Program with array of structs crashes

Question

I have an array with multiple structs. When i ask the user to enter data the first time everything works but when i ask again for the next position in the array the program crashes. If this method doesn't work souldn't the program crash in the beginning? Is something wrong with malloc?

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



struct student {
    char name[50];
    int semester;
};

struct prof {
    char name[50];
    char course[50];
};

struct student_or_prof {
    int flag;
    int size;
    int head;
    union {
        struct student student;
        struct prof prof;
    }
}exp1;
struct student_or_prof *stack;


void init(int n)
{
   stack = malloc(n);


}

int push(struct student_or_prof **pinx,int *head,int n)
{
    char name[50];

    printf("\nn= %d\n",n);
     printf("\nhead= %d\n",*head);
    if(*head==n)
    {
        printf("Stack is full.\n");
        return 1;
    }


    char x;
    printf("Student or Professor? [s/p] ");
     getchar() != '\n';
    scanf("%c",&x);

    if(x=='s')
    {

        getchar() != '\n';
       pinx[*head]->flag = 0;

       printf("\n\nGive student's name: ");
       fgets(pinx[*head]->student.name,sizeof(pinx[*head]->student.name),stdin);

       printf("\nGive student's semester: ");
       scanf("%d",&(pinx[*head]->student.semester));

       printf("\nName = %s\tSemester = %d",pinx[*head]->student.name,pinx[*head]->student.semester);

    }
    else if(x=='p')
    {
        getchar() != '\n';
       pinx[*head]->flag = 1;

       printf("\n\nGive professor's name: ");
       fgets(pinx[*head]->prof.name,sizeof(pinx[*head]->prof.name),stdin);

       printf("\nGive course: ");
       fgets(pinx[*head]->prof.course,sizeof(pinx[*head]->prof.course),stdin);

       printf("\nName = %s\tCourse = %s\n",pinx[*head]->prof.name,pinx[*head]->prof.course);
    }



    (*head)++;
    printf("\nhead= %d\n",*head);



}


int main()
{
    int n,i;
  printf("Give size: ");
  scanf("%d",&n);

  init(n);

 for(i=0;i<n;i++)
   push(&stack,&exp1.head,n);

    return 0;
}

How many student_or_prof nodes do you think will fit in the memory allocated with this stack = malloc(n); ? — WhozCraig
– WhozCraig, Commented Mar 27, 2014 at 21:42

brokenfoot · Accepted Answer · 2014-03-27 22:06:07Z

1

You need to malloc the structure not n

malloc(sizeof(struct student_or_prof)*n)

EDIT:

And your code crashes again because pinx is a double pointer, so this operation is not valid:

pinx[*head]->flag = 0;

this is equivalent to:

*(pinx + *head)->flag = 0;

Since you are not changing what stack points to, you are better off using a single pointer instead of a double pointer.

So instead you should change your push API:

int push(struct student_or_prof *pinx,int *head,int n)

and call it like:

push(stack,&exp1.head,n);

edited Mar 27, 2014 at 22:06

answered Mar 27, 2014 at 21:43

brokenfoot

11.7k10 gold badges66 silver badges85 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

brokenfoot Over a year ago

where does it crash? Use GDB or put some printfs to pin-point the line which is causing the crash.

Manuel Pap Over a year ago

it crashes there pinx[*head]->flag = 0; EDIT: stack = malloc(sizeof(struct student_or_prof)*n); is it wrong?

Manuel Pap Over a year ago

I don't want something like that. I called as push(&stack,&exp1.head,n); and double pointer in pinx because i want the changes to remain in stack array.

brokenfoot Over a year ago

They will remain in the stack. You are passing the address of stack and then doing changes on that address. So, this can be done using a single pointer. Double pointer is an overkill.

thumbmunkeys · Accepted Answer · 2014-03-27 21:43:12Z

1

malloc allocates the given number of bytes.

You have to multiply n with the size of your struct, to allocate enough memory.

answered Mar 27, 2014 at 21:43

thumbmunkeys

20.8k8 gold badges70 silver badges111 bronze badges

1 Comment

Manuel Pap Over a year ago

Of course you are right but i've tried that and it crashes again

M.M · Accepted Answer · 2014-03-27 22:22:53Z

0

pinx does not point to an array, so pinx[*head] is going to access invalid memory unless *head is zero.

I think you meant (*pinx)[*head] , which accesses the N-th element of the array you allocated via malloc. For example (*pinx)[*head].prof.name etc.

BTW, your head number doesn't seem to be used at all, except for exp1.head, maybe it'd be better to remove head from the struct, and just have a single variable head?

answered Mar 27, 2014 at 22:22

M.M

143k26 gold badges223 silver badges400 bronze badges

2 Comments

Manuel Pap Over a year ago

No i really need head there. Everything is good now actually , brokenfoot gave me what i need

M.M Over a year ago

OK, good to hear. However if you ever decide you want push to be able to increase the capacity of the stack (e.g. you want to allow people to enter names without having to specify the count beforehand), you may need to return to the pointer-to-pointer version.

Collectives™ on Stack Overflow

Program with array of structs crashes

3 Answers 3

4 Comments

1 Comment

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

1 Comment

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related