Hi, mpiuni.h does not define MPI_UNSIGNED_LONG_LONG, which leads to a problem in defining MPIU_SIZE_T here in petscsys.h. #if (PETSC_SIZEOF_SIZE_T) == (PETSC_SIZEOF_INT) #define MPIU_SIZE_T MPI_UNSIGNED #elif (PETSC_SIZEOF_SIZE_T) == (PETSC_SIZEOF_LONG) #define MPIU_SIZE_T MPI_UNSIGNED_LONG #elif (PETSC_SIZEOF_SIZE_T) == (PETSC_SIZEOF_LONG_LONG) #define MPIU_SIZE_T MPI_UNSIGNED_LONG_LONG #else #error "Unknown size for size_t! Send us a bugreport at [email protected]" #endif In case this is important, I'm on windows, no mpi and can send more details if needed. I'm using a slightly older version of petsc, but I see that the next branch does not contain the required definition in mpiuni.h https://bitbucket.org/petsc/petsc/src/d93c087c991ba51c9078189a5f0b6f06a51774... Chetan