Even simpler than this, we could just forward all calls to the MPI_Count interface (see below).  The int count argument should type convert to MPI_Count without issue.  Note that it still needs to be a function-like macro so that function pointers work.

#include <stdio.h>

  

typedef int MPI_Datatype;

typedef int MPI_Comm;

int MPI_Send(const void* buf, int count, MPI_Datatype datatype, int dest,

             int tag, MPI_Comm comm)

{

    printf("MPI_Send(count = %d)\n", count);

    return 0;

}

int MPI_Send_x(const void* buf, long long count, MPI_Datatype datatype, int dest,

               int tag, MPI_Comm comm)

{

    printf("MPI_Send_x(count = %lld)\n", count);

    return 0;

}

#define MPI_Send(buf, count, datatype, dest, tag, comm) MPI_Send_x(buf, count, datatype, dest, tag, comm)

int main(int argc, char *argv[]) {

    /* 8589934592LL == 2^33 */

    long long i = 8589934592LL + 11;

    int ret;

    int (*snd_ptr)(const void*, int, MPI_Datatype, int, int, MPI_Comm) = &MPI_Send;

    ret = MPI_Send(NULL, i, 0, 0, 0, 0);

    ret = MPI_Send(NULL, 5, 0, 0, 0, 0);

    ret = (*snd_ptr)(NULL, i, 0, 0, 0, 0);

    ret = (*snd_ptr)(NULL, 5, 0, 0, 0, 0);

    return 0;

}