c ++ chrono duration_cast到毫秒会以秒为单位 [英] c++ chrono duration_cast to milliseconds results in seconds

问题描述

我想有自纪元以来的毫秒数。一个流行的解决方案如下（此问题的解决方案之一获取时间自纪元以来的毫秒数，最好使用C ++ 11 chrono ）

  #include< iostream> 
 #include< chrono> 
 
 int main（）{
 auto millitime = std :: chrono :: duration_cast< std :: chrono :: milliseconds> 
（std :: chrono :: system_clock :: now（）。time_since_epoch（））count（）; 
 std :: cout<<海上< std :: endl; 
 return 0; 
} 
  

通过调用 g ++ 像 g ++ -std = c ++ 11 main.cpp -o timetest 导致输出

  1372686001 
  

秒。

这是glibc中的错误吗？在g ++？我的错误？

  g ++（Debian 4.7.3-4）4.7.3 
 ldd（Debian EGLIBC 2.17-6 ）2.17 
  

---

更新：它使用g ++ 4.8时工作。那么这是一个gcc错误？

  g ++  -  4.8（Debian 4.8.1-2）4.8.1 
 

我想发生的是你正在使用GCC 4.7进行编译，但是运行-time链接器使用来自不同GCC版本的 libstdc ++。so ，并且为 std :: chrono：system_clock 。如果使用 LD_LIBRARY_PATH 或合适的链接器选项来确保您使用GCC 4.7 和编译，请使用 libstdc ++。 c $ c>，则结果应正确。

例如：

  $ $ HOME / gcc / 4.7.1 / bin / g ++ -std = c ++ 11 t.cc 
 $ ./a.out 
 1372693222 
 $ LD_LIBRARY_PATH = $ HOME /gcc/4.7.1/lib64 ./a.out 
 1372693225128 
  

因为对 system_clock :: now（）的调用位于 libstdc ++。so 库中，因此结果取决于哪个库在运行时使用，但从该值到毫秒的 duration_cast 转换由内联模板完成，在编译时。如果编译时转换与运行时调用不一致，则结果不一致。

对于GCC 4.8.1， system_clock 实现改进为总是使用 clock_gettime 系统调用，如果它可用，这不是4.7的情况，所以它一贯使用高精度时钟无论GCC是如何配置的，这可能解释了为什么你看不到4.8.1的问题。

您应该始终确保正确的版本 libstdc ++。因此在运行时使用。

I want to have the number of milliseconds since epoch. A popular solution looks like follows (one of the solutions of this question asked here Get time since epoch in milliseconds preferably using C++11 chrono )

#include <iostream>
#include <chrono>

int main() {
    auto millitime = std::chrono::duration_cast<std::chrono::milliseconds>
        (std::chrono::system_clock::now().time_since_epoch()).count();
    std::cout << millitime << std::endl;
    return 0;
}

compiling this with a call to g++ like g++ -std=c++11 main.cpp -o timetest results in the output

1372686001

which is equal to the number of seconds since epoch!

Is this a bug in the glibc? in g++? my mistake?

g++ (Debian 4.7.3-4) 4.7.3
ldd (Debian EGLIBC 2.17-6) 2.17

---

Update: it works when using the g++ 4.8. So it is a gcc bug?!

g++-4.8 (Debian 4.8.1-2) 4.8.1

解决方案

I think what's happening is that you are compiling with GCC 4.7 but the run-time linker is using the libstdc++.so from a different GCC version, and they are configured with different precision for std::chrono:system_clock. If you use LD_LIBRARY_PATH or suitable linker options to ensure you compile with GCC 4.7 and use its libstdc++.so then the results should be correct.

For example:

$ $HOME/gcc/4.7.1/bin/g++ -std=c++11 t.cc
$ ./a.out
1372693222
$ LD_LIBRARY_PATH=$HOME/gcc/4.7.1/lib64 ./a.out
1372693225128

The difference happens because the call to system_clock::now() is in the libstdc++.so library so the result depends on which library is used at run-time, but the duration_cast conversion from that value to milliseconds is done by inline templates which are instantiated at compile-time. If the compile-time conversion is not consistent with the run-time call then the results are inconsistent.

For GCC 4.8.1 the system_clock implementation was improved to always use the clock_gettime system call if it's available, which was not the case for 4.7, so it consistently uses the high-precision clock no matter how GCC was configured, which probably explains why you don't see the problem with 4.8.1.

You should always ensure the right version of libstdc++.so is used at run-time.

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