Re: Robustifying pretty-printers

[Vladimir Prus <vladimir at codesourcery dot com>]

On Saturday 13 June 2009 you wrote:

> 
> While playing with pretty-printers and KDevelop, I've got GDB
> to "hang", with the below backtrace:
> 
> 	(gdb) where
> 	#0  0xb7d0ed2a in strcmp () from /lib/tls/i686/cmov/libc.so.6
> 	#1  0x081be1a7 in install_variable (var=0x191b0620) at /home/ghost/Work/CodeSourcery/Projects/egdb/gdb-cvs/gdb/varobj.c:1731
> 	#2  0x081beef7 in create_child_with_value (parent=0x8a6e158, index=514255, name=0xb7bf8e14 "[514255]", value=0x191b03d0)
>     at /home/ghost/Work/CodeSourcery/Projects/egdb/gdb-cvs/gdb/varobj.c:1859
> 
> The '514255' above is sufficient to understand what happened. Our beloved GCC fails to emit
> proper debug info, therefore a vector is considered in scope before constructor is executed,
> and the vector is full of random bits.
> 
> Now, what are the best strategies for fix this (assuming GCC won't be
> fixed for 10 years to come)? One approach is to make IDE check at what
> line the variable is declared, and don't try to pretty-print it unless
> the current line is above that. Alternatively, we might need to revive
> the code to limit the number of children to fetch, and use some reasonable
> limit, like 10. Comments?

In fact, there's yet another case:

	class StdStringPrinter:
    "Print a std::basic_string of some kind"

    def __init__(self, encoding, val):
        self.encoding = encoding
        self.val = val

    def to_string(self):
        # Look up the target encoding as late as possible.
        encoding = self.encoding
        if encoding == 0:
            encoding = gdb.parameter('target-charset')
        elif encoding == 1:
            encoding = gdb.parameter('target-wide-charset')
        elif isinstance(encoding, WideEncoding):
            encoding = encoding.value
        return self.val['_M_dataplus']['_M_p'].string(encoding)

I am not quite sure where the 'string' method is defined, so the
question is -- assuming I know the expected size of the string.
How do I make the 'string' method not to fetch more than that?

- Volodya