c++ - 读取/sys/block/$DEVICE/stat 会导致竞争条件吗？

Question

每一秒钟，我的程序都会读取/sys/block/$DEVICE/stat。

减去以前保存的值，然后保存当前值。所以我知道硬盘事件。 但有时我的值太大(在上一个和当前之间)(5GB/秒)。

1. 它可以是竞争条件吗(如果内核正在写入文件 当应用程序同时读取它时)？

2. 是否存在避免这种情况的标准解决方案？

   const int HDD_READ_POS     = 2;
   const int HDD_WRITE_POS    = 6;
   const int UNIX_SECTOR_SIZE = 512;
   
   std::tuple<uint64_t, uint64_t> hddStatus(const std::string &name)
   {
       std::ifstream in("/sys/block/"+name+"/stat");
   
       auto readVal_ = static_cast<uint64_t>(0);
       auto writeVal_= static_cast<uint64_t>(0);
   
       if ( ! in.is_open() ) {
           return std::tuple<uint64_t, uint64_t> (readVal_, writeVal_);
       }
   
       std::string line;
       std::regex rgx ( "\\d+" );
       std::regex_token_iterator<std::string::iterator> end;
   
       while (std::getline(in, line) ){
   
           std::regex_token_iterator<std::string::iterator> iter( line.begin(), line.end(), rgx, 0 );
           int pos_ = 0 ;
   
           while ( iter != end ) {
   
               if ( pos_ == HDD_READ_POS){
                   readVal_ = std::stoul( *iter ) ;
               }
   
               if ( pos_ == HDD_WRITE_POS){
                   writeVal_ = std::stoul( *iter ) ;
               }
   
               ++iter;
               ++pos_;
           }
       }
   
       return std::tuple<uint64_t, uint64_t> (readVal_, writeVal_);
   
   }
   

Answer 1

完整列表

#include <iostream>
#include <fstream>
#include <regex>
#include <tuple>
#include <chrono>
#include <thread>
#include <cstdint>

const int HDD_READ_POS     = 2;
const int HDD_WRITE_POS    = 6;
const int UNIX_SECTOR_SIZE = 512;
uint64_t prevRead  = static_cast<uint64_t>(0);
uint64_t prevWrite = static_cast<uint64_t>(0);

std::tuple<uint64_t, uint64_t> hddStatus(const std::string &name="sda"){
  std::ifstream in("/sys/block/"+name+"/stat");

auto readVal_ = static_cast<uint64_t>(0);
auto writeVal_= static_cast<uint64_t>(0);

if ( ! in.is_open() ) {
    return std::tuple<uint64_t, uint64_t> (readVal_, writeVal_);
}

std::string line;
std::regex rgx ( "\\d+" );
std::regex_token_iterator<std::string::iterator> end;

while (std::getline(in, line) ){

    std::regex_token_iterator<std::string::iterator> iter( line.begin(), line.end(), rgx, 0 );
    int pos_ = 0 ;

    while ( iter != end ) {

        if ( pos_ == HDD_READ_POS){
            readVal_ = std::stoul( *iter ) ;
        }

        if ( pos_ == HDD_WRITE_POS){
            writeVal_ = std::stoul( *iter ) ;
        }

        ++iter;
        ++pos_;
    }
}

return std::tuple<uint64_t, uint64_t> (readVal_, writeVal_);

}


void init()
{

    auto values = hddStatus();
    prevRead  = std::get<0>( values ) * UNIX_SECTOR_SIZE;
    prevWrite = std::get<1>( values ) * UNIX_SECTOR_SIZE;
}

int main(int argc, char const *argv[])
{
init();


while(true){

std::ofstream stat("statistics.txt", std::fstream::out | std::fstream::app);
if ( stat.is_open() ){    

        auto values = hddStatus();
        auto read  = std::get<0>( values ) * UNIX_SECTOR_SIZE;
        auto write = std::get<1>( values ) * UNIX_SECTOR_SIZE;

         // stat<<"Current Read: "<< read<<" Write: "<<write<<'\n';
        if (read > prevRead){
            stat<<"Diff Read: "<< read - prevRead <<'\n';
            std::cout<<"Diff Read: "<< read - prevRead <<'\n';
        }

        if ( write > prevWrite){
            stat<<"Diff Write: "<<write - prevWrite <<'\n';
            std::cout<<"Diff Write: "<<write - prevWrite <<'\n';
        }

        prevRead  = read;
        prevWrite = write;

        std::this_thread::sleep_for(std::chrono::milliseconds(1));

    }
}

return 0;

}