Why dynamic allocation of array of pointers crashes?

Question

The malloc part crashes in the following code, but only in VS and not in CodeBlocks. As I have learned, that means that I am probably triggering some undefined behavior. But I can't figure out why...

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

#define TableLength 29
typedef int info;
typedef int tipkey;
typedef struct element
{
    tipkey key;
    info info;
} element;

typedef struct node* nodepointer;
typedef struct node
{
    element element;
    nodepointer next;
} tipnod;
typedef nodepointer table[TableLength];

int main()
{
    table table;

    for (int i = 0; i < TableLength; i++)
    {
        table[i] = NULL;
    }

    for (int i = 0; i < TableLength; i++)
    {
        element el = { i, i };
        table[i] = (nodepointer)malloc(sizeof(nodepointer));
        table[i]->element = el;
        table[i]->next = NULL;
    }

    getch();
    return 0;
}
}```

I strongly encourage you to use fewer typedefs. They make it really awkward to read the code. — Bill Lynch
– Bill Lynch, Commented May 16, 2021 at 22:52
You could avoid this error by using either of the canonical patterns p = malloc(sizeof *p); or p = (T *)malloc(sizeof(T));. (Most of us prefer the first one since it is guaranteed to be correct; the latter can be wrong if you get T wrong , but will at least cause a compiler diagnostic in that case) — M.M
– M.M, Commented May 17, 2021 at 3:50

dbush · Accepted Answer · 2021-05-16 22:53:44Z

2

You're not allocating enough memory:

table[i] = (nodepointer)malloc(sizeof(nodepointer));

You're allocating space for a nodepointer instead of a tipnod (?) a.k.a struct node. As a result, you're writing past the end of allocated memory when you write to the struct, triggering undefined behavior.

You want to use that for the size instead.

table[i] = malloc(sizeof(tipnod));

Note also that you shouldn't cast the return value of malloc as that can hide other errors in your code.

answered May 16, 2021 at 22:53

dbush

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1 Comment

Gabriela Over a year ago

Thank you! Your answer felt like turning on the lightbulb in my brain. It really helped me clarify my misconception. :)

thirdeye · Accepted Answer · 2021-05-17 01:09:14Z

1

Tested on VSCode: Extending @dbush response, you allocating less memory than required and trying to access unallocated memory. you can refer this What happens if I use malloc with the wrong size?

Tested on your code:

printf("size of nodepointer: %lu\n", sizeof(nodepointer));
printf("size of node: %lu\n", sizeof(struct node));

Output:

size of nodepointer: 8
size of node: 16

answered May 17, 2021 at 1:09

thirdeye

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Comments

chux · Accepted Answer · 2021-05-17 04:30:12Z

0

Avoid allocation size mistakes.

Allocate to the size of the refenced object and drop the unneeded cast.

ptr = malloc(sizeof *ptr);

It is that simple.

In OP's case

//table[i] = (nodepointer)malloc(sizeof(nodepointer));
table[i] = malloc(sizeof (*table[i]));
// or 
table[i] = malloc(sizeof *table[i]);
// or 
table[i] = malloc(sizeof table[i][0]);

answered May 17, 2021 at 4:30

chux

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