C改变结构内数组的大小

Question

#include <stdio.h>
#include <stdlib.h>

#define TRUE 1

struct Cases
    {
        char name[3];
        int cases;
    };



int main()
{
    int n=5;
    struct Cases Real;
    Real.name[0] = 'a';
    Real.name[1] = 'b';
    Real.name[2] = 'c';
    printf("Size before: %d \n", sizeof Real.name);
    Real.name = (char *)malloc(5 * sizeof(char));
    printf("Size after: %d \n", sizeof Real.name);


    return 0;
}

Hi, i am trying to change the size of the array inside structure, but it gives error. I tried to do it first empty array but that didn't work out aswell. Any help ?

Answer 1

Hi, i am trying to change the size of the array inside structure

You cannot change the size of an array, it's a constant. If you try to assign to a variable of any array type, you'll see error, as an array is not a modifiable lvalue.

As an alternative: You can define a pointer, and allocate memory dynamically using the allocator functions (as you tried).

Answer 2

This is impossible without the use of pointers , an example for you to be able to do this with pointers :

#include <stdio.h>

int main ()
{
     char **real = malloc (3 * sizeof (* array));
     real[0] = "a";
     real[1] = "b";
     real[2] = "c";

     printf ("% My array in position 0 n", real[0]);

     char ** tmp = realloc (real, 5 * sizeof (* array));
     real = tmp;
     real[1] = "G";

     printf ("% s \ n", real [0]);
     printf ("% s \ n", real [1]);
         

     free (array);
     return 0;
}

Answer 3

You could do something close to that with a flexible array :

struct Cases
{
        int cases;
        char name[]; //Must be the last element in the struct   
};

int main()
{
    struct Cases * real = (struct Cases *)malloc(sizeof(struct Cases) + 3 * sizeof(char));
    real->length  = 3;
    real->name[0] = 'a';
    real->name[1] = 'b';
    real->name[2] = 'c';
    printf("%d\n", real->length);           //=> 3
    
    real = (struct Cases *)realloc(real, sizeof(struct Cases) + 5 * sizeof(char));
    real->length  = 5;
    real->name[3] = 'd';
    real->name[4] = 'e';
    printf("%d\n", real->length);           //=> 5

    printf("%ld\n", sizeof(real));          //=> 8 (the size of the pointer)
    printf("%ld\n", sizeof(struct Cases));  //=> 8 (the size of the structure without the array)
    
    free(real);
    return 0;
}

However you must memorize the size by yourself (I added a field to the structure for that purpose).

Answer 4

As you've defined it, name is an array of three elements, inside a struct . As such, its size cannot be changed. If you want to be able to change the size dynamically, you can declare it as a char pointer, then allocate the storage dynamically (and free it when done).

Here's an example of what this might look like:

#include <stdio.h>
#include <stdlib.h>

struct Cases
{
    char *name;
    int cases;
};

int main(void)
{
    struct Cases Real;

    Real.name = malloc(3 * sizeof(char));
    if (Real.name == NULL) {
        fprintf(stderr, "Malloc failed.\n");
        exit(1);
    }

    Real.name[0] = 'a';
    Real.name[1] = 'b';
    Real.name[2] = 'c';

    Real.name = realloc(Real.name, 5 * sizeof(char));
    if (Real.name == NULL) {
        fprintf(stderr, "Realloc failed.\n");
        exit(1);
    }

    Real.name[3] = 'd';
    Real.name[4] = 'e';

    free(Real.name);

    return 0;
}

In this example, name is now a char pointer, which is first set with malloc to be able to hold 3 elements. It is later grown with realloc to be able to hold 5 elements. Finally, it is freed by calling free .

Under normal circumstances, this example doesn't print anything out. It will print an error message if either malloc or realloc fails. You should always check the result of malloc or realloc to see if it's NULL before attempting to use it.

Note that these arrays do not have a terminating null character in the example (although you could certainly add one if desired, provided you've allocated space for it). Also note that sizeof is no longer useful when used on name , since name is now a pointer. You need to keep track of the size yourself. If needed, you could add a size field to Cases , setting it to the current size.