Writing array of structs to binary file using fwrite

Question

I have:

pointfile = fopen("points.bin", "wb");
void    savepoints(point points[], int n, FILE f){
    fwrite(&points, sizeof(point), n, &f);
    return;
}
fclose(pointfile);

where typedef struct {float x; float y;} point;

and called by savepoints(buffer, npoints, *pointfile);

But nothing is written to the file. Can anyone spot my mistake? I can't see how to go about this, and other's I find searching either don't relate, or have merely got me this far.

does it make a difference, since I had ...*pointfile and ...&f? — OJFord
– OJFord, Commented Dec 14, 2013 at 10:24
Shouldn't it be fwrite(points instead of fwrite(&points? — starrify
– starrify, Commented Dec 14, 2013 at 10:26
@OllieFord Pass FILE* as suggested. FILE can be copied, but because it contains things like the current stream position, you may run into unexpected issues if you copy it, write using that file, then use the original copy outside of savepoints. — user2428400
– user2428400, Commented Dec 14, 2013 at 10:39

amdixon · Accepted Answer · 2013-12-14 11:34:12Z

Need to pass a FILE * as parameter like:

test.c

#include <stdio.h>
#include <stdlib.h>

typedef struct 
{
  float x,
        y;
}point;

/* function to save points data to myfile */
void save_points(point *points, int num_points, FILE *myfile)
{
  /* have to use points not (&points) here */
  fwrite(points, sizeof(point), num_points, myfile);
}

int main()
{
  FILE *pointfile;
  point *points; int num_points, index;
  pointfile = fopen("points.txt", "w");
  if(!pointfile)
  {
    fprintf(stderr, "failed to create file 'points.txt'\n");
    goto err0;
  }
  num_points = 10;
  points = malloc(num_points * sizeof(point));
  if(!points)
  {
    fprintf(stderr, "failed to alloc `points`\n");
    goto err1;
  }
  /* points are uninitialized but can still write uninitialized memory to file to test.. */
  save_points(points, num_points, pointfile);
  free(points);
  fclose(pointfile);
  return 0;

err1:
  fclose(pointfile);
err0:
  return 0;
}

Results

$ ./test
$ ls -l points.txt 
-rw-r--r-- 1 me me 80 Dec 14 22:24 points.txt

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