Using fwrite to write 1D array as a single block versus element-wise

Question

I wrote some code which simply writes an array of doubles to a binary file

#include<stdio.h>

void main(void) {
    // Open binary files
    FILE *pFileResultsBin1;
    FILE *pFileResultsBin2;
    pFileResultsBin1 = fopen("results1.bin","wb+");
    pFileResultsBin2 = fopen("results2.bin","wb+");
    // Create array
    double arr[3];
    arr[0] = 1.0;
    arr[1] = 2.0;
    arr[2] = 3.0;
    // Write array to binary files 10 times
    int i;
    // Method using sizeof(arr)
    for(i=0;i<10;i++){
        fwrite(&arr, sizeof(arr),1,pFileResultsBin1)
    }
    // Method using sizeof(arr[0])
    for(i=0;i<10;i++){
        fwrite(&arr, sizeof(arr[0]),3,pFileResultsBin2)
    }
    // Close files
    fclose(pFileResultsBin1);
    fclose(pFileResultsBin2);
}

When reading examples on the internet for writing arrays usually the method of sizeof(arr[0]) is used, but is there any disadvantage in using sizeof(arr)? I tried both methods and both were read correctly using MATLAB:

FID1 = fopen(result1.bin);
FID2 = fopen(result1.bin);
A = fread(FID1,[3 inf],'*double','ieee-le');
B = fread(FID2,[3 inf],'*double','ieee-le');

Kind regards,

EJG

PS. I did not try to compile this specific example since I'm on a computer without compiler.

Answer 1

Have a look at the man-page http://www.freebsd.org/cgi/man.cgi?query=fwrite&apropos=0&sektion=0&manpath=FreeBSD+10.0-RELEASE&arch=default&format=html.

The return value of fwrite is the number of objects written. Hence, the first fwrite in your code will return 1 and the second 3, if the write is successful.

Besides that the calls are equivalent.