string.c:198
VALUE
rb_str_buf_new(capa)
    long capa;
{
    VALUE str = str_alloc(rb_cString);

    if (capa < STR_BUF_MIN_SIZE) {
	capa = STR_BUF_MIN_SIZE;
    }
    RSTRING(str)->ptr = 0;
    RSTRING(str)->len = 0;
    RSTRING(str)->aux.capa = capa;
    RSTRING(str)->ptr = ALLOC_N(char, capa+1);
    RSTRING(str)->ptr[0] = '\0';

    return str;
}
