Invalid application of sizeof to incomplete type with a struct

Question

I have a struct where I put all the information about the players. That's my struct:

struct player{
   int startingCapital;
   int currentCapital;
   int startingPosition;
   int currentPosition;
   int activePlayer; 
   int canPlay;      
};

And that's my main:

#include <stdio.h>
#include <stdlib.h>
#include "header.h"


int main(int argc, char *argv[])
{  int s,i,numOfPlayers;
   struct player *players;
    printf("Give the number of players: \n");
    scanf("%d",&numOfPlayers);

    players = (struct player *)calloc(numOfPlayers,sizeof(struct player));


   system("PAUSE"); 
  return 0;
}

I'm asking the user to give the number of players and then I try to allocate the needed memory. But I'm getting this compiler error that I can't figure out:

invalid application of `sizeof' to incomplete type `player'  

Answer 1

It means the file containing main doesn't have access to the player structure definition (i.e. doesn't know what it looks like).

Try including it in header.h or make a constructor-like function that allocates it if it's to be an opaque object.

EDIT

If your goal is to hide the implementation of the structure, do this in a C file that has access to the struct:

struct player *
init_player(...)
{
    struct player *p = calloc(1, sizeof *p);

    /* ... */
    return p;
}

However if the implementation shouldn't be hidden - i.e. main should legally say p->canPlay = 1 it would be better to put the definition of the structure in header.h.

Answer 2

Your error is also shown when trying to access the sizeof() of an non-initialized extern array:

extern int a[];
sizeof(a);
>> error: invalid application of 'sizeof' to incomplete type 'int[]'

Note that you would get an array size missing error without the extern keyword.

Answer 3

I am a beginner and may not clear syntax. To refer above information, I still not clear.

/*
 * main.c
 *
 *  Created on: 15 Nov 2019
 */

#include <stdio.h>
#include <stdint.h>
#include <string.h>

#include "dummy.h"

char arrA[] = {
        0x41,
        0x43,
        0x45,
        0x47,
        0x00,
};

#define sizeA sizeof(arrA)

int main(void){

    printf("\r\n%s",arrA);
    printf("\r\nsize of = %d", sizeof(arrA));
    printf("\r\nsize of = %d", sizeA);

    printf("\r\n%s",arrB);
    //printf("\r\nsize of = %d", sizeof(arrB));
    printf("\r\nsize of = %d", sizeB);


    while(1);

    return 0;
};

/*
 * dummy.c
 *
 *  Created on: 29 Nov 2019
 */


#include <stdio.h>
#include <stdint.h>
#include <string.h>

#include "dummy.h"


char arrB[] = {
        0x42,
        0x44,
        0x45,
        0x48,
        0x00,
};

/*
 * dummy.h
 *
 *  Created on: 29 Nov 2019
 */

#ifndef DUMMY_H_
#define DUMMY_H_

extern char arrB[];

#define sizeB sizeof(arrB)

#endif /* DUMMY_H_ */

15:16:56 **** Incremental Build of configuration Debug for project T3 ****
Info: Internal Builder is used for build
gcc -O0 -g3 -Wall -c -fmessage-length=0 -o main.o "..\\main.c" 
In file included from ..\main.c:12:
..\main.c: In function 'main':
..\dummy.h:13:21: **error: invalid application of 'sizeof' to incomplete type 'char[]'**
 #define sizeB sizeof(arrB)
                     ^
..\main.c:32:29: note: in expansion of macro 'sizeB'
  printf("\r\nsize of = %d", sizeB);
                             ^~~~~

15:16:57 Build Failed. 1 errors, 0 warnings. (took 384ms)

Both "arrA" & "arrB" can be accessed (print it out). However, can't get a size of "arrB".

What is a problem there?

1. Is 'char[]' incomplete type? or
2. 'sizeof' does not accept the extern variable/ label?

In my program, "arrA" & "arrB" are constant lists and fixed before to compile. I would like to use a label(let me easy to maintenance & save RAM memory).

Answer 4

I think that the problem is that you put #ifdef instead of #ifndef at the top of your header.h file.

Answer 5

Really late to the party here, but a special case of the reasons for this error cited above would simply be to reference a structure with sizeof() above where the structure is defined:

int numElements = sizeof(myArray)/sizeof(myArray[0]);
.
.
.
myArray[] = 
{
    {Element1},
    {Element2},
    {Element3}
};

Answer 6

The cause of errors such as "Invalid application of sizeof to incomplete type with a struct ... " is always lack of an include statement. Try to find the right library to include.