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From: Lew Pitcher <lew.pitcher@digitalfreehold.ca>
Newsgroups: comp.lang.c
Subject: Re: how copy file on linux?
Date: Fri, 28 Jun 2024 23:25:20 -0000 (UTC)
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On Sat, 29 Jun 2024 00:04:00 +0100, Malcolm McLean wrote:

> On 28/06/2024 19:15, Bonita Montero wrote:
>> int fromFile = open( from, O_RDONLY | O_NOATIME );
>>      if( !fromFile )
>>          return false;

open() returns -1 on failure, and some non-negative number
on success. That non-negative number /can be/ 0 and still
be a valid success value.

The above test /will not/ detect an open() failure, as !(-1) == 0
However, the above /will/ falsely call a valid open() that returns
fd 0 (slim chance unless you've previously closed stdin).


>>      invoke_on_destruct closeFrom( [&] { close( fromFile ); } );
>>  //    int toFile = open( to, O_CREAT );
>>  //    if( !toFile )
>>  //        return false;
>>      invoke_on_destruct closeTo( [&] { close( toFile ); } );
>> //     invoke_on_destruct delTo( [&] { unlink( to ); } );
>>      for( int64_t remaining = attrFrom.st_size; remaining > 0; remaining 
>> -= 0x100000 )
>>      {
>>          size_t n = (size_t)(remaining >= 0x100000 ? 0x100000 : remaining);
>>          buf.resize( n );
>>          if( read( fromFile, buf.data(), n ) )
>>          {
>>              // this branch is taken
>>              cout << strerror( errno ) << endl;
>>              return false;
>>          }
> 
> So it claims the file opens, then won't read it?
> 
> Check the call. Is 0 the success return?
> 
> Then I must admit I'm stumped. "to" doesn't alias "from" I suppose?
> 
> I'd also wonder if somehow the C++ compiler has invoked the on destruct 
> code, and closed the file.




-- 
Lew Pitcher
"In Skills We Trust"