传递struct数组以使用malloc运行

Question

I'am writing a program where I need to add students to a array of structs.

Well, at the beginning I have the following struct array:

struct student *students[4];

After the declaration I add students like this to the array (just to have examples ...):

  students[0] = malloc(sizeof(struct student));
  students[0]->firstname = "Max";
  students[0]->secondname = "Taler";
  students[0]->number = 123456l;

...

Now I have to write a additional function, where I can pass the array as a parameter and add a new student.

So my function looks like this:

void add_student(struct student *students[],char *fristname, char *secondname, long number)
{
    int i=0;
    int new_position=0;
    int return_value = 0;
    // Search for the first available position for a new student 
    for(i=0; i<sizeof(students); i++)
    {
        if(students[i]==NULL)
        {
            new_position=i;
            return_value = 1;
            break;
        }
    }

    struct student *new_student = malloc(sizeof(struct student));
    students[new_position] = new_student;

    students[new_position]->fristname = fristname;
    students[new_position]->secondname = secondname;
    students[new_position]->number = number;

   return return_value; 
}

I call the function with this code:

add_student(students, "Anni", "Karls", 123232);

But now my issue : In my "add_student" function I get an array that has a strange structur: it includes itself as the first element and every other element is shifted by 1 element.

Cant figure out what the problem is ... Can somebody please help?

EDIT: Somebody asked me, how this can be. How the array can include "itself" as the first element.

Well, here are screenshots of the debugger (xCode):

Before entering "add_student":

After entering "add_student" -> IN the function "add_student":

As you see, "students" has now 5 elements ...

Answer 1

Pass the length of the array to the function and change this

for(i=0; i<sizeof(students); i++)

to this

for(i=0; i<arrayLen; i++)

It seems you are inserting an element to some position in an array which has been allocated before hand; inserting is maybe more proper term than adding, but that is opinion.

You were applying sizeof operator on array type inside function - this will likely give you size of pointer in bytes because array name decays to pointer.

Answer 2

It's a little strange to statically allocate the array but then dynamically allocate the individual structures. You might consider dynamically allocating them all:

int num_students = 4;
struct student **students = malloc(sizeof(struct student) * num_students);

And then change your add_student function to look like this:

void add_student(struct student *students[], int arr_size, char *firstname, char *secondname, long number)