Gdb Pretty Printer: *(char*){hex_address} equivalent in python

Question

I have C++ classes in following format (copying just important parts):

class my_stringimpl {
public:
static sample_string* create(const char* str, int len) {
    my_stringimpl* sample = static_cast<my_stringimpl*>(malloc(sizeof(my_stringimpl) + len*sizeof(char)));
    char* data_ptr = reinterpret_cast<char*>(sample+1);
    memset(data_ptr, 0, len);
    memcpy(data_ptr, str, len);
    return new (sample) my_stringimpl(len);
}   
private:
    int m_length;
};
class my_string {
public:
    my_string(const char* str, int len)
        : m_impl(my_stringimpl::create(str, len)) { }
    ~my_string() {
        delete m_impl;
    }
private:
    my_stringimpl* m_impl;
};

For this my_string class I am adding pretty printer. I added the following defs in a python script (which I am including in my .gdbinit file) - just func defs copied here:

def string_to_char(ptr, length=None):
    error_message = ''
    if length is None:
        length = int(0)
        error_message = 'null string'
    else:
        length = int(length)
    string = ''.join([chr((ptr + i).dereference()) for i in xrange(length)])
    return string + error_message

class StringPrinter(object):
    def __init__(self, val):
        self.val = val 

class StringImplPrinter(StringPrinter):
    def get_length(self):
        return self.val['m_length']

    def get_data_address(self):
        return self.val.address + 1

    def to_string(self):
        return string_to_char(self.get_data_address(), self.get_length())

class MyStringPrinter(StringPrinter):
    def stringimpl_ptr(self):
        return self.val['m_impl']

    def get_length(self):
        if not self.stringimpl_ptr():
            return 0
        return StringImplPrinter(self.stringimpl_ptr().dereference()).get_length()

    def to_string(self):
        if not self.stringimpl_ptr():
            return '(null)'
        return StringImplPrinter(self.stringimpl_ptr().dereference()).to_string()

But, on usage I am getting the below error -

Python Exception <class 'gdb.error'> Cannot convert value to int.: 

If I try to change the value in 'ptr' to int and then do the arthimetic before casting back to char (like in def above), it gives below error:

Python Exception <type 'exceptions.AttributeError'> 'NoneType' object has no attribute 'cast':

Can anybody tell what is that I am doing wrong? I am really struck here. :(. In nutshell, I am trying to achieve the following c/c++ expr equivalent,

*(char*){hex_address}

in python. How can I do it?

Answer 1

It's better to post full stack traces or at least to indicate exactly which lines throws the exception. Python provides this...

Your string_to_char function can be replaced by Value.string or Value.lazy_string, which were designed for exactly this use. I imagine the error is coming from there; in which case this ought to remove it.

Also, your printers should implement "hint" methods that return "string".