Ravi Joshi Ravi Joshi - 25 days ago 5
C++ Question

Why does the order of passing parameters to g++ matter

Recently, I was trying to build an application, which uses some libraries, available in form of shared object files. I wasted lot of time in compiling the CPP code and it didn't work.

Below is the command, previously I was trying to compile the code-

g++ -I/opt/ros/indigo/include/ -I/usr/include/eigen3/ -L/opt/ros/indigo/lib/ -lorocos-kdl -lkdl_parser test.cpp -o test


The above command always shows many
undefined references
errors. Just for the curiosity, I changed the order of parameters. Below is the command, which is working-

g++ -L/opt/ros/indigo/lib -I/opt/ros/indigo/include -I/usr/include/eigen3 test.cpp -lorocos-kdl -lkdl_parser -o test


I posted the complete code and solution here.

My question is why does the order of passing parameters to g++ matter? Is there any alternative to avoid such problems in future?

Answer

Generally the order of arguments doesn't mater, but there are of course exceptions. For example if you provide multiple -O flags it will be the last one that is used, the same for other flags.

Libraries are a little different though, because for them the order is significant. If object file or library A depends on library B, then A must come before B on the command line. This is because of how the linker scans for symbols: When you use a library the linker will check if there are any symbols that could be resolved. Once this scan is over the library is discarded and will not be searched again.

This means when you have -lorocos-kdl -lkdl_parser test.cpp the linker will scan the libraries orocos-kdl and kdl_parser first, notice that there aren't dependencies on these library, no symbols from the libraries are needed, and continue with the object file generated by the source file.

When you change the order to test.cpp -lorocos-kdl -lkdl_parser the linker will be able to resolve the undefined symbols referenced by test.cpp when it comes to the libraries.