Notes on char arrays

Working with Derived Datatypes in MPI and char arrays. in C requires:

Using the computation of displacements as in the code containing this link.

Encoding displacements taking into account char array buffering, as in the code below.

#include "mpi.h"
#include <stdio.h>
#include <string.h>

struct mytype {
    char name[29]; /*30 or 31 or 32 . if necessary, all have padding to make it divisable by 4*/
    int tests[2];
    char report[31];
    int final_grade[1] ;
};

int main(int argc, char *argv[])
{
    struct mytype astudent;
    int i, j, myrank;
    MPI_Init(&argc, &argv);
    MPI_Status status;
    MPI_Datatype MPI_MYTYPE;
    MPI_Datatype type[4] = { MPI_CHAR,MPI_INT, MPI_CHAR,MPI_INT};
    int blocklen[4] = {29, 2 ,31,1};
    MPI_Aint disp[4]={0,32,(32+2*sizeof(int)),(64+2*sizeof(int))};
    MPI_Type_create_struct(4, blocklen, disp, type, &MPI_MYTYPE);
    MPI_Type_commit(&MPI_MYTYPE);
    MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
    if (myrank == 0) {
        struct mytype bstudent; /* Just known to rank 0 */
        strncpy(bstudent.name, "Joe Jones", 30);
        bstudent.tests[0]=90;       bstudent.tests[1]=93;
        strncpy(bstudent.report, "excellent", 30);
        bstudent.final_grade[0]=95;
        printf("length of bstudent.name = %i\n",sizeof(bstudent.name));
        printf("length of bstudent.tests = %i\n",sizeof(bstudent.tests));
        printf("length of bstudent.report = %i\n",sizeof(bstudent.report));
        printf("length of bstudent.final_grade = %i\n",sizeof(bstudent.final_grade));
        printf("length of bstudent = %i\n\n",sizeof(bstudent));

        MPI_Send(&bstudent, 1, MPI_MYTYPE, 1, 123, MPI_COMM_WORLD);
    } else if (myrank == 1) {
        struct mytype cstudent; /* Just known to rank 1 */
        MPI_Recv(&cstudent, 1, MPI_MYTYPE, 0, 123, MPI_COMM_WORLD, &status);
        printf("length of cstudent.name = %i\n",sizeof(cstudent.name));
        printf("length of cstudent.tests = %i\n",sizeof(cstudent.tests));
        printf("length of cstudent.report = %i\n",sizeof(cstudent.report));
        printf("length of cstudent.final_grade = %i\n",sizeof(cstudent.final_grade));
        printf("length of cstudent = %i\n\n",sizeof(cstudent ));
        printf("Name: %s \n",cstudent.name);
        printf("Test Grades: %i   %i\n",cstudent.tests[0],cstudent.tests[1]);
        printf("Performance: %s \n",cstudent.report);
        printf("Grade on Final: %i \n",cstudent.final_grade[0]);
    }
    MPI_Finalize();
    return 0;
}

A Run


length of bstudent.name = 29
length of bstudent.tests = 8
length of bstudent.report = 31
length of bstudent.final_grade = 4
length of bstudent = 76

length of cstudent.name = 29
length of cstudent.tests = 8
length of cstudent.report = 31
length of cstudent.final_grade = 4
length of cstudent  = 76

Name: Joe Jones
Test Grades: 90   93
Performance:  excellent
Grade on Final: 95