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1                                  _   _ ____  _
2                              ___| | | |  _ \| |
3                             / __| | | | |_) | |
4                            | (__| |_| |  _ <| |___
5                             \___|\___/|_| \_\_____|
6
7                          libcurl's binary interface
8
9ABI - Application Binary Interface
10----------------------------------
11
12  "ABI" describes the low-level interface between an application program and a
13  library. Calling conventions, function arguments, return values, struct
14  sizes/defines and more.
15
16  [Wikipedia has a longer description](http://en.wikipedia.org/wiki/Application_binary_interface)
17
18Upgrades
19--------
20
21  In the vast majority of all cases, a typical libcurl upgrade does not break
22  the ABI at all. Your application can remain using libcurl just as before,
23  only with less bugs and possibly with added new features. You need to read
24  the release notes, and if they mention an ABI break/soname bump, you may
25  have to verify that your application still builds fine and uses libcurl as
26  it now is defined to work.
27
28Version Numbers
29---------------
30
31  In libcurl land, you really can't tell by the libcurl version number if that
32  libcurl is binary compatible or not with another libcurl version.
33
34Soname Bumps
35------------
36
37  Whenever there are changes done to the library that will cause an ABI
38  breakage, that may require your application to get attention or possibly be
39  changed to adhere to new things, we will bump the soname. Then the library
40  will get a different output name and thus can in fact be installed in
41  parallel with an older installed lib (on most systems). Thus, old
42  applications built against the previous ABI version will remain working and
43  using the older lib, while newer applications build and use the newer one.
44
45  During the first seven years of libcurl releases, there have only been four
46  ABI breakages.
47
48  We are determined to bump the SONAME as rarely as possible.  Ideally, we
49  never do it again.
50
51Downgrades
52----------
53
54  Going to an older libcurl version from one you're currently using can be a
55  tricky thing. Mostly we add features and options to newer libcurls as that
56  won't break ABI or hamper existing applications. This has the implication
57  that going backwards may get you in a situation where you pick a libcurl
58  that doesn't support the options your application needs. Or possibly you
59  even downgrade so far so you cross an ABI break border and thus a different
60  soname, and then your application may need to adapt to the modified ABI.
61
62History
63-------
64
65  The previous major library soname number bumps (breaking backwards
66  compatibility) have happened the following times:
67
68  0 - libcurl 7.1,   August 2000
69
70  1 - libcurl 7.5    December 2000
71
72  2 - libcurl 7.7    March 2001
73
74  3 - libcurl 7.12.0 June 2004
75
76  4 - libcurl 7.16.0 October 2006
77