Reading and writing one block at a time

/* 2104.c: Reading and writing one block at a time */
#include <stdio.h>

enum {SUCCESS, FAIL, MAX_LEN = 80};

void BlockReadWrite(FILE *fin, FILE *fout);
int ErrorMsg(char *str);

main(void)
{
    FILE *fptr1, *fptr2;
    char filename1[]= "test1.txt";
    char filename2[]= "test2.txt";
    int reval = SUCCESS;

    if ((fptr1 = fopen(filename1, "w")) == NULL){
        reval = ErrorMsg(filename1);
    }
    else if ((fptr2 = fopen(filename2, "r")) == NULL){
        reval = ErrorMsg(filename2);
    }
    else {
        BlockReadWrite(fptr2, fptr1);
        fclose(fptr1);
        fclose(fptr2);
    }

    return reval;
}
/* function definition */
void BlockReadWrite(FILE *fin, FILE *fout)
{
    int num;
    char buff[MAX_LEN + 1];

    while (!feof(fin)){
        num = fread(buff, sizeof(char), MAX_LEN, fin);
        buff[num * sizeof(char)] = '\0'; /* append a null character */
        printf("%s", buff);
        fwrite(buff, sizeof(char), num, fout);
    }
}
/* function definition */
int ErrorMsg(char *str)
{
    printf("Cannot open %s.\n", str);
    return FAIL;
}