间t的位被划分为代表年份、月、日等的组)，那么-操作符可以产生无意义的结果。
The difftime() function, on the other hand, "knows" how a time_t represents a time, and uses that information to compute the difference in seconds.
另一方面，difftime()函数“知道”time_t是如何表示时间的，并使用该信息来计算秒的差值。
On most implementations, simple subtraction and difftime() happen to do the same thing -- but only difftime() is guaranteed to work correctly on all implementations.
在大多数实现中，简单的减法和difftime()恰好做了相同的事情——但是只有difftime()才能保证在所有实现上正确工作。
Another difference: difftime() returns a result of the floating-point type double, while "-" on time_t values yields a result of type time_t. In most cases the result will be implicitly converted to the type of whatever you assign it to, but if time_t happens to be an unsigned integer type, subtraction of a later time from an earlier time will yield a very large value rather than a negative value. Every system I've seen implements time_t as a 32-bit or 64-bit signed integer type, but using an unsigned type is permitted -- one more reason that simple subtraction of time_t values isn't necessary meaningful.
另一个区别是:difftime()返回浮点类型double的结果，而“-”在time_t值上的结果是类型time_t的结果。在大多数情况下，结果将隐式地转换为您所赋值的类型，但是如果time_t恰好是无符号整数类型，则从较早时间减去较晚时间将产生一个非常大的值，而不是一个负值。我看到的每个系统都将time_t实现为32位或64位有符号整数类型，但是允许使用无符号类型——这也是为什么简单地减去time_t值没有意义的原因之一。
#2
1
difftime() returns a floating point double, just subtracting them doesn't unless you cast them to double first.
source: here
difftime()返回一个浮点值为double的浮点数，除非你先将它们转换为double，否则仅仅减去它们是不可能的。来源:这里
#1
28
The language specifies that time_t is an arithmetic type capable of representing times. It doesn't require it to represent times in any particular way.
该语言指定time_t是一种能够表示时间的算术类型。它不要求它以任何特定的方式表示时间。
If time_t represents time as the number of seconds since some moment, the - operator will correctly compute the difference in seconds between two time_t values.
如果time_t表示时间为某个时刻之后的秒数，那么-操作符将正确地计算两个time_t值之间的秒数。
If it doesn't (say, if the granularity is one millisecond, or if the bits of a time_t are divided into groups representing years, months, days, etc.), then the - operator can yield meaningless results.
如果它没有(例如，如果粒度是1毫秒，或者如果时间t的位被划分为代表年份、月、日等的组)，那么-操作符可以产生无意义的结果。
The difftime() function, on the other hand, "knows" how a time_t represents a time, and uses that information to compute the difference in seconds.
另一方面，difftime()函数“知道”time_t是如何表示时间的，并使用该信息来计算秒的差值。
On most implementations, simple subtraction and difftime() happen to do the same thing -- but only difftime() is guaranteed to work correctly on all implementations.
在大多数实现中，简单的减法和difftime()恰好做了相同的事情——但是只有difftime()才能保证在所有实现上正确工作。
Another difference: difftime() returns a result of the floating-point type double, while "-" on time_t values yields a result of type time_t. In most cases the result will be implicitly converted to the type of whatever you assign it to, but if time_t happens to be an unsigned integer type, subtraction of a later time from an earlier time will yield a very large value rather than a negative value. Every system I've seen implements time_t as a 32-bit or 64-bit signed integer type, but using an unsigned type is permitted -- one more reason that simple subtraction of time_t values isn't necessary meaningful.
另一个区别是:difftime()返回浮点类型double的结果，而“-”在time_t值上的结果是类型time_t的结果。在大多数情况下，结果将隐式地转换为您所赋值的类型，但是如果time_t恰好是无符号整数类型，则从较早时间减去较晚时间将产生一个非常大的值，而不是一个负值。我看到的每个系统都将time_t实现为32位或64位有符号整数类型，但是允许使用无符号类型——这也是为什么简单地减去time_t值没有意义的原因之一。
#2
1
difftime() returns a floating point double, just subtracting them doesn't unless you cast them to double first.
source: here
difftime()返回一个浮点值为double的浮点数，除非你先将它们转换为double，否则仅仅减去它们是不可能的。来源:这里