ctypes in python failed to honor c_int

[Gabriel Genellina]

On 18 mar, 04:12, Jerry Fleming <jerry.flem... at saybot.com> wrote:
> Gabriel Genellina wrote:
> > On 17 mar, 23:57, Jerry Fleming <jerry.flem... at saybot.com> wrote:
>
> >> I have a binary file written with c structures. Each record contains a
> >> null-terminated string followed by two 4-bytes integers. I wrote a small
> >> segment of python code to parse this file in this way:
> >> [coe]
> >> #!/usr/bin/python
>
> >> from ctypes import *
>
> >> class Entry(Structure):
> >>         _fields_ = ('w', c_char_p), ('s', c_uint, 32), ('l', c_uint, 32)
>
> >> idx = open('x.idx', 'rb')
> >> str = idx.read(1000)
> >> obj = Entry(str)
> >> print obj.w
> >> print obj.s
> >> print obj.l
> >> [/code]
> >> where the field w is the string, and s and l are the integers. Problem
> >> is that, I can only get the strings, not the integers. Well, I did got
> >> integers, but they are all zeros. What should I do to get the real numbers?
>
> > So the string has a variable length? For "Hello" you have
> > 'h','e','l','l','o', a zero byte, followed by the two integers? This
> > is somewhat unusual for a C struct (in fact you can't declare it in
> > C). Perhaps the string is actually a char[n] array with a declared
> > maximum size?
>
> Yes, it has a variable length. The C version of the structure is
> something like this:
> [code]
> struct entry {
>      char *str,
>      int start,
>      int length}
>
> [/code]

But this doesn't match the file contents. There are no pointers in the
file.

> And adding repr() would print something like this:
> [code]
> '(as) mad as a
> hatter\x00\x00\x00\x00\x00\x00\x00\x00\x1f-ish\x00\x00\x00\x00\x1f\x00\x00\­x00 at -ism\x00\x00\x00\x00_\x00\x00\x00B-ist\x00\x00\x00\x00\xa1\x00\x00\x00J­.AU\x00\x00\x00\x00\xeb\x00\x00\x00P.EXE\x00\x00\x00\x01;\x00\x00\x00`.GIF\­x00\x00\x00'
> (as) mad as a hatter
> 0
> 0
> [/code]
> where the first line is the result of repr(). We can find that, after
> the null-terminated string '(as) mad as a hatter', there are two
> integers, 0 and 31 (0x1f). But python treat 31 as zero.

Ah, but it doesn't "treat 31 as zero". Entry(str) is the same as
Entry(w=str), that is, you are initializing the w attribute alone,
leaving the other two integers as 0.
I don't know how to use ctypes to read the structure (nor if it is
possible at all), I would read it normally with Python code and build
the struct afterwards (in case it is used to call any C code).

w, data = data.split('\x00', 1)
s, l = struct.unpack("ll", data[:8])
data= data[8:]

--
Gabriel Genellina