C 结构中的丢失数据

Question

mat.h

struct cell {
    int first;
    int second;
};
struct mat {
    struct cell cells[ROWS][COLS];
};

buf.h

struct act {
    void (*fct)();
};

struct q {
    int head;
    int top;
    int size;
    struct act act[];
};

buf.c

void add(struct q *b, void (*o)()) {
    b->actions[b->top].fct = o;
    b->top++;
    
    if(b->top == b->size) b->top = 0;
}

Main.c

int main(int argc, char **argv) {
    struct q b;
    struct mat m;

    add(&b, init);
    get(&b, &m);
}

mat.c

void init(struct q *b, struct mat *m) {
    srand(time(NULL));

    for(int i = 0; i < ROWS; i++) {
        for(int j = 0; j < COLS; j++) {
            m->cells[i][j].first = rand() %5;
            m->cells[i][j].second = rand() %2;
        }
    }

    addQueue(b, printM);
}

Hi,

I lose data in a structure array when I add function pointer on it. I have a buffer which has a void function pointer. With add function, I add the function address and execute it on main.c. But the problem is, every time I add a new function, there is a cell loses its data (m.cells[0][3].first). I did not find how to avoit it.

Thanks.

Answer 1

A variable of type struct q has no room to store any elements of the flexible array member actions . In order to be useful, the struct q needs to part of a larger block of memory with enough room at the end for the number of required elements.

You could create a function to allocate a struct q from dynamically allocated storage with enough space for a specified number of elements:

#include <stddef.h>
#include <stdlib.h>

struct q *create_q(int size) {
    struct q *b = malloc(offsetof(struct q, actions[size]));

    if (b) {
        b->head = 0;
        b->top = 0;
        b->size = size;
    }
    return b;
}

For consistency, a function can be added to free the struct q . That is pretty simple now, but having a specific function allows the internals of the implementation to be changed at a later time if need be:

void free_q(struct q *b) {
    free(b);
}