Question

How do I fwrite a struct containing an array

#include <iostream>
#include <cstdio>

typedef struct {
  int ref; 
  double* ary;
} refAry;

void fillit(double *a,int len){
  for (int i=0;i<len;i++) a[i]=i;
}

int main(){
  refAry a;
  a.ref =10;
  a.ary = new double[10];
  fillit(a.ary,10);
  FILE *of;
  if(NULL==(of=fopen("test.bin","w")))
     perror("Error opening file");
  fwrite(&a,sizeof(refAry),1,of);

  fclose(of);
  return 0;
}

The filesize of test.bin is 16 bytes, which I guess is (4+8) (int + double*). The filesize should be 4+10*8 (im on 64bit)

~$ cat test.bin |wc -c
16
~$ od -I test.bin 
0000000                   10             29425680
0000020
~$ od -fD test.bin -j4
0000004   0,000000e+00   7,089709e-38   0,000000e+00
                     0       29425680              0
0000020

thanks

Answer 1

You are writing the pointer (a memory address) into the file, which is not what you want. You want to write the content of the block of memory referenced by the pointer (the array). This can’t be done with a single call to fwrite. I suggest you define a new function:

void write_my_struct(FILE * pf, refAry * x)
{
    fwrite(&x->ref, sizeof(x->ref), 1, pf);
    fwrite(x->ary, sizeof(x->ary[0]), x->ref, pf);
}

You’ll need a similar substitute for fread.