The btree library uses low level I/O. On POSIX-like systems that means read/write/lseek. lseek() won't support large files on systems where long is 32 bits. POSIX-like operating systems get around this problem by supply functions with names like llseek or lseek64 that traffic in long long instead of long. So there is a line of code: new_offset = ::lseek(handle(), offset, whence); // offset is a variable of type int64_t That needs to change to something like: #if defined(SOMEOS) new_offset = ::|lseek(handle(), offset, whence); #elif defined(ANOTHEROS) new_offset = ::whatever(handle(), offset, whence); #else new_offset = ::lseek(handle(), offset, whence); #endif Up at the front of the source file there may also have to be special headers included or symbols defined, depending on the operating system. I'd appreciate some help from folks familiar with particular non-Windows operating systems. What is the preferred name for the 64-bit seek function and what is the preferred #if defined(symbol)? Do special headers have to be included or special symbols defined? Thanks, --Beman