Using malloc properly

Question

I'm trying to put my directory pathname on a string variable, like this:

int main(int ac, char **av)
{
  char*dir;

  if(ac > 2)
    {
      dir = malloc(sizeof(*dir) * 512);
      getcwd(dir, sizeof(*dir));
      printf("dat dir is:\n");
      printf("%s\n", dir);
    }
}

I got a blank printed, but when I do something like that

int main(int ac, char **av)
{
  char dir[512];

  if(ac > 2)
    {
      // dir = malloc(sizeof(dir) * 512);                                                                
      getcwd(dir, sizeof(dir));
      printf("dat dir is:\n");
      printf("%s\n", dir);
    }
}

it is printed properly, why? Isn't my first malloc suppose to make my variable like dir[512]

Answer 1

In

getcwd(dir, sizeof(*dir))

the sizeof is producing a 1 because *dir refers to a single char. This isn't what you want. If you replace this with 512, all ought to work fine.

The idiomatic coding would be something like this:

int main(int ac, char **av)
{
  const int buf_size = 512;
  char *dir;

  if(ac > 2)
    {
      dir = malloc(buf_size);
      getcwd(dir, buf_size);
      printf("dat dir is:\n");
      printf("%s\n", dir);
    }
}

Note multiplying by sizeof(*dir) is a no-op because it always returns 1. That is, malloc allocates in units of char. So I've omitted the no-op.