Understanding pointer structs in C

Question

I am trying to understand an assignment I have before I have to take a final. I am trying to understand what exactly I am declaring. So in a given file the typedef struct's are declared as so:

(Struct Declaration)

/** The following two structs must be defined in your <gamename>.c file **/
typedef struct game_position_t *game_position;

/* move struct must code enough information to reverse the move, given the resulting position */
typedef struct move_t *move;

I have then built the structs out as so (yes this has to be separated just because it is interfaced programming):

(Struct Definition)

/** The following two structs must be defined in your <gamename>.c file **/
struct game_position_t {
  int mathy;
  int numrows;
  int *sizes;
};

/* move struct must code enough information to reverse the move, given the resulting position */
struct move_t {
  int rownum;
  int move_size;
};

Then an example of a functions and declaration of game_position for example is:

(Example Function)

/* return the starting position, NULL if error */
game_position starting_position(int me_first, int argc, char **argv) {

  if (argc < 3) {
    printf("\n\nToo few arguments, see help below\n\n");
    game_help(argv[0]);
    return NULL;
  }

  int mathy;
  if (strcmp(argv[2],"search")==0)
    mathy = 0;
  else if (strcmp(argv[2],"mathy")==0)
    mathy = 1;
  else {
    printf("\n\nSecond argument must be \"search\" or \"mathy\", see help below\n\n");    
    game_help(argv[0]);
    return NULL;
  }    

  int play_default = (argc==3);

  if (play_default) printf("\n\nOK, we will play the default game of 7 5 3 1\n\n");

  int defaultgame[4] = {7,5,3,1};
  game_position result = malloc(sizeof(struct game_position_t)*1);

  result->mathy = mathy;
  if (result) {
    result->numrows = (play_default ? 4 : argc-3);
    result->sizes = malloc(sizeof(int)*(result->numrows));
    int row;
    for (row=0; row<(result->numrows); row++)
      (result->sizes)[row] = (play_default ? defaultgame[row] : strlen(argv[row+2]));
  }

  return result;
}

So my main misunderstanding is when using a struct declaration in this manner, specifically putting the * before the name like this, typedef struct move_t *move;. Is that previous line saying move it a struct pointer or dereferencing move? Continuing from that. When defining them I just use the struct name such as struct move_t. I don't fully understand how they are linking together and in what matter. Then inside the function I just declare game_position, but still need to use a derefencer, 'p->`, to access it fields. So if someone could explain to me when these struct variables are points to structs and when they are the actual struct.

An example of my misunderstanding is that in the Example Function after result was declared. I first thought to use the . operator to access and set it's fields. I then changed it due to compiler errors, but now I want to understand my misunderstanding. And why did I I have to malloc game_position_t and not game_position?

pjhades · Accepted Answer · 2015-05-03 02:06:12Z

2

typedef defines a type, so typedef struct move_t *move defines a new type named move, which is a pointer type, pointing to struct move_t. So after this if you define a variable with move ptr, ptr will have a pointer type so that you should use the syntax of accessing members through a pointer. When allocating memory for it, of course you have to specify the exact size of the structure other than the size of a pointer, that's sizeof(struct move_t)

answered May 3, 2015 at 2:06

pjhades

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2 Comments

napkinsterror Over a year ago

Thanks. So helpful. would sizeof(*move) work? Just curious.

pjhades Over a year ago

@napkinsterror You may try it out if that works. There's a book called Expert C Programming that may help you more.

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