Gentoo Archives: gentoo-portage-dev

From: Paul de Vrieze <pauldv@g.o>
To: gentoo-portage-dev@l.g.o
Subject: Re: [gentoo-portage-dev] DB and binary dependency
Date: Fri, 24 Mar 2006 13:38:34
In Reply to: Re: [gentoo-portage-dev] DB and binary dependency by tvali
1 On Friday 24 March 2006 12:40, tvali wrote:
2 >
3 > Interface can be made somewhat automatically checkable.
4 >
5 > For example:
6 > void a(int);
7 > void b(int, int);
8 > void b(int, char);
9 >
10 > Is compatible with:
11 > void a(int);
12 > void b(int, int);
13 >
14 Unfortunately, your wrong. This only makes sure that you have the right
15 slots to put your squares, triangles and circles in. It does not say that
16 b(int,int) from the first lib actually does the same thing as b(int,int)
17 from the second library.
19 To make it worse, suppose that header file 1 defines the following macro:
20 #define c(x,y) { int A = x; int B = y; if (A<B) { b(A,B); } else
21 { b(B,A); }
23 And header file 2 defines the following macro
24 #define c(x,y) { int A = x; int B = y; if (A>B) { b(A,B); } else
25 { b(B,A); }
27 And the semantics of B are such that one is supposed to use c(x,y) to call
28 b, and b1 accordingly expects the first argument to be smaller, and b2
29 expects the second one to be smaller. On the source level, this macro
30 makes it transparent. The libraries, although they may seem different,
31 are incompatible. While they implement the same source interface, they do
32 NOT implement the same binary interface. On a binary (read compiled)
33 level, they can not be exchanged for eachother.
35 While the above example is clearly broken design, this does happen enough
36 in actual libraries in way more subtle ways. And that is disregarding the
37 fact that the linux/elf ABI does not include argument lists in symbol
38 linking. As such b(int,char) is indistinguishable from b(int,int). To
39 overcome this C++ uses name mangling which creates names based on the
40 signature.
42 > And this can be checked automatically - real header must have all
43 > headings that interface describes. Microsoft has actually done good
44 > work in C# on that topic -- first place where i saw something like
45 > that after thinking about it myself :) This MS version is feature of a
46 > language, of course, not a feature of compiler. Anyway, that might
47 > give an idea.
49 There is a reason that MS implements it on the language level, in C#. It
50 is impossible to do while keeping the (admittedly extremely poor)
51 semantics of C and C++. Unfortunately the linux ABI (Application Binary
52 Interface) is based on C. As such one has to take C into account.
54 > I'm actually sure that this all can be calculated up from sourcecode
55 > and bindep would be after that a check if cpu didnt calculate
56 > something wrong :P Another question, how difficult it is and is it
57 > worth the time.
59 Perhaps you should read up your knowledge of the C language. After you
60 found that the C language is a mess, try C++, it makes things worse.
61 After that's finished take a look at solving this problem for ALL
62 languages.
64 Automatic source analysis for dependency calculation is a dead end. Even
65 if you manage to find the proper interfaces (oops, the package had it's
66 own gl.h instead of using the system one), you don't know anything about
67 the semantics of those interfaces. Two things with the same name may very
68 well have very different behaviours.
70 Paul
72 --
73 Paul de Vrieze
74 Gentoo Developer
75 Mail: pauldv@g.o
76 Homepage:


Subject Author
Re: [gentoo-portage-dev] DB and binary dependency tvali <qtvali@×××××.com>