The address will always evaluate as true warning

Code:

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#include <stdio.h>
#include <stdbool.h>


#define M 5
#define N 5


// 2 - free spot ; -1 - vertical line ; -2 - horizontal line ; 1 - X ; 0 - 0


int board[M][N];




void printBoard(int board[M][N])
{
    int i,j;


    for(i = 0; i < 5; i=i+2)
    {
        for(j = 1; j <= 3; j = j+2)
        {
            board[i][j] = -1;
        }
    }


    for(j = 0; j < 5; j++)
    {
        board[1][j] = -2;
        board[3][j] = -2;
    }


    for(i = 0; i <= 4; i=i+2)
    {
        for(j= 0; j <= 4; j=j+2)
        {
            board[i][j] = 2;
        }
    }


    for(i = 0; i < 5; i++)
    {
        for(j = 0; j < 5; j++)
        {
            if(board[i][j] == -1)
            {
                putchar('|');
            }
            else if(board[i][j] == 2)
            {
                putchar(' ');
            }
            else if(board[i][j] == -2)
            {
                putchar('-');
            }
            else if(board[i][j] == 1)
            {
                putchar('X');
            }
            else if(board[i][j] == 0)
            {
                putchar('0');
            }
        }
        putchar('\n');
    }
}




bool isEmpty(int board[M][N], int i, int j)
{
    if(board[i][j] == 2)
        return true;
    else
        return false;
}


void place_X(int board[M][N], int i, int j)
{
    if(isEmpty(board,i,j))
        board[i][j] = 1;
    else
        printf("The block has already been used!\n");
}


void place_zero(int board[M][N], int i, int j)
{
    if(isEmpty(board,i,j))
        board[i][j] = 0;
    else
        printf("The block has already been used!\n");
}


bool checkBoardFull(int board [M][N])
{
    int i,j;


    for(i = 0; i < 5; i++)
    {
        for(j = 0; j < 5; j++)
        {
            if(isEmpty(board, i, j))
                return false;
        }
    }


    return true;
}




int main(int argc, char **argv)
{
    bool running = true;
    char user_1[20], user_2[20];
    int i,j;


    printf("Enter the name of player 1 (X):\n");
    scanf("%19s", user_1);
    printf("Enter the name of player 2 (0):\n");
    scanf("%19s", user_2);
    printf("\n\n");
    printf("\t\t\t||------------------------------------||\n");
    printf("\t\t\t||------------------------------------||\n");
    printf("\t\t\t||                                    ||\n");
    printf("\t\t\t||                                    ||\n");
    printf("\t\t\t||                                    ||\n");
    printf("\t\t\t||          %s  vs  %s          ||\n", user_1, user_2);
    printf("\t\t\t||                                    ||\n");
    printf("\t\t\t||                                    ||\n");
    printf("\t\t\t||                                    ||\n");
    printf("\t\t\t||------------------------------------||\n");
    printf("\t\t\t||------------------------------------||\n");
    printf("\n\n");


    printBoard(board);


    while(running)
    {
        if(!checkBoardFull)
        {
            printf("Pick i position:\n");
            scanf("%d", &i);
            printf("Pick j position:\n");
            scanf("%d", &j);
       
            place_X(board, i , j);
            printBoard(board);
        }
        running = false;
        printBoard(board);
    }
    return 0;
}

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I'm trying to create a tic tac toe game, but I'm getting a strange warning:
warning: the address of ‘checkBoardFull’ will always evaluate as ‘true’ [-Waddress]
137 | if(!checkBoardFull)
Any idea why?

You forgot the argument list on that function call. Change line 157 to:

Code:

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              if (!checkBoardFull(board))

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That should get you farther along in your testing.

PS: The reason for that warning is that a C converts function name alone (without a parenthesized argument list) to a pointer to the function. That is usually implemented as the memory address of the function at runtime, and any non-NULL pointer evaluates as "true" when used in a "boolean context" (if or while conditions, or anywhere else that a "true/false" test is needed.)