malloc function disturbs array items (C)

Question

I'm writing a program that need to scan numbers into an array, and I know that the number of items on my array will be a multiple of 5. I can't use the realloc function, only malloc, But my program mess the 6th item, and after 10 items just crashes. can you help me find my error here? Thanks!

#include <stdio.h>
#include <stdlib.h>
#define K 5
int main(){
    int counter=0;
    int enteredNum;
    int *p=malloc(K*sizeof(int));
    int *pmore=NULL;
    printf("Please enter the series : \n");
    scanf("%d",&enteredNum);
    while(enteredNum!=0){
            p[counter]=enteredNum;
            if(counter%K==0&&counter!=0){
                pmore=malloc(((counter)+K)*sizeof(int));

                for(int i=0;i<counter;i++){
                    pmore[i]=p[i];
                //for
                free(p);
                p=pmore;
                pmore=NULL;
            }//if
            counter++;
            scanf("%d",&enteredNum);
    }


    for(int i=0;i<counter;i++)
        printf("%d\t",p[i]);
}

If counter is 6 the first time you first write to the array and then create a new one that fits the element in. You're "lucky" you didn't cause it to crash on the 6th element. Move the p[counter] after the if and before the counter++ — apokryfos
– apokryfos, Commented Jan 26, 2017 at 11:37

David Schwartz · Accepted Answer · 2017-01-26 11:37:45Z

2

You set p[counter] before you enlarge p. So when K is 5, counter%K won't be zero until counter is 5, but by then it's too late, you've already stored six elements in p (0, 1, 2, 3, 4, and 5), which only had enough space for 5.

answered Jan 26, 2017 at 11:37

David Schwartz

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