指向函数中结构体数组的指针

Question

I had a problem with the pointers. I wanna read a binary file with a function, and then, use the read data in the main. The problem is that I had to pass a pointer to array of struct to can use the data in main.

The code is:

#define TMOLDEO 8
#define TAM 41


struct fichpiezas{
    int codPieza;
    float dimPieza;
    float costePieza[TMOLDEO];
};

int leer_fichero(struct fichpiezas *vpiezas[]);

int main(int argc, const char * argv[]) {

    struct fichpiezas vpiezas[TAM];

    leer_fichero(&vpiezas);

    for(int i = 0; sizeof(vpiezas)/sizeof(struct fichpiezas); i++){
        printf("Codigo pieza : %d\n", vpiezas[i].codPieza);
    }

    return 0;
}


int leer_fichero (struct fichpiezas *vpiezas[]){

    FILE *fich;
    struct fichpiezas pieza;
    int error_dev = 0, i = 0;
    if ((fich = fopen("piezas.bin", "rb")) == NULL){
        printf ( "Error en apertura del fichero para lectura \n " );
        error_dev = 1;
    } else{
        //Bucle mientras no sea fin de fichero
        while (! feof(fich)){
            fread (&pieza, sizeof(pieza), 1, fich);
            vpiezas[i] = &pieza;
            i++;
        }

        fclose (fich);
    }

    return error_dev;
}

Answer 1

Just change this

int leer_fichero (struct fichpiezas *vpiezas[])

to

int leer_fichero (struct fichpiezas *vpiezas)

and in your main()

leer_fichero(&vpiezas);

to

leer_fichero(vpiezas);

the array will automatically decay to a pointer when passed to the function. So you don't need to pass it's address to the function.

You have another problem, this assignment

vpiezas[i] = &pieza;

is a problem, because you are storing the address of the local variable pieza in the array, and the data will be gone when this function returns.

Aditionally the value of pieza is being overwritten in each iteration bu fread() and since you store the address of pieza instead of it's value, all the elements of the array will have the same value if it succeeded this way.

You need to copy the struct into the array element, and this line if you follow my advice above should change to

vpiezas[i] = pieza;

or the program will fail to compile, because the type of &pieza is struct fichpiezas * and that of vpiezas[i] now is struct fichpiezas after changing the function prototype.

Also, about while (! feof(fich)) there was an answer which explains in detail why that is wrong.

And one more thing, add a check i < TAM because you are in risk of overflowing the array otherwise.

Answer 2

This

int leer_fichero(struct fichpiezas * vpiezas[]);

defines vpiezas to be a pointer to a pointer to struct fichpiezas , as

int leer_fichero(struct fichpiezas * vpiezas[]);

is equivalent to

int leer_fichero(struct fichpiezas ** vpiezas);

To address the elements of the array you pass by doing

leer_fichero(&vpiezas);

do like this

(*vpiezas)[index]

inside of leer_fichero() .

---

It would be more straight forward to define

int leer_fichero(struct fichpiezas * vpiezas);

and pass

leer_fichero(vpiezas);

then you can address the elements as you do:

vpiezas[index]