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If we try to compile with gcc, we get

gcc hwp.c -o hwp
mpi/hwp.c:1:10: fatal error: mpi.h: No such file or directory
 #include "mpi.h"
          ^~~~~~~
compilation terminated.

However, we can compile with mpicc

mpicc hwp.c -o hwp

Simply running the executable will run the serial program.

./hwp
Hello from rank 0 / 1

But when with mpirun, we can set the number of processes.

mpirun -np 3 ./hwp
Hello from rank 0 / 3
Hello from rank 1 / 3
Hello from rank 2 / 3