Question

I want to allocate memory for strucs such below

typedef struct {
int *buffer;
int length;
int dsn;
int handle;
} myStr;

which one is correct?

myStr *pStr = malloc(sizeof(myStr)+lenOfBuff);

or

myStr *pStr = malloc(sizeof(myStr));

I saw in some Examples that use the first one, but it seems a little strange for me. Does it have to allocate memory for data Buffer at the same time??

Answer 1

It depends.

If you want to change the length of the buffer after creation, you need to allocate it separately. If not, you can co-allocate the two blocks in a single malloc() call, which might be slightly more efficient.

The co-allocation would look like so:

myStr * mystr_create(size_t initial_size)
{
  myStr *ptr;

  ptr = malloc(sizeof *ptr + initial_size * sizeof *myStr->buffer);
  ptr->buffer = (int *) (ptr + 1);
  ptr->length = initial_size;
  ptr->dsn = ... something ...
  ptr->handle = ... something ... *

  return ptr;
}