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 10## Upgrades 11 12 A libcurl upgrade does not break the ABI or change established and documented 13 behavior. Your application can remain using libcurl just as before, only with 14 fewer bugs and possibly with added new features. 15 16## Version Numbers 17 18 In libcurl land, you cannot tell by the libcurl version number if that 19 libcurl is binary compatible or not with another libcurl version. As a rule, 20 we don't break the ABI so you can *always* upgrade to a later version without 21 any loss or change in functionality. 22 23## Soname Bumps 24 25 Whenever there are changes done to the library that will cause an ABI 26 breakage, that may require your application to get attention or possibly be 27 changed to adhere to new things, we will bump the soname. Then the library 28 will get a different output name and thus can in fact be installed in 29 parallel with an older installed lib (on most systems). Thus, old 30 applications built against the previous ABI version will remain working and 31 using the older lib, while newer applications build and use the newer one. 32 33 During the first seven years of libcurl releases, there have only been four 34 ABI breakages. 35 36 We are determined to bump the SONAME as rarely as possible. Ideally, we 37 never do it again. 38 39## Downgrades 40 41 Going to an older libcurl version from one you're currently using can be a 42 tricky thing. Mostly we add features and options to newer libcurls as that 43 won't break ABI or hamper existing applications. This has the implication 44 that going backwards may get you in a situation where you pick a libcurl that 45 doesn't support the options your application needs. Or possibly you even 46 downgrade so far so you cross an ABI break border and thus a different 47 soname, and then your application may need to adapt to the modified ABI. 48 49## History 50 51 The previous major library soname number bumps (breaking backwards 52 compatibility) happened the following times: 53 54 0 - libcurl 7.1, August 2000 55 56 1 - libcurl 7.5 December 2000 57 58 2 - libcurl 7.7 March 2001 59 60 3 - libcurl 7.12.0 June 2004 61 62 4 - libcurl 7.16.0 October 2006 63