Allocating dynamic array of struct on C.

Question

Allocating dynamic array of struct on C. Valgrind found this error: Use of uninitialised value of size 8. The error pops up while trying to access the struct member.

What is the way to avoid that?

void find_rate()
{
  int num_lines = 0;
  FILE * in;
  struct record ** data_array;
  double * distance;
  struct record user_record;

  in = open_file();

  num_lines = count_lines(in);

  allocate_struct_array(data_array, num_lines);

  data_array[0]->community_name[0] = 'h';       // the error is here
  printf("%c\n", data_array[0]->community_name[0]);

  fclose(in);
}

FILE * open_file()
{
  ..... some code to open file
  return f;
}

int count_lines(FILE * f)
{
  .... counting lines in file
  return lines;
}

Here is the way I allocate the array:

void allocate_struct_array(struct record ** array, int length)
{
  int i;

  array = malloc(length * sizeof(struct record *));

  if (!array)
    {
      fprintf(stderr, "Could not allocate the array of struct record *\n");
      exit(1);
    }

  for (i = 0; i < length; i++)
    {
      array[i] = malloc( sizeof(struct record) );

      if (!array[i])
    {
      fprintf(stderr, "Could not allocate array[%d]\n", i);
      exit(1);
    }
    }
}

which struct error? Can you also add the valgrind output?

IanNorton
– IanNorton

2011-12-10 07:31:36 +00:00
Commented Dec 10, 2011 at 7:31 — IanNorton
– IanNorton, Commented Dec 10, 2011 at 7:31
Can you paste the definition of the struct record?

Louis
– Louis

2011-12-10 07:51:09 +00:00
Commented Dec 10, 2011 at 7:51 — Louis
– Louis, Commented Dec 10, 2011 at 7:51

codaddict · Accepted Answer · 2011-12-10 07:28:11Z

3

Since you are passing the address of array to the function allocate_struct_array

You need:

*array = malloc(length * sizeof(struct record *));

And in the calling function you need to declare data_array as:

struct record * data_array;

and pass its address as:

allocate_struct_array(&data_array, num_lines);

answered Dec 10, 2011 at 7:28

codaddict

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4 Comments

AusCBloke Over a year ago

sizeof(struct record *)? Should be sizeof(struct record) or sizeof(**array).

user1090944 Over a year ago

if you meant like here - stackoverflow.com/q/8460874/1090944, didn't work.

user1090944 Over a year ago

@AusCBloke sizeof(struct record *) means allocation memory in size of pointer to struct record.

AusCBloke Over a year ago

@user1090944: I thought you asked for "Allocating dynamic array of struct ", not allocating an array of pointers to struct. And *array is of type struct record*, therefore the elements it's pointing to should be of type struct record and not struct record*.

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