[libcxx-dev] problem with seekg() on a wfstream

[Giorgio Dal Molin via libcxx-dev]

Hallo,

I'm having surprising problems trying to seek back
a wfstream after reading its contents.
I'm working on linux with the current llvm 'release_80'.

Here is the test.cpp and the input file:

#include <iostream>
#include <fstream>

int main( int argc, const char *argv[] )
{
	std::wfstream ws;
	std::wstring line;

	if( argc > 1 )
		ws.open( argv[1] );

	if( !ws.is_open() ) {
		std::wcout << L"errors opening the stream.\n";
		return -1;
	}

	std::wcout << L"read the file:\n";
	while( std::getline( ws, line ) )
		std::wcout << L"line: " << line << std::endl;

	std::wcout << L"done reading the file.\n";

	ws.clear();
	ws.seekg( 0 );

	std::wcout << L"read the file again:\n";
	while( std::getline( ws, line ) )
		std::wcout << L"line: " << line << std::endl;

	std::wcout << L"done reading the file again.\n";
	return 0;
};

The input file:

abc
1 2 3


Compiling the program with:

# make test CXX=clang++ CXXFLAGS="-O2 -march=native -stdlib=libc++
-std=c++17" LDFLAGS="-fuse-ld=lld"

And here are the results:

# ./test test.txt
read the file:
line: abc
line: 1 2 3
done reading the file.
read the file again:
done reading the file again.

The second time I want to read the file it does not work because the
wfstream is in 'fail' state after the call to seekg( 0 ).

The same source linked against the libstdc++ works as expected:

# make test CXX=clang++ CXXFLAGS="-O2 -march=native -stdlib=libstdc++
-std=c++17" LDFLAGS="-fuse-ld=lld"

# ./test test.txt
read the file:
line: abc
line: 1 2 3
done reading the file.
read the file again:
line: abc
line: 1 2 3
done reading the file again.

giorgio

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