WilliamKF WilliamKF - 2 months ago 10
C++ Question

How to get a file descriptor from a std::basic_ios for clang on OS X?

I'm porting some code to Darwin OS X and as part of the change, we go from gcc to the clang compiler.

In the code, there is a function dating from 2005 and posted several places on the Internet. It provides functionality for several different old versions of GCC, I've edited out all but the last it offers, v3.4.0 or later. The code depends upon two GCC specific classes:

__gnu_cxx::stdio_filebuf
and
__gnu_cxx::stdio_sync_filebuf
.

//! Similar to fileno(3), but taking a C++ stream as argument instead of a
//! FILE*. Note that there is no way for the library to track what you do with
//! the descriptor, so be careful.
//! \return The integer file descriptor associated with the stream, or -1 if
//! that stream is invalid. In the latter case, for the sake of keeping the
//! code as similar to fileno(3), errno is set to EBADF.
//! \see The <A HREF="http://www.ginac.de/~kreckel/fileno/">upstream page at
//! http://www.ginac.de/~kreckel/fileno/</A> of this code provides more
//! detailed information.
template <typename charT, typename traits>
inline int
fileno_hack(const std::basic_ios<charT, traits>& stream)
{
// Some C++ runtime libraries shipped with ancient GCC, Sun Pro,
// Sun WS/Forte 5/6, Compaq C++ supported non-standard file descriptor
// access basic_filebuf<>::fd(). Alas, starting from GCC 3.1, the GNU C++
// runtime removes all non-standard std::filebuf methods and provides an
// extension template class __gnu_cxx::stdio_filebuf on all systems where
// that appears to make sense (i.e. at least all Unix systems). Starting
// from GCC 3.4, there is an __gnu_cxx::stdio_sync_filebuf, in addition.
// Sorry, darling, I must get brutal to fetch the darn file descriptor!
// Please complain to your compiler/libstdc++ vendor...
#if defined(__GLIBCXX__) || defined(__GLIBCPP__)
// OK, stop reading here, because it's getting obscene. Cross fingers!
# if defined(__GLIBCXX__) // >= GCC 3.4.0
// This applies to cin, cout and cerr when not synced with stdio:
typedef __gnu_cxx::stdio_filebuf<charT, traits> unix_filebuf_t;
unix_filebuf_t* fbuf = dynamic_cast<unix_filebuf_t*>(stream.rdbuf());
if (fbuf != NULL) {
return fbuf->fd();
}

// This applies to filestreams:
typedef std::basic_filebuf<charT, traits> filebuf_t;
filebuf_t* bbuf = dynamic_cast<filebuf_t*>(stream.rdbuf());
if (bbuf != NULL) {
// This subclass is only there for accessing the FILE*. Ouuwww, sucks!
struct my_filebuf : public std::basic_filebuf<charT, traits> {
int fd() { return this->_M_file.fd(); }
};
return static_cast<my_filebuf*>(bbuf)->fd();
}

// This applies to cin, cout and cerr when synced with stdio:
typedef __gnu_cxx::stdio_sync_filebuf<charT, traits> sync_filebuf_t;
sync_filebuf_t* sbuf = dynamic_cast<sync_filebuf_t*>(stream.rdbuf());
if (sbuf != NULL) {
return fileno(sbuf->file());
}
# endif
#else
# error "Does anybody know how to fetch the bloody file descriptor?"
return stream.rdbuf()->fd(); // Maybe a good start?
#endif

errno = EBADF;
return -1;
}


The question is, for clang 5.1 on OS X Mavericks, what would be the way to computed the file descriptor for a
std::basic_ios
?

Answer

If you don't mind getting your hands really dirty poking round in private implementation details the following code works:

#include <iostream>
#include <fstream>

// Generate a static data member of type Tag::type in which to store
// the address of a private member. It is crucial that Tag does not
// depend on the /value/ of the the stored address in any way so that
// we can access it from ordinary code without directly touching
// private data.
template < class Tag >
struct stowed
{
  static typename Tag::type value;
};

template < class Tag >
typename Tag::type stowed< Tag >::value;

// Generate a static data member whose constructor initializes
// stowed< Tag >::value. This type will only be named in an explicit
// instantiation, where it is legal to pass the address of a private
// member.
template < class Tag, typename Tag::type x >
struct stow_private
{
  stow_private() { stowed< Tag >::value = x; }
  static stow_private instance;
};
template < class Tag, typename Tag::type x >
stow_private< Tag, x > stow_private< Tag, x >::instance;

struct filebuf_file { typedef FILE*( std::filebuf::*type ); };
template struct stow_private< filebuf_file, &std::filebuf::__file_ >;

FILE* c_file( std::filebuf& fb )
{
  return fb.*stowed< filebuf_file >::value;
}

int main(int argc, const char * argv[])
{
  std::ofstream fs("test.txt");
  FILE* file = c_file(*fs.rdbuf());
  std::cout << file->_file << "\n";
  return 0;
}
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