I found it. C++ takes the BUFSIZ from C (see ). The files fstream and fstream.tcc includes the class basic_filebuf.
Note: LIBSTDC++ from GCC
from file fstream
#include <istream>
#include <ostream>
#include <bits/codecvt.h>
#include <cstdio> // For BUFSIZ
#include <bits/basic_file.h> // For __basic_file, __c_lock
#ifdef __GXX_EXPERIMENTAL_CXX0X__
#include <string> // For std::string overloads.
#endif
/**
* Actual size of internal buffer. This number is equal to the size
* of the put area + 1 position, reserved for the overflow char of
* a full area.
*/
size_t _M_buf_size;
from file fstream.tcc
template<typename _CharT, typename _Traits>
basic_filebuf<_CharT, _Traits>::
basic_filebuf() : __streambuf_type(), _M_lock(), _M_file(&_M_lock),
_M_mode(ios_base::openmode(0)), _M_state_beg(), _M_state_cur(),
_M_state_last(), _M_buf(0), _M_buf_size(BUFSIZ),
_M_buf_allocated(false), _M_reading(false), _M_writing(false), _M_pback(),
_M_pback_cur_save(0), _M_pback_end_save(0), _M_pback_init(false),
_M_codecvt(0), _M_ext_buf(0), _M_ext_buf_size(0), _M_ext_next(0),
_M_ext_end(0)
{
if (has_facet<__codecvt_type>(this->_M_buf_locale))
_M_codecvt = &use_facet<__codecvt_type>(this->_M_buf_locale);
}