what is error in this C code using realloc? - c

Question

I am trying to have some kind of dynamically growing array/data structure in C. Below is the C code I have for it. But after it prints the array, it gives a run-time error as shown below in the snapshot. What is going wrong? It is being compiled using MS-Visual C++ 2010 (Free version) on a Windows-7.
#include <stdio.h>
#include <stdlib.h>
int main(void) {
int *a;
int i = 5;
if((a = (int *)malloc(i * sizeof(int))) == NULL) {
fprintf(stderr, "Error: failed malloc\n");
return 1;
}
for(i = 0; i < 5; i++)
a[i] = i;
printf("-- array after malloc\n");
for(i = 0; i < 5; i++)
printf(" a[%d] = %d\n", i, a[i]);
if((a = (int *)realloc(a, i * sizeof(int))) == NULL) {
fprintf(stderr, "Error: failed realloc\n");
return 1;
}
for(i = 0; i < 10; i++)
a[i] = i;
printf("\n-- array after realloc\n");
for(i = 0; i < 10; i++)
printf(" a[%d] = %d\n", i, a[i]);
free(a);
return 0;
}

Answer 1

//<important>
//weren't you supposed to do i = 10 here ????
//</important>
if((a = (int *)realloc(a, i * sizeof(int))) == NULL) {
fprintf(stderr, "Error: failed realloc\n");
return 1;
}
for(i = 0; i < 10; i++)
a[i] = i;
I think this was supposed to resize array to 10 and use it, but you never changed i to 10 so i is still 5 and you go out of range

Answer 2

On this line:
if((a = (int *)realloc(a, i * sizeof(int))) == NULL) {
i still contains the value 5. So basically you are realloc'ing for 5 integers while you assign 10 integers in the next loop: an error.
This would be the fixed code:
#include <stdio.h>
#include <stdlib.h>
int main(void) {
int *a;
int i;
if((a = (int *)malloc(5 * sizeof(int))) == NULL) {
fprintf(stderr, "Error: failed malloc\n");
return 1;
}
for(i = 0; i < 5; i++)
a[i] = i;
printf("-- array after malloc\n");
for(i = 0; i < 5; i++)
printf(" a[%d] = %d\n", i, a[i]);
if((a = (int *)realloc(a, 10 * sizeof(int))) == NULL) {
fprintf(stderr, "Error: failed realloc\n");
return 1;
}
for(i = 0; i < 10; i++)
a[i] = i;
printf("\n-- array after realloc\n");
for(i = 0; i < 10; i++)
printf(" a[%d] = %d\n", i, a[i]);
free(a);
return 0;
}