[boost] Re: boost.filesystem - rationale for copy_file

From: Lars Gullik Bjønnes (larsbj_at_[hidden])
Date: 2005-01-28 09:55:54

Jonathan Wakely <cow_at_[hidden]> writes:

| On Fri, Jan 28, 2005 at 10:58:24AM +0000, Jonathan Wakely wrote:
>
>> Looking at the implementation of copy_file() I'm wondering why it isn't
>> done like this (this provides noclobber support too):
>> 
>>     std::fstream out;
>>     if (noclobber)
>>     {
>>         out.open(to_file_ph.string().c_str(), std::ios::in);
>>         if (out.is_open())
>>         {
>>             // file exists
>>             boost::throw_exception( filesystem_error(
>>                 "boost::filesystem::copy_file",
>>                 from_file_ph, to_file_ph, fs::detail::system_error_code() ) );
>>         }
>>     }
>>     std::ifstream in(from_file_ph.string().c_str());
>>     out.open(to_file_ph.string().c_str(), std::ios::out);
>>     out << in.rdbuf();
>
| Hmm, this has a race-condition,

In that the target can come into existance after checked for?
lots and lots of races like that possible when working with
filesystems.

| which is a good reason to stick with
| open(2) and O_CREAT|O_EXCL and read(2)/write(2).
>
| However, in that case there's a comment about the implementation:
>
|       // TODO: Ask POSIX experts if this is the best way to copy a file
>
| Now, I'm no POSIX expert, but I believe the loop used:

Well, you can expect the implementators of your stdlib to be experts on
the system it is implemented for. And that speaks in favour of writing
a C++ solution.

Pity that there is no ios::nocreate in the standard.

And in the case where noclobber is false there is no race.

|       ssize_t sz;
|       while ( (sz = ::read( infile, buf.get(), buf_sz )) > 0
|           && (sz = ::write( outfile, buf.get(), sz )) > 0 ) {}

exactly the kindo thing that has been solved of the implementators for
the 
    out << in.rdbuf();
case.

-- 
	Lgb