How to give a size using realloc? [closed] - c

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I want to give a random size to pointer and I want to add some elements to the pointer. I want to give place to the pointer enough size. For example, if I have 10 elements in pointer, I want to give place 40 byte to pointer. But when I looked to my code, initially I gave so much size for the element.
I want to see as an output:
67
11
64
7
67
11
64
7
23
81
88
35
12
5
7
The size of memory=40
#include <stdio.h>
#include <stdlib.h>
int main() {
int *k;
int i=0,j=0,t=0;
int array[20]={67,11,64,7};
k=malloc(5*sizeof(int));
k=array;
for(i=0;i<4;i++){
printf("%d\n",k[i]);
}
k=realloc(k,50*sizeof(int));
k[4]=23;
k[5]=81;
k[6]=88;
k[7]=35;
k[8]=12;
k[9]=5;
k[10]=7;
for(j=0;k[j]!='\0';j++){
printf("%d\n",k[j]);
}
k=realloc(k,(j+1)*sizeof(int));
for(t=0;k[t]!='\0';t++){
printf("%d\n",k[t]);
}
printf("the size of the memory=%d \n",j*4);
printf("the size of the memory=%d \n",t*4);
return 0;
}

Answer 1

You can't resize an automatic variable like array which is what you are trying to do since you do k=array; (and leak the memory previously allocated).
You can copy the data from array into the allocated memory pointed out by k instead:
int array[20] = {67, 11, 64, 7};
int *k = malloc(sizeof array);
memcpy(k, array, sizeof array);
The loop condition you use is a tad confusing though:
for (j = 0; k[j] != '\0'; j++)
I suggest comparing with 0 instead.
The "the size of the memory" that you print out at the end is however not accurate. The size of the allocated memory at the end is (j+1)*sizeof(int), that is, 12 * sizeof(int), since k[11] is the first element being 0.
A word of caution is also in place: Your program currently has defined behavior because the array does contain 0:s in the positions [11, 19]. Just be aware that the extra memory allocated by realloc contains indeterminate values, so had your original array been only 11 elements big, you had read those indeterminate values when you compare with \0 (or preferably 0) and you would never know what the result would be. It could continue searching for \0 out of bounds.
Note: Don't forget to free(k); when you're done with it.