Modifying data in a function which was passed by another function-CodePudding

My goal is to modify data1 and data2 using a function that was passed as an argument from another function. I tried but failed. Here below is my last attempt. Could you please tell me what I am doing wrong?

typedef struct data_structs
{
    int data1;
    int data2;
} DATA_STRUCT;

int inner_func(void** user_data)
{
    DATA_STRUCT *inner_data = (DATA_STRUCT *) (*user_data);
    inner_data->data1 = 1;
    inner_data->data2 = 2;

    return 0;
}

void func(void **user_data, int (*passed_func)(void**) )
{
    passed_func( (void**) &user_data );
}

int main(void)
{
    DATA_STRUCT *data = malloc(sizeof(DATA_STRUCT));
    func( (void**) &data, inner_func );
    printf("I expect 1 and 2 %d, %d\n", data->data1, data->data2);
}

CodePudding user response：

As has already been stated in the comments, your problem probably is with the void **.

Try this code instead (untested):

typedef struct data_structs
{
    int data1;
    int data2;
} DATA_STRUCT;

int inner_func(void *user_data)
{
    DATA_STRUCT *inner_data = (DATA_STRUCT *)user_data;
    inner_data->data1 = 1;
    inner_data->data2 = 2;

    return 0;
}

void func(void *user_data, int (*passed_func)(void *))
{
    passed_func(user_data);
}

int main(void)
{
    DATA_STRUCT data;
    func(&data, inner_func );
    printf("I expect 1 and 2 %d, %d\n", data.data1, data.data2);
}

CodePudding user response：

if you want to use a double-pointer, you should initialize your data structure with a pointer.

here's a way to do it, but I didn't test it, so it might have typos or mistakes:

typedef struct data_structs
{
    int data1;
    int data2;
} DATA_STRUCT;

void inner_func(DATA_STRUCT **inner_data)
{
    (*inner_data)->data1 = 1;
    (*inner_data)->data2 = 2;

    return;
}

void func(DATA_STRUCT **user_data, void (*passed_func)(DATA_STRUCT  **))
{
    passed_func(user_data);
}

int main(void)
{
    DATA_STRUCT *data= (DATA_STRUCT *)malloc(sizeof(DATA_STRUCT));
    func(&data, inner_func );
    printf("I expect 1 and 2 %d, %d\n", data->data1, data->data2);
    free(data);
}