placement of keyword struct in C

Question

I have few questions connected with struct and typedef, there is piece of code and I have marked some places where I'm not sure if the syntax is correct. I use Eclipse editor and it shows me when there is problem in compilation. I just don't understand why is sometimes keyword struct needed and sometimes not. I may have also some mistakes of using this keyword. So plese help me to understand it.

let's have struct

 typedef struct player
    {
        char *name; 
        int  gameId; 
        int  points; 
        int socketfd; //socket descriptor of player
        int state;
    } player_struct;

lets have another struct

#define PLAYERSLEN 2
typedef struct game{
    struct player_struct *players[PLAYERSLEN]; //PLACE1
    //some code

} game_struct;

let's have function

player_struct *create_player()  //PLACE2
    {
    player_struct *player;  //PLACE3

    //alokace pameti
    player = (player_struct *) malloc(sizeof(player_struct)); //PLACE4
    //PLACE5

    player->gameId = -1;
    player->points = 0;
    player->state = 0;

    return player;
    }

let's have function? In fact what does this definition mean?

void *( player_struct *player) //PLACE6
        {

        //some code

        }

Questions references:
PLACE1 - is this correct? why can't I use just player_struct *players[PLAYERSLEN]; ??
PLACE2 - it looks like there is not needed struct before player_struct , is it correct? why?
PLACE3 - it looks like there is struct also not needed, is it correct? why?
PLACE4 - it looks like there is struct also not needed, is it correct? why?
PLACE4 & PLACE5 I may should handle errors there, cause there is malloc so I should probably put all the line with PLACE4 to if and if the malloc fails I should put at PLACE 5 free(player). Am I right?
PLACE6 what could have mean this function or whatever it is? The code inside brackets which is not included here should delete the player .. I just don't understand the syntax of wrote function - what does it mean?
PLACE6 - again similar as previous why is not necessary put the keyword struct before player_struct at this line? is it correct?

Really thanks you for your time.

Answer 1

You're doing two things in the initial definition:

1. You're defining the structure with the type name struct player.
2. You're creating a typedef called player_struct, which is just an alias for struct player.

As such, either struct player or player_struct is correct (and more-or-less equivalent), but struct player_struct refers to a completely separate type, and is probably incorrect unless you're trying to confuse people.

Now, handling your six cases in order:

• PLACE1: As noted above, this code is actually wrong as it stands.
• PLACE2, PLACE3, PLACE4: All correct; see above for an explanation.
• PLACE5: Yes, you should probably check that player != NULL here just to be sure.
• PLACE6: Again, this is correct, and the reason why is above. I'm not sure what void *(...) is supposed to be, though -- if * is actually a function name then it's probably fine though.

Answer 2

It's because the labels are in two separate namespaces (yes, even in C). There's a special namespace for structs.

I like to declare the label in both namespaces, using this:

typedef struct mon_struct {
    int a;
} mon_struct;

then you can use either/both struct mon_struct mon = ... or just mon_struct mon = ....

Update

If you want to use just one label, then you can use one of the following:

struct mon_struct { int a; };
// requires namespace resolution using struct tag:
void f(struct mon_struct p);

// -= OR =-

// slightly awkward declaration
typedef struct { int a; } mon_struct;
// but the type exists in the global namespace,
// so we don't need to use the struct tag:
void f(mon_struct p);

but this becomes messy if you are using the struct in both C and C++, particularly when the declaration or implementations move between C and C++ translations without proper guarding (extern "C" { ...stuff... }).

so you might opt to declare it in both namespaces to minimise breakage:

typedef struct mon_struct { int a; } mon_struct;
void f(struct mon_struct p); // << ok
void f2(mon_struct p); // << ok

Answer 3

You don't need struct in PLACE1... as long the definition of player_struct is known, ie, declared before it, or from a .h before it.

Answer 4

1. No. player_struct is a typedef for struct player. You don't need the struct keyword in this case.
2. Correct. You created a type definition called player_struct which means struct player.
3. Same reason as 2.
4. Same reason as 2 & 3.
5. Yes, you should check to see if malloc() returns NULL. In addition, don't cast the return value from malloc(). Doing so can hide #include errors.
6. That's not a valid function definition. You don't need struct because of the typedef - same as above.

You may find this all makes more sense if you remove the words typedef, player_struct, and game_struct from your code entirely. Then once you get used to how that all works, you can reintroduce the typedefs and maybe cut down on some typing. As a quick example, you can break down your first definition into its components:

struct player
{
  char *name; 
  int  gameId; 
  int  points; 
  int socketfd; //socket descriptor of player
  int state;
};

typedef struct player player_struct;

Maybe that will help you make sense of it?