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build: Add a symbol version to all exported symbols for glibc
The --default-symver linker option attaches a default version definition (the SONAME) to every exported symbol. It is supported since at least GNU binutils 2.22 in 2011 (older versions not tested). With this version definition, newly-linked binaries that depend on the jansson shared library will refer to its symbols in a versioned form, preventing their references from being resolved to a symbol of the same name exported by json-c or json-glib if those libraries appear in dependency search order before jansson, which will usually result in a crash. This is necessary because ELF symbol resolution normally uses a single flat namespace, not a tree like Windows symbol resolution. At least one symbol (json_object_iter_next()) is exported by all three JSON libraries. Linking with -Bsymbolic is not enough to have this effect in all cases, because -Bsymbolic only affects symbol lookup within a shared object, for example when parse_json() calls json_decref(). It does not affect calls from external code into jansson, unless jansson was statically linked into the external caller. This change will also not prevent code that depends on json-c or json-glib from finding jansson's symbols and crashing; to prevent that, a corresponding change in json-c or json-glib would be needed. Adding a symbol-version is a backwards-compatible change, but once added, removing or changing the symbol-version would be an incompatible change that requires a SONAME bump. Resolves: akheron/jansson#523 (when combined with an equivalent change to json-c). Signed-off-by: Simon McVittie <smcv@collabora.com>
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