Link errors while using G++ for MPI code - g++

Question

My code is as simple as this:
#include <mpi.h>
int main(int argc, char**args) {
MPI_Init(&argc, &args);
int mpiSize;
int mpiRank;
MPI_Comm_size(MPI_COMM_WORLD, &mpiSize);
MPI_Comm_rank(MPI_COMM_WORLD, &mpiRank);
MPI_Finalize();
}
I first compile it to object file:
g++ -c src/mpitest.cpp -o src/mpitest.o
Then I can easily use mpicxx:
mpicxx src/mpitest.o -o mpi
But I want to use g++ instead, because It's easier for automake, so I tried:
mpicxx src/mpitest.o -o mpi -show
It prints out:
g++ src/mpitest.o -o mpi -I/usr/local/include -L/usr/local/lib -L/usr/local/lib -lmpichcxx -lmpich -lopa -lpthread -lrt
And yes, that command actually does the same thing successfully. However, If I tried to change it to (I just change the object file and output to the end):
g++ -I/usr/local/include -L/usr/local/lib -L/usr/local/lib -lmpichcxx -lmpich -lopa -lpthread -lrt src/mpitest.o -o mpi
Which is what automake does when it adds LDFLAGS, the g++ start complaining...
src/mpitest.o: In function `main':
..src/mpitest.cpp:5: undefined reference to `MPI_Init'
../src/mpitest.cpp:8: undefined reference to `MPI_Comm_size'
../src/mpitest.cpp:9: undefined reference to `MPI_Comm_rank'
../src/mpitest.cpp:10: undefined reference to `MPI_Finalize'
collect2: ld returned 1 exit status
What happens with g++ ? I couldn't figure out. Please enlighten me. Why the order here matter at all ? What is the proper way to do what I want from the beginning ?
Thanks a lot
p/s : g++ --version
g++ (Ubuntu 4.4.3-4ubuntu5) 4.4.3
mpi is mvapich2

Answer 1

I didn't really remember, maybe since version 4, library to link with -l must be passed after all source and object files.