Question

   typedef struct {
      void *elems;//address of the memory block
      int elemSize; //
      int logicLen;//number of existing elements in vector
      int allocLen;//allocated space for the vector
  } vector;

  static void InsertNumbers(vector *numbers, long n, long d)
  {
    long k;
    long residue;

    for (k = 0; k < d; k++) {
      residue = (long) (((long long)k * (long long) n) % d);
      VectorAppend(numbers, &residue);
    }
  }



void VectorAppend(vector *v, const void *elemAddr)
{
   void *target=(char*)v->elems + (v->logicLen * v->elemSize);

   if(v->logicLen==v->allocLen){
    v->allocLen*=2;
    v->elems=realloc(v->elems,v->allocLen*v->elemSize);
    assert(v->elems!=NULL);
   }
   memcpy(target,elemAddr,v->elemSize);
   v->logicLen++;
}

Then, I use the following sentence to call InsertNumbers()

vector aVector;
VectorNew(&aVector, sizeof(long),4);
long first=139269,second=3021377;
InsertNumbers(&aVector,first , second);

It seems like 3021377 is too big…
in v->elems=realloc(v->elems,v->allocLen*v->elemSize); I find that when v->allocLen=4096, the program crashes and says:This may be due to a corruption of the heap
why?

Answer 1

This is not a problem with the code you posted, this is a problem somewhere else.

What happens is your program corrupts the heap, and then realloc detects that the heap is corrupted.

You will want to detect the corruption as follows:

1. Make sure you enable debugging symbols

2. Run your program through Valgrind

Edit: There is a serious error is in the code you added.

void VectorAppend(vector *v, const void *elemAddr)
{
    void *target = (char *) v->elems + v->logicLen * v->elemSize;

    if (v->logicLen == v->allocLen) {
        v->allocLen *= 2;
        // Once you call 'realloc', the value of 'elems' might change
        // This means that 'target' is now INVALID
        // 'target' is based on the old value of 'elems'
        v->elems = realloc(v->elems,v->allocLen*v->elemSize);
        assert(v->elems != NULL);
    }
    memcpy(target, elemAddr, v->elemSize);
    v->logicLen++;
}

To fix it, move the calculation for target below the reallocation:

void VectorAppend(vector *v, const void *elemAddr)
{
    if (v->logicLen == v->allocLen) {
        v->allocLen *= 2;
        v->elems = realloc(v->elems,v->allocLen*v->elemSize);
        assert(v->elems != NULL);
    }
    void *target = (char *) v->elems + v->logicLen * v->elemSize;
    memcpy(target, elemAddr, v->elemSize);
    v->logicLen++;
}

Another error: There is an error in your comments, which is part of the code and I’d recommend taking comments seriously.

VectorNew(&aVector, sizeof(long), 4); // allocate 4*4 bytes 

The comment should not say, “allocate 4*4” bytes because that is misleading: someday you’ll compile the program on a 64-bit system that isn’t Windows, and it will be 8×4 bytes. You’re better off removing the comment and just reading the code.