g++ linking order dependency when linking c code to c++ code

Question

Prior to today I had always believed that the order that objects and libraries were passed to g++ during the linking stage was unimportant. Then, today, I tried to link from c++ code to c code. I wrapped all the C headers in an extern "C" block but the linker still had difficulties finding symbols which I knew were in the C object archives.

Perplexed, I created a relatively simple example to isolate the linking error but much to my surprise, the simpler example linked without any problems.

After a little trial and error, I found that by emulating the linking pattern used in the simple example, I could get the main code to link OK. The pattern was object code first, object archives second eg:

g++ -o serverCpp serverCpp.o algoC.o libcrypto.a

Can anyone shed some light on why this might be so?. I've never seen this problem when linking ordinary c++ code.

Answer 1

The order you specify object files and libraries is VERY important in GCC - if you haven't been bitten by this before you have lead a charmed life. The linker searches symbols in the order that they appear, so if you have a source file that contains a call to a library function, you need to put it before the library, or the linker won't know that it has to resolve it. Complex use of libraries can mean that you have to specify the library more than once, which is a royal pain to get right.