Problems with realloc, makes program crash - c

Question

Hello i'implementing a smart vector in c, and i'm having problems with the reallocation of the buffer.
this is the struct that contains the array and its infos:
struct _vector
{
item* vec;
size_t elements;
size_t size;
};
item is just a typedef that in this case happens to be int.
I made several function to manage the array, but the one that should resize it, gives me problems.
(Vector is also a typedef for struct _vector* by the way)
This is the function:
void insertVector(const Vector vec,const int pos,const item a)
{
if(vec->elements==vec->size)
{
item* temp=realloc(vec->vec,(vec->size*2)*sizeof(item));
if(temp==NULL)
{
puts("Error: space unavailable");
return;
}
//vec->vec=realloc(vec->vec,(vec->size*2)*sizeof(item));
vec->vec=temp;
vec->size*=2;
}
int size=vec->elements;
if(pos>=0&&pos<=size)
{
for(int i=size;i>pos;i--)
{
vec->vec[i]=vec->vec[i-1];
}
vec->vec[pos]=a;
vec->elements+=1;
printf("size is %lu\nelements are %lu\n",vec->size,vec->elements);
}
}
I just shift the contents to make space for the new element, and it works fine, the problem is when the array is reallocated.
when the number of valid elements is equal to the actual size of the array,
i do a realloc to double the actual size.
As soon as that if activates though the realloc makes the program crash with this error:incorrect checksum for freed object.
The problem is in the if, because it only crashes when the size and elements are equal, if i comment out that section, everything works
I don't know what could it be.
EDIT:
The functions that i used to create and the initialise the instance i'm working with are:
Vector newVector(void)
{
Vector new=malloc(sizeof(*new));
new->vec=NULL;
new->elements=0;
new->size=0;
return new;
}
and
void initVector(const Vector vec,const size_t size)
{
vec->vec=calloc(size,sizeof(item));
vec->elements=size;
vec->size=size*2;
}

Answer 1

Based of your comment
I created a new vector setting to zero every field, then i used this function:
void initVector(const Vector vec,const size_t size)
{
vec->vec=calloc(size,sizeof(item));
vec->elements=size;
vec->size=size*2;
}
I think you are treating the size and the number of elements incorrectly. The
initVector function just allocates memory for the vec->vec array, so
vec->elements should be 0, not size. And vec->size should be size, not
size*2. So the correct function should be
// remove the const, you are modifying the data vec is pointing to
int initVector(Vector vec, size_t size)
{
if(vec == NULL)
return 0;
vec->vec = calloc(size, sizeof *vec->vec);
if(vec->vec == NULL)
return 0;
vec->elements = 0;
vec->size = size;
return 1;
}
Now the insertVector would only allocate new space, when all allocated spaces
are used.
And I suggest you use memmove to copy the memory:
// again, remove the const here
int insertVector(Vector vec, const size_t pos, const item a)
{
if(vec == NULL)
return 0;
if(vec->elements==vec->size)
{
item* temp=realloc(vec->vec,(vec->size*2)*sizeof *temp);
if(temp==NULL)
{
fprintf(stderr, "Error: space unavailable\n");
return 0;
}
vec->vec=temp;
vec->size*=2;
}
// I use vec->elements as upper limit,
// otherwise you could have "holes" in the array and
// you wouldn't realize it.
if(pos < 0 || pos > vec->elements)
{
fprintf(stderr, "invalid position\n");
return 0;
}
memmove(vec->vec + pos + 1, vec->vec + pos, (vec->elements - pos) * sizeof *vec->vec);
vec->vec[pos] = a;
vec->elements += 1;
printf("size is %lu\nelements are %lu\n",vec->size,vec->elements);
return 1;
}

Answer 2

In your initVector function, you set the size incorrectly, to two times what you allocated with calloc. This memory then gets overwritten as you are adding new elements and this is the reason the free fails when you finally invoke realloc. Change initVector to:
void initVector(const Vector vec,const size_t size)
{
vec->vec=calloc(size,sizeof(item));
vec->elements=size;
vec->size=size;
}