Dynamic Memory allocation of array inside structure in C

Question

I'm doing dining-philosopher problem in C for assignment. And got stuck very begining of my code.

I decided each philosopher to be structure, and forks to be int array.

But I can't use global variable in this assignment.

So, I have to include shared variable in philosopher structure to pass them for arguments of thread routine.

Here is my problem - how to include int array in structure if I can't know proper size of them when initializing?

My plan is just include pointer variable in structure then allocate array's address using &.

But It doesn't work :

#include <stdlib.h>

/* inside structure*/
typedef struct s_share {
    int **forks;
} t_share;

/* outside structure */
typedef struct s_philo {
    t_share *share;
} t_philo;


int main(void)
{
    t_philo *philo;
    int     *forks;
    int     i;

    i = 0;

    /* malloc structure arrary philo, size = 10 */
    philo = (t_philo *)malloc(sizeof(t_philo) * 10);

    /* malloc int arrary forks, size = 100 */
    forks = (int *)malloc(sizeof(int) * 100);
    while (i < 10)
    {
        philo[i].share->forks = &forks; //error
        i++;
    }

}

Output : segmentation fault

I tested share->forks size like this :

printf("size of forks : %ld\n", sizeof(philo->share->forks));

Output was 8. It's enough size to store int * pointer. Through this I know It's not the memory allocation problem.

Then what is problem? Can someone check this for me?

Edit : When I try to malloc directly philo->share->forks, I got same error.

typedef struct s_share {
    int *forks;
} t_share;

typedef struct s_philo {
    t_share *share;
} t_philo;


int main(void)
{
    t_philo *philo;
    int     *forks;
    int     i;

    i = 0;
    philo = (t_philo *)malloc(sizeof(t_philo) * 10);
    while (i < 10)
    {
       philo[i].share->forks = (int *)malloc(sizeof(int) * 100); //error
        i++;
    }
}

I thought it's because when philo initialized, sizeof operator calculated forks's memroy to be 8 - which required for pointer.

Is there something wrong?

Edit 2 : To clear my question,

It's easy to solve this problem, if I write size of array in structure definition.

typedef struct s_share {
    int forks[100];
} t_share;

typedef struct s_philo {
    t_share *share;
} t_philo;

but according to my assignmet I have to get philosopher's number from cmd. So I can't do that. Above is simple version of my origin code

Sorry, Edit 2 is wrong :


typedef struct s_share {
    int forks[100];
} t_share;

typedef struct s_philo {
    t_share *share;
} t_philo;


int main(void)
{
    t_philo *philo;
    t_share *share;
    int     *forks;
    int     i;

    i = 0;
    philo = (t_philo *)malloc(sizeof(t_philo) * 10);
    while (i < 10)
    {
        philo[i].share->forks[i] = 1;
        i++;
    }
}

Output

zsh: segmentation fault  ./a.out

I still got segfault when I write array size in struct definition.

I used calloc to initialize all member in my struct but same error occurs :

typedef struct s_share {
    int **forks;
} t_share;

typedef struct s_philo {
    t_share *share;
} t_philo;


int main(void)
{
    t_philo *philo;
    t_share *share;
    int     *forks;
    int     i;

    i = 0;
    philo = (t_philo *)calloc(10, sizeof(t_philo));
    forks = (int *)calloc(100, sizeof(int));
    while (i < 10)
    {
        philo[i].share->forks = &forks; //error
        i++;
    }
}

Edit 4: I finally found error. It's because I didn't malloc 'share' struct in philo struct

typedef struct s_share {
    int **forks;
} t_share;

typedef struct s_philo {
    t_share *share;
} t_philo;


int main(void)
{
    t_philo *philo;
    int     *forks;
    int     i;

    i = 0;
    philo = (t_philo *)malloc(sizeof(t_philo) * 10);
    forks = (int *)malloc(sizeof(int) * 100);
    while (i < 10)
    {
        philo[i].share = (t_share *)malloc(sizeof(t_share)); //here
        philo[i].share.forks = &forks;
        i++;
    }
}

That one line -allocating struct share- solved problem.

Or, I can modify philo struct definition like this :

typedef struct s_philo {
    t_share share; //not pointer, just struct
} t_philo;

In this way, I can automatically malloc struct share.

I got confused in this point. Thanks for helping!

Why do you even need structs for the philosophers? You can have an array that is the forks and the value represents which philosopher is using the fork? — LtWorf
– LtWorf, Commented Jan 18, 2022 at 6:40
I edited my question after Sarker's comment. It was my first effort but failed — acho
– acho, Commented Jan 18, 2022 at 6:42
I have to use structure because I get sevral arguments form cmd that affect philosophers's state - like eating time. I posted simple version in my question because it's too complicated — acho
– acho, Commented Jan 18, 2022 at 6:44

akatz · Accepted Answer · 2022-01-18 07:18:53Z

0

this line

philo[i].share->forks

Is dereferencing the pointer 'share' which is not set. You called malloc and did not set any values, so the data inside your allocated buffer is 'garbage' data.

answered Jan 18, 2022 at 7:18

akatz

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3 Comments

acho Over a year ago

Oh, I didn't catch that! Thanks alot. But I still don't know how to modify my code. I try to declare t_share * variable and allocate it to philo.share, but it doesn't work. Can you help?

paladin Over a year ago

Just use calloc instead of malloc and you get zero initialized data.

acho Over a year ago

I malloced philo->share and code works well. Thank you both!

Neo · Accepted Answer · 2022-01-18 09:08:18Z

0

// add begin
t_share* new_share = (t_share*)malloc(sizeof(t_share));
philo[i].share = new_share;
// add end

// don't use &forks
philo[i].share->forks = forks; //error
i++;
// need forks++
forks++;

answered Jan 18, 2022 at 9:08

Neo

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