在gdb漂亮的打印机中检查内存

Question

I'm trying to figure out how GDB pretty printing works in order to create some pretty printers that will display data structures in a more compact, readable form, but the documentation seems pretty thin. As a starting exercise, I tried to create a pretty printer for a sockaddr_in -- instead of printing umpteen different union variations in an unreadable form, just print it in normal dotted notation.

I put the following in my .gdbinit file:

python
class sockaddr_in_Printer(object):
    "Print a sockaddr_in"
    def __init__(self, val):
        self.val = val
    def to_string(self):
        addr = self.val['sin_addr'].address().cast(gdb.lookup_type("unsigned char *"))
        port = self.val['sin_port'].address().cast(gdb.lookup_type("unsigned char *"))
        rv = "" + addr.dereference()
        for x in range(0,3):
            addr += 1
            rv += "."
            rv += addr.dereference()
        pnum = port.dereference() * 256
        port += 1
        pnum += port.dereference()
        rv += ":"
        rv += pnum
        return rv;
def find_pp(val):
    if val.type.tag == 'sockaddr_in':
        return sockaddr_in_Printer(val)
    return None
gdb.pretty_printers.append(find_pp)
end

This seems to load ok, but when I try to print a sockaddr_in, I get an opaque error message:

(gdb) p srcaddr
Python Exception <type 'exceptions.RuntimeError'> Value is not callable (not TYPE_CODE_FUNC).: 
$2 = 
(gdb)

Any ideas as to what is going wrong?

Anyone have any good pointers to documentation about writing/using/debugging gdb pretty printing functions? Much of the above is cribbed from examples found around the web, as that appears to be the only 'documentation' available.

edit

changing the addr/port stuff to

 addr = self.val['sin_addr'].address.cast(gdb.lookup_type("unsigned char").pointer())
 port = self.val['sin_port'].address.cast(gdb.lookup_type("unsigned char").pointer())

fixes that exception, but leads to

(gdb) p src
Python Exception <class 'gdb.error'> Argument to arithmetic operation not a number or boolean.: 
$1 = 

..still no line number info to indicate where the problem is.

edit

after lots of random twiddling of the code, I figured out that I need:

python
class sockaddr_in_Printer(object):
    "Print a sockaddr_in"
    def __init__(self, val):
        self.val = val
    def to_string(self):
        ptr_type = gdb.lookup_type("unsigned char").pointer()
        addr = self.val['sin_addr'].address.cast(ptr_type)
        port = self.val['sin_port'].address.cast(ptr_type)
        rv = str(int(addr.dereference()))
        for x in range(0,3):
            addr += 1
            rv += "."
            rv += str(int(addr.dereference()))
        pnum = port.dereference() * 256
        port += 1
        pnum += port.dereference()
        rv += ":"
        rv += str(pnum)
        return rv;
def find_pp(val):
    if val.type.tag == 'sockaddr_in':
        return sockaddr_in_Printer(val)
    return None
gdb.pretty_printers.append(find_pp)
end

Answer 1

Value.address is a property, not a function. So when you write ".address()", you are telling gdb to try to make an inferior function call. Instead write ".address".

Please file bug reports in gdb bugzilla for any documentation issues. The current documentation is written in a "reference" style, but adding examples is probably worth doing.