Linux共享库中缺少函数

Question

I'm supporting a Linux program that allows users to write their own .so file based plugins that are loaded at runtime with dlopen(). Sometimes a user will forget to include a C++ file in the makefile that contains necessary function definitions. When the plugin manager uses dlsym() to load an exported function from the plugin that depends on one of these missing function definitions, I get a null function address.

My question: How can I detect this error ahead of time? I've tried the -z defs and -Wno-undef linker flags, but they don't generate an error. I've tried the nm, readelf, and objdump commands with various flags but the undefined functions don't show up in the symbol tables at all. Any suggestions? Thanks!

Answer 1

i suspect the null function address is mostly the result of using some third party library in the .so file and when the linux program does dlopen(), it can not locate the third party and then null function address is returned

i suggest try nm command on both "good" and "bad" .so files and look for the pattern of symbols with ' U ' (undefined symbol) "bad" .so file would have more ' U '

see also Unresolved external symbol in object files and Linux shared library that uses a shared library undefined symbol

Answer 2

--no-undefined / -z defs documentation:

Undefined symbols in shared libraries are still allowed.

Use another flag: --no-allow-shlib-undefined :

Allows or disallows undefined symbols in shared libraries. This switch is similar to --no-undefined except that it determines the behaviour when the undefined symbols are in a shared library rather than a regular object file. It does not affect how undefined symbols in regular object files are handled.