使用共享库时对函数的 C++ 代码未定义引用

Question

I am trying to compile a simple c++ program that calls some functions from the NDPI C library. When compiling using the shared library (.so file) it can't find some of the functions in the library (like set_ndpi_flow_malloc) but seems to be able to find another function called set_ndpi_malloc.

Note: I believe the functions it can't find were newly added to the library when it updated from version 1.8 to 2.0, but the library files I am using were all compiled from source of version 2.0.

Strangely when I compile using the static library (.a) it has no problem finding these functions.

See the image linked above to see the compiler output each time and the code. The two compile attempts were made within a minute of each other and nothing was changed except for one line in the makefile to compile with the static library instead of the shared library.

Update

This has been identified as an issue with the library not exporting all the symbols.

See the issue on github here: https://github.com/ntop/nDPI/issues/459

Answer 1

I made some effort to reproduce your steps.

1. Clone (download/unpack) the library source from their Github
2. Enter the source directory of the library
3. ./autogen.sh (as per README.md , also: libpcap-dev package is needed to be installed in the system)
4. ./configure (as per README.md )
5. make (as per README.md )
6. Create a file in the home directory called test.cpp and place your code there (however, I think that include statement shall be reworded as simply #include "ndpi_api.h" )
7. g++ -c -O3 -Wall -I ./src/include ~/test.cpp (as per your wish)
8. g++ -O3 -Wall -o test test.o -L ./lib -lndpi (as per your wish)

So, indeed, there are undefined symbols in this case. I used nm -gC ./lib/libndpi.so to read the symbol list and didn't find set_ndpi_flow_malloc and set_ndpi_flow_free . Not mentioned in either way.

I then inspected ./src/include/ndpi_api.h and ./src/lib/ndpi_main.c and couldn't wrap my head around what might have been wrong there. Everything seemed to be OK since these two functions were implemented and defined similar to their counterparts without _flow_ . Nevertheless, the difference certainly exists and could be observed in ./libndpi.sym file. As you might see, set_ndpi_malloc and set_ndpi_free are listed there whilst their _flow_ counterparts are not.

So, I deleted the library source and followed the steps 1 and 2 one more time. Then I added these two lines to ./libndpi.sym just after non- _flow_ function names:

set_ndpi_flow_malloc
set_ndpi_flow_free

Next I had to proceed with the steps 3 - 8 and the last one turned to be successful. Also, nm -gC ./lib/libndpi.so shows the symbols of interest just fine.

I may suppose that forgetting to add the symbol names to .sym file is some sort of mistake. Here is the commit by Vito Piserchia . Perhaps it would be better if you contacted that person or, maybe, lucaderi , who makes the impression of an avid contributor, to tell them about the issue with the symbols. As you may learn from libndpi.sym commit history , they often "add missing symbols", etc.

That's it.