Domenico Andreoli, the Debian curl and libcurl maintainer posted to the curl-library list about the recent libcurl soname bump and the related issue of ABI breakage.

I responded, explaining my view on why the soname bump from 3 to 4 was valid, but I’ve also gone back in my mail archives and checked out a private discussion I had with Bjorn Reese over this subject a long time ago and I feel it is about time it hits the air.

An ABI breakage occurs when one or more of the following changes are made

Functions:

• Change function names
• Remove existing functions
• Change parameters
• Change behavior
• Change undefined behavior
• Add dependence on other functions (e.g. A() must be called before B())

Data structures:

• Change the order of elements
• Add elements
• Remove elements
• Change the data type or size
• Change alignment
• Change element values (e.g. change bits in flag variables)

Global variables:

• Change variable names or types
• Remove variables

Update: Dan Fandrich posted on the curl-library list and mentioned Mike Hearn’s great related page: Writing shared libraries.