为什么 difftime() return 是双倍的？
Why does difftime() return a double?
double difftime(time_t time1, time_t time0);
The difftime() function returns the number of seconds elapsed between
time time1 and time time0, represented as a double.
既然'number of seconds'不需要浮点数，为什么这个函数return是double？
这点documentation说的比较清楚：
On POSIX systems, time_t is measured in seconds, and difftime is equivalent to arithmetic subtraction, but C and C++ allow fractional units for time_t.
虽然 POSIX requires time_t to be an integer type，但对于非 POSIX 系统，这可能会 return 小数秒。
C 允许使用各种标量数（整数、浮点数）来表示时间。它需要是“......能够代表时间的真实类型”C11§7.27.1 3，
The range and precision of times representable in clock_t and time_t are
implementation-defined. C11dr §7.27.1 4
2 个 time_t 值之间的差异，因为 double 提供了广泛的范围和精度。
OP, "Since 'number of seconds' doesn't require floating-point numbers, why does this function return a double?
[编辑] Linux/posix 可能不会使用几分之一秒，但其他系统已经这样做了。定义 difftime() 的 C 标准选择 double 并容纳秒的整数累加以及其他 OS 实现。
double difftime(time_t time1, time_t time0);The
difftime()function returns the number of seconds elapsed between timetime1and timetime0, represented as adouble.
既然'number of seconds'不需要浮点数，为什么这个函数return是double？
这点documentation说的比较清楚：
On POSIX systems, time_t is measured in seconds, and difftime is equivalent to arithmetic subtraction, but C and C++ allow fractional units for time_t.
虽然 POSIX requires time_t to be an integer type，但对于非 POSIX 系统，这可能会 return 小数秒。
C 允许使用各种标量数（整数、浮点数）来表示时间。它需要是“......能够代表时间的真实类型”C11§7.27.1 3，
The range and precision of times representable in
clock_t and time_tare implementation-defined. C11dr §7.27.1 4
2 个 time_t 值之间的差异，因为 double 提供了广泛的范围和精度。
OP, "Since 'number of seconds' doesn't require floating-point numbers, why does this function return a double?
[编辑] Linux/posix 可能不会使用几分之一秒，但其他系统已经这样做了。定义 difftime() 的 C 标准选择 double 并容纳秒的整数累加以及其他 OS 实现。