std::strstreambuf::setbuf (3) - Linux Man Pages
virtual streambuf<char>* setbuf(char* s, std::streamsize n);
If s is a null pointer and n is zero, this function has no effect.
Otherwise, the effect is implementation-defined: some implementations do nothing, while some implementations deallocate the dynamic member array used as the buffer and begin using the user-supplied character array of size n, whose first element is pointed to by s.
This function is protected virtual, it may only be called through pubsetbuf() or from member functions of a user-defined class derived from std::strstreambuf.
s - pointer to the first byte in the user-provided buffer
n - the number of bytes in the user-provided buffer
Implementation test to check if setbuf() is supported on a dynamic strstream (output obtained with Sun Studio)
// Run this code
pubsetbuf (public member function of std::basic_streambuf<CharT,Traits>)
setbuf replaces the buffer with user-defined array, if permitted
setbuf attempts to replace the controlled character sequence with an array
setbuf provides user-supplied buffer or turns this filebuf unbuffered