Decreasing realloc

问题

I have a few questions about understanding realloc behavior.

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int *str;

    /* Initial memory allocation */
    str = malloc(5 * sizeof(int));
    *str = 1;
    *(str + 1) = 2;
    *(str + 2) = 3;
    *(str + 3) = 4;
    *(str + 4) = 5;

    /* Reallocating memory */
    int i, j;

    for (i = 5; i > 0; i--) {
        str = realloc(str, i * sizeof(int));

        for (j = 0; j < i; j++) {
            printf("%d", *(str + j));
        }  

        printf("\n");
    }

    free(str);

    return(0);
}

1. In this code example, can I be sure that a smaller realloc will drop the highest number?

2. Is the realloc is only freeing the last memory and keeping the same address in str? Or may the address change even though it's getting smaller and for sure have space in the current place?

回答1:

1. Yes. If you have a memory block p of size N and you do realloc(p, M), then (assuming realloc succeeds), the result will contain the first min(N, M) bytes from p.

2. The address can change, even if the new size is smaller than the old one. realloc simply makes no guarantees about this case (it can even fail).

来源：https://stackoverflow.com/questions/38927320/decreasing-realloc