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