Reading and writing binary data

/* 2202.c: Reading and writing binary data */
#include <stdio.h>

enum {SUCCESS, FAIL, MAX_NUM = 3};

void DataWrite(FILE *fout);
void DataRead(FILE *fin);
int ErrorMsg(char *str);

main(void)
{
    FILE *fptr;
    char filename[]= "double.bin";
    int reval = SUCCESS;

    if ((fptr = fopen(filename, "wb+")) == NULL){
        reval = ErrorMsg(filename);
    }
    else {
        DataWrite(fptr);
        rewind(fptr); /* reset fptr */
        DataRead(fptr);
        fclose(fptr);
        }

return reval;
}
/* function definition */
void DataWrite(FILE *fout)
{
    int i;
    double buff[MAX_NUM] = {123.45,567.89,100.11};

    printf("The size of buff: %d-byte\n", sizeof(buff));
    for (i=0; i<MAX_NUM; i++){
        printf("%5.2f\n", buff[i]);
        fwrite(&buff[i], sizeof(double), 1, fout);
    }
}
/* function definition */
void DataRead(FILE *fin)
{
    int i;
    double x;

    printf("\nRead back from the binary file:\n");
    for (i=0; i<MAX_NUM; i++){
        fread(&x, sizeof(double), (size_t)1, fin);
        printf("%5.2f\n", x);
    }
}
/* function definition */
int ErrorMsg(char *str)
{
    printf("Cannot open %s.\n", str);
    return FAIL;
}