source: CIVL/examples/contracts/contractsMPI/civl_mpi_collectives/gather.c

#include<mpi.h>
#include<civl-mpi.cvh>
#include<civlc.cvh>
#include<string.h>
#include<stdio.h>

#pragma CIVL ACSL

/*@ 
  @ \mpi_collective(comm, P2P) :
  @   requires \mpi_agree(root) && \mpi_agree(sendcount * \mpi_extent(sendtype));
  @   requires sendcount * \mpi_extent(sendtype) >= 0;
  @   requires recvcount * \mpi_extent(recvtype) >= 0;
  @   requires 0 <= root && root < \mpi_comm_size;
  @   behavior imroot_not_inplace:
  @     assumes  \mpi_comm_rank == root && sendbuf != MPI_IN_PLACE;
  @     requires \mpi_valid(sendbuf, sendcount, sendtype);
  @     requires \mpi_valid(recvbuf, recvcount * \mpi_comm_size, recvtype);
  @     requires recvcount * \mpi_extent(recvtype) == sendcount * \mpi_extent(sendtype);
  @     assigns  \mpi_region(recvbuf, recvcount * \mpi_comm_size, recvtype);
  @     ensures  \mpi_equals(\mpi_region(\mpi_offset(recvbuf, root * recvcount, recvtype), recvcount, recvtype),
  @                          \mpi_region(sendbuf, sendcount, sendtype));
  @     waitsfor (0 .. \mpi_comm_size-1);
  @   behavior imroot_inplace:
  @     assumes  \mpi_comm_rank == root && sendbuf == MPI_IN_PLACE;
  @     requires \mpi_valid(recvbuf, recvcount * \mpi_comm_size, recvtype);
  @     assigns  \mpi_region(recvbuf, recvcount * \mpi_comm_size, recvtype);
  @     ensures  \mpi_equals(\old(
  @                            \mpi_region(\mpi_offset(recvbuf, root * recvcount, recvtype), recvcount, recvtype)),
  @                            \mpi_region(\mpi_offset(recvbuf, root * recvcount, recvtype), recvcount, recvtype));
  @     waitsfor (0 .. \mpi_comm_size-1);
  @   behavior not_root:
  @     assumes  \mpi_comm_rank != root;
  @     requires \mpi_valid(sendbuf, sendcount, sendtype);
  @     requires \on(root, recvcount * \mpi_extent(recvtype)) == sendcount * \mpi_extent(sendtype);
  @     assigns  \nothing;
  @     ensures  \mpi_equals(\mpi_region(sendbuf, sendcount, sendtype), 
  @                          \mpi_region(\mpi_offset(\on(root, recvbuf), \mpi_comm_rank * sendcount, sendtype), sendcount, sendtype));
  @*/
int gather(void* sendbuf, int sendcount, MPI_Datatype sendtype, 
           void* recvbuf, int recvcount, MPI_Datatype recvtype,
           int root, MPI_Comm comm){
  int rank, nprocs;
  MPI_Status status;
  int tag = 998;

  MPI_Comm_rank(comm, &rank);
  MPI_Comm_size(comm, &nprocs);
  if(root == rank) { 
    if(sendbuf != MPI_IN_PLACE) {
      void *ptr;
      
      ptr = $mpi_pointer_add(recvbuf, root * recvcount, recvtype);
      memcpy(ptr, sendbuf, recvcount * sizeofDatatype(recvtype));
    }
  } else
    MPI_Send(sendbuf, sendcount, sendtype, root, tag, comm);
  if(rank == root) {
    int real_recvcount;
    int offset;

    for(int i=0; i<nprocs; i++) {
      if(i != root) {
        void * ptr;

        offset = i * recvcount;
        ptr = $mpi_pointer_add(recvbuf, offset, recvtype);
        MPI_Recv(ptr, recvcount, recvtype, i, tag, comm,
                 &status);
      }
    }
  }
  return 0;
}