CODEWITHSUNDEEP
ciofwrite
OJFord
OJFord

Reputation: 11140

Writing array of structs to binary file using fwrite

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.

Upvotes: 2

Views: 10523

Answers (1)

amdixon
amdixon

Reputation: 3833

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

Upvotes: 2

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