gtod：调用标准的gettimeofday。
Precision: gettimeofday precision is fixed to the microsecond.
精度：gettimeofday 精度是固定在微秒级别上的。
Using gettimeofday, the implementation of a nanosecond calculation is as follows
使用gettimeofday，纳秒计算的实现如下
For that purpose, POSIX libraries provide timing functions, such as gettimeofday, which give an easy-to-use interface to application programmers.
为了实现这个目的，posix库提供了一些与时间有关的函数，例如gettimeofday，它为应用程序开发人员提供了一个简单易用的接口。
For that purpose, POSIX libraries provide timing functions, such as gettimeofday, which give an easy-to-use interface to application programmers.
为了实现这个目的，posix库提供了一些与时间有关的函数，例如gettimeofday，它为应用程序开发人员提供了一个简单易用的接口。
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