Thread: Dynamic array help

Hello all, I am creating a program that uses a dynamic array to store students. My code compiles but i keep getting a runtime error which prints out a bunch of stuff, but the main thing being:

*** glibc detected *** ./a.out: realloc(): invalid pointer: 0xb7fc2e00 ***

so apparently there is a problem with my realloc but I just can't seem to figure out what it is. As far as I can see it should work. Any help with this would be greatly appreciated.

Here is my main which is pointless right now. Basically it's just set up to test whether the add function works.

Code:

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

int main()
{
	
	student *array;
	student std1;
	int number_of_elements = 0;
	int array_size = 0;
	
	std1.lastname = "smith";
	std1.firstname = "bob";
	std1.score = 50;

	add(array, std1, &number_of_elements, &array_size);
	
	free(array);
	return 0;
}

And my header file

Code:

#include <stdlib.h>

#ifndef STUDENT_C
#define STUDENT_C
typedef struct student
{
	char *lastname;
	char *firstname;
	int score;
} student;

void add(student*, student, int*, int*);

#endif

and finally the important one. The student file which contains the dynamic memory function. Again the error seems to be on the realloc line.

Code:

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

void add(student *array, student item, int *number_of_elements, int *array_size)
{
	if (*number_of_elements == *array_size) /*resize array*/
	{
		if (*array_size == 0)
		{
			*array_size = 1;
		}
		else
		{
			*array_size = *array_size * 2;
		}
		array = (student*)realloc(array, ((*array_size) * sizeof(student)));
		
		if (array == 0)
		{
			printf("no space for memory");
			exit(1);
		}
	}
	array[*number_of_elements] = item;
	(*number_of_elements)++;
}

Maybe the problem is the extra "student" in the first line of the definition:

Code:

typedef struct student
{
	char *lastname;
	char *firstname;
	int score;
} student;

thanks for the reply. hmmm.. removed that and it doesn't seem to make a difference

I just realized -- you're using realloc without ever having used malloc, so the pointer that you're giving to realloc doesn't point to any memory -- ergo invalid.

Originally Posted by xmariux

try this , define student *array=NULL; array must be NULL to allocate memory with realloc

that worked! Thanks a million, I can't believe it was something so trivial.

Originally Posted by laserlight

Try calling the add() function more than once in your main() function. You have a problem in that the array parameter is just a pointer, so this merely changes the local copy of the pointer:

Code:

array = (student*)realloc(array, ((*array_size) * sizeof(student)));

You should declare array to be a pointer to a student*. (Maybe you should also rename it to students.)

sorry but I don't think I'm quite following, you mean that array should be a double pointer??

Originally Posted by Perogy

sorry but I don't think I'm quite following, you mean that array should be a double pointer??

Yes, a student**. But not in main(), only in the add() function, so when you call the function from main the first argument will be &array.

ahhh ok I see what you mean, just so we can change the original it needs to be passed in as an address. I did that and now it seems to work perfectly for the first time I use add but if I use it again it segfaults. This is my new Student.

Code:

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

void add(student **array, student item, int *number_of_elements, int *array_size)
{
	if (*number_of_elements == *array_size) /*resize array*/
	{
		if (*array_size == 0)
		{
			*array_size = 1;
		}
		else
		{
			*array_size = *array_size * 2;
		}
		*array = (student*)realloc(*array, ((*array_size) * sizeof(student)));
		
		if (*array == 0)
		{
			printf("no space for memory");
			exit(1);
		}
	}
	*array[*number_of_elements] = item;
	(*number_of_elements)++;
}

Can you post the updated code.

Originally Posted by Perogy

.... but if I use it again it segfaults...

It shouldn't -- unless you have an error in main. I suspect that in the second call to add() you forgot to make the first argument a pointer (&array, ...)

Originally Posted by quzah

Also, 'item' isn't a pointer, so once your function ends, the copy of that structure that you've just passed to the function no longer exists.
Quzah.

Not true. The array is an array of structures, not an array of pointers, so when the new 'item' is assigned to the array, its contents are copied into the memory allocated to the array. Even if 'item' itself is dynamically allocated, copied into the array and then 'item' is freed, the data in the array would not be lost.

Originally Posted by quzah

It's good practice to not assume a pointer you're getting actually points to something. Check to make sure your pointers aren't NULL. Also, 'item' isn't a pointer, so once your function ends, the copy of that structure that you've just passed to the function no longer exists.
Quzah.

I don't see why item would have to be a pointer though, it just gets stored in the array and that's that. Let me know if I'm wrong.

I'll post the entire updated code shortly.

Originally Posted by Perogy

I don't see why item would have to be a pointer though, it just gets stored in the array and that's that. Let me know if I'm wrong.

It doesn't. Did you read post #13?