why cp fails to copy /proc/stat file?

Question

In my linux machine if i try to copy /proc/stat it is creating 0 byte file. but if i do cat /proc/stat it has data. but the size always shows as 0.

 cp /proc/stat statfile

is creating zero byte file. If i write a program to copy then it worked. why is it so ?

  int main() 
  {
    std::ifstream procFile("/proc/stat");
    std::ofstream outfile("statfile");
    char buf[1024];
    while (!procFile.eof() && procFile.is_open())
    {
            procFile.getline(buf, 1024);
            outfile << buf<<endl;
    }

    procFile.close();
    outfile.close();
    return 0;
  }

Answer 1

/proc is a pseudo-filesystem. It indicates a size of 0 for the file /proc/stat.

This is why copying doesn't work (cp does first look at the size of the file it has to copy), but you can easily read the information and write it back to a file.

$> cat /proc/stat > statfile

This was fixed in GNU coreutils 7.3.