Thread: fwriting and freading binary (structs)

Here is my struct:

Code:

struct location {
 int houseNumber;
 char streetName[STRING_BUF];
 char city[STRING_BUF];
 char state[STATE_BUF];
 int zip;
};

struct person {
 char nameFirst[STRING_BUF];
 char nameLast[STRING_BUF];
 char number[STRING_BUF];
 struct location address;
 struct person *next;
} entry[100];

typedef struct person PERSON;
typedef PERSON *PERSON_PTR;

And here is the fwrite:

Code:

void write_to_disk (PERSON_PTR first)
{
 PERSON_PTR cur_ptr;
 cur_ptr = first;

 if ( (fp = fopen(filename, "wb") ) == NULL )
 {
  fprintf(stderr, "Error opening file %s\n", filename);
  exit(1);
 }

 while (cur_ptr != NULL)
 {
  if ( fwrite(cur_ptr, sizeof(PERSON), 1, fp) != 1)
  {
   fprintf(stderr, "Error writing to %s.", filename);
   exit(1);
  }
  cur_ptr = cur_ptr->next;
 }

 fclose(fp);
}

But now how to read the file? I can post my attempt but wanted to keep it streamline. I actually don't even know if I'm writing to the file correctly.

You could do something like this:

Code:

int write_persons_list( char *filename, PERSON_PTR pptr )
{
  PERSON_PTR p;
  FILE *f;
  uint32_t count;
  
  if ( ! ( f = fopen( filename "w" ) ) )
    return 0;

  count = 0;
  if ( fwrite( &count, sizeof count, 1, f ) != 1 )
  {
error:
    fclose( f );

    // delete the file in case of error!
    unlink( filename );

    return 0;
  }

  p = pptr;
  while ( p )
  {
    // don't write p->next!
    if ( fwrite( p, offsetof(PERSON,next), 1, f ) != 1 )
      goto error;

    p = p->next;
    count++;
  }

  // Write the number of elements...
  fseek( f, 0, SEEK_SET );
  if ( fwrite( &count, sizeof count, 1 f ) != 1 )
    goto error;

  fclose( f );

  return 1;
}

// Example call:
//
//  PERSON_PTR *plist;
//  if ( ! read_persons_list( "./list.bin", &plist ) ) { ... error handling... }
int read_persons_list( char *filename, PERSON_PTR *pp )
{
  PERSON *p;
  FILE *f;
  uint32_t count;

  if ( ! ( f = fopen( filename, "r" ) ) )
    return 0;

  if ( fread( &count, sizeof count, 1, f ) != 1 )
  {
error:
    fclose( f );
    return 0;
  }

  if ( count == 0 )
    goto error;

  if ( ! ( *pp = malloc( count * sizeof( PERSON ) ) )
    goto error;

  p = *pp;
  while ( count-- )
  {
    // don't read p->next!
    if ( fread( p, offsetof( PERSON, next ), 1, f ) != 1 )
    {
      free( *pp );
      goto error;
    }

    p->next = ++p;
  }

  // the last one must be NULL.
  (p - 1)->next = NULL;

  fclose( f );

  return 1;
}

Thanks all. I wrote this read attempt before viewing your advice. I don't expect you to go through it. I will look over your code, @flp1969, as mine errors out during fread. I am obviously a beginner. Thanks for taking the time.

Code:

PERSON_PTR read_file (PERSON_PTR first)
{
 PERSON_PTR cur_ptr;
 cur_ptr = first;

if ( (fp = fopen(filename, "r+b") ) == NULL )
 {
  fprintf(stderr, "Error opening file %s\n", filename);
  exit(1);
 }

 while (!feof(fp))
 {
  if ( fread(cur_ptr, sizeof(PERSON), 1, fp) != 1)
  {
   fprintf(stderr, "Error reading from %s.", filename);
   exit(1);
  }
strcpy(entry[i].nameFirst, cur_ptr->nameFirst);
  strcpy(entry[i].nameLast, cur_ptr->nameLast);
  strcpy(entry[i].number, cur_ptr->number);
  entry[i].address.houseNumber = cur_ptr->address.houseNumber;
  strcpy(entry[i].address.streetName, cur_ptr->address.streetName);
  strcpy(entry[i].address.city, cur_ptr->address.city);
  strcpy(entry[i].address.state, cur_ptr->address.state);
  entry[i].address.zip = cur_ptr->address.zip;

  first = add_to_list (entry[i], first);
  i++;
 }
 fclose(fp);

 return (first);
}