How to fix valgrind memory?

Question

So I have a structure and when I initiate one, I use malloc as so:

typedef struct node{ 
    void *value;
    struct node *next;
} node;

typedef struct QueueADT{
    int (*cmp)(const void*a, const void*b);
    struct node *front;
    int len;
    struct node *back;
} * QueueADT;




QueueADT que_create( int (*cmp)(const void*a, const void*b) ) {
    printf("%lu\n",sizeof(QueueADT));
    QueueADT q = (QueueADT)malloc(sizeof(QueueADT));
    if (q == NULL) {return NULL;}
    q->cmp = cmp;
    q->len = 0;
    return q;
}

valgrind spits out:

Invalid write of size 4
Address 0x5204490 is 8 bytes after a block of size 8 alloc'd

write error pertains to q->len = 0;

I cannot tell what the problem is, am I allocating an incorrect amount of bytes?

Please provide a [mcve], especially the declaration of QueueADT. — Yunnosch, Mar 29 '18 at 18:03

dbush · Accepted Answer · 2018-03-29T18:04:47.933

It looks like QueueADT is a typedef for a pointer type. That means sizeof(QueueADT) evaluates to the size of the pointer, not what it points to. Since it seems that a pointer is 8 bytes on your system and that the struct in question is larger than that, you write past the end of allocated memory.

What you want instead is:

QueueADT q = malloc(sizeof(*q));

This allocates enough space for what q points to. Also, don't cast the return value of malloc.

It's also bad practice to hide a pointer behind a typedef, as it is not obvious that you're working with a pointer which can confuse the reader, and is probably what tripped you up in this case.

thanks for the quick reply! Nice assumption! It's all good now. also thanks for the explanation about typedef pointers, ill take that into consideration — Adam G., Mar 29 '18 at 18:08

How to fix valgrind memory?

1 Answers1