[Bug python/18779] New: collection of issues handling C strings in python

["dje at google dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18779

            Bug ID: 18779
           Summary: collection of issues handling C strings in python
           Product: gdb
           Version: unknown
            Status: NEW
          Severity: normal
          Priority: P2
         Component: python
          Assignee: unassigned at sourceware dot org
          Reporter: dje at google dot com
  Target Milestone: ---

Wanting to extract a nul-terminated string from memory given just an ELF symbol
I ran into a few warts/bugs.
This bug is to record what I find, though I may not list them all in this
initial report (I *could* file a separate report per bug, but I'm not sure how
many there will be and since they're all related I wanted to keep them
together).

Given:

const char string[] = "hello";

bug #1:

(gdb) py print
gdb.parse_and_eval("&string[0]").string(length=7,encoding="ASCII")
Traceback (most recent call last):
  File "<string>", line 1, in <module>
gdb.MemoryError: Cannot access memory at address 0x40075c
Error while executing Python code.

One can certainly argue that's an error, but not a memory access error.

bug #2:

(gdb) py print gdb.parse_and_eval("&string[0]").lazy_string()
<gdb.LazyString object at 0x7f3444f8a750>

Question: what should str(lazy_string_object) be?
If one wants the above there is "repr".
The only useful thing to me seems to be the value returned as a python string.
IOW I expected to see "hello".

feature request #1:

IWBN if one could specify that the string is nul-terminated,
which we can by specifying a length of -1, but also specify
a maximum length (for those wanting more robustness).

maybe bug #3:

I don't have a string opinion on whether this works or not,
in c or c++, just filing this for reference sake since it seems odd:

(gdb) py print gdb.parse_and_eval("&string").string()
Traceback (most recent call last):
  File "<string>", line 1, in <module>
gdb.error: Trying to read string with inappropriate type `const char (*)[6]'.
Error while executing Python code.

maybe bug #4:

If we know the array's length, how come the lazy string length is always -1?

(gdb) pt &string
type = const char (*)[6]
(gdb) py print gdb.parse_and_eval("&string[0]").lazy_string().length
-1
(gdb) py print gdb.parse_and_eval("&string").lazy_string().length
-1
(gdb) py print gdb.parse_and_eval("string").lazy_string().length
-1

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