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    1: ABI - 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: 
   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: 
   20: Version 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: 
   26: Soname 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: 
   43: Downgrades
   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: 
   54: History
   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