gdb user defined function

[Sinbad]

i am trying to write a user-defined function in gdb.
while analyzing the core is it possible to see if a
pointer variable is accessible or not inside a gdb
user-defined function. for example.
(gdb) p *ptr
cannot access memory at address 0x35336
is there any way to find this inside a gdb function.
i need something as follows
define sub
   is_accessible(ptr)
end