Error with memcpy and memset

Question

Could anyone tell me why this problem occurs below? It does not copy the last position of the pointer vector, with the value 57, to another pointer vector. This intrigues me, why not copy this last position?

int main()
{
    int i;
    int *v;
    int *k;
    k = malloc(100*sizeof(int));
    memset(k,0,sizeof(k));
    k[0]=78;
    k[1]=63;
    k[2]=57;
    v = malloc (10 * sizeof (int));
    memset(v,0,sizeof(v));


    printf("Valor que não quer imprimir no vetor:%d\n",k[2]);
    //*(v+i) = 789;
    //v[i] = 789;
    for (i = 0; i < 2; ++i)
    {
        printf("\nDigite um valor:");
        scanf ("%d", &v[i]);
    }
    for (i = 0; i < 2; ++i) printf("%d\n",v[i]);

    printf("\nResultado:");
    memcpy(&v[2],k,sizeof(k));
    for (i = 0; i < 5; ++i) printf("\n%d",v[i]);

    return 0;
}

As a result, this output:

Valor que não quer imprimir no vetor:57

Digite um valor:1

Digite um valor:2
1
2

Resultado:
1
2
78
63
758396511
Process returned 0 (0x0)   execution time : 12.095 s
Press any key to continue.

In memset, is not zeroing all items... Can someone give me a hand? Thank you very much!

Answer 1

The program is confused, I couldn’t quite understand what it should do, but in relation to the memset the correct is:

...
k = malloc(100*sizeof(int));
memset(k, 0, 100*sizeof(int));
...
v = malloc(10*sizeof(int));
memset(v, 0, 10*sizeof(int));

Answer 2

You are not initiating all memory allocated with malloc(), the call of memset() is filling with zeros only the first bytes of your buffer.

The operator sizeof() will not return the memory region size pointed by k, and yes the size in bytes that the pointer k occupies in the mémoria.

The right thing would be:

int * k;
k = malloc( 100 * sizeof(int) );
memset( k, 0, sizeof(int) * 100 );

The function calloc() of the standard library stdlib.h is able to allocate memory and fill the allocated region with zeros, which dispenses with the use of memset(), greatly simplifying your code:

int * k; 
k = (int*) calloc( 100, sizeof(int) );

In the present case, the dynamic allocation of memory makes no sense, the memory can be allocated statically, which would dispense with the use of malloc(), look at you:

int k[100];
memset( k, 0, sizeof(k) );