Allocating memory through calloc in C [closed] - c

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I am simply testing what would be the output if I try to dereference a pointer, which points to out of range of dynamically created memory using calloc() and expecting memory fault or some garbage value instead. It is giving 0 as output, which is basic feature of calloc() that it intializes allocated memory with '0', or is it just due to undefined behavior?
#include <stdio.h>
#include <stdlib.h>
int main()
{
int *p = NULL;
p = (int *)calloc(10, 8);
if(p != NULL)
{
printf("\n successfully allocated memory ");
}
else
EXIT_FAILURE;
printf("\n Thirty first element of memory is %d \n",p[30]);
}
Result :
successfully allocated memory
Thirty first element of memory is 0

Answer 1

You are trying to access p + 30 * sizeof(int) which is p + 120 while you have allocated only 8 * 10 = 80 bytes, so max address you can access is p[19]

Answer 2

From a language perspective, accessing out of bounds memory is undefined behavior, meaning that anything can happen.
One possible explanation for that particular behavior is that most operating systems will zero out any memory before they give it to your process, independently of what function you where using internally (this is a security measure, so you can't read data that was used by other processes).
To get garbage data, you usually have to have used and freed the same memory before inside your program (in which case it retains its old value). Or of course, if your index is so far off that you are indexing into a neighboring data structure.
The reason your program didn't just crash is that malloc (and similar functions) usually request much more memory from the os, than what you are requiring and use the surplus memory the next time you call malloc. So from the OS perspective you are accessing valid memory, and there is no reason to terminate your program.

Answer 3

Accessing out of range memory is Undefined Behaviour. Hence the value which you get can be 0 or 1 or Batman
The point is, you cannot ask any explanation or reason for things that are undefined. Anything can happen in such a scenario.
Also, you would get a seg fault if you try to access a memory location which your program is not allowed to access. That may not be the case always when you access out of bounds memory. It can lie well inside the user memory area of the program.

Answer 4

I think windows zeroes a freed page before making available for allocation. Have fun with the code below.
#include<stdio.h>
#include<stdlib.h>
#include <windows.h>
#include <signal.h>
int g = 0;
unsigned long long* p=NULL;
void signal_handler(int ssc)
{
printf("Error %d \n", ssc);
printf("segfault after %llu bytes",g*8);
printf("should i go backwards? press enter");
getch();
g=0;
do
{
printf("Virtual Adress:%llu is %llu\n",p+g,*(p+g) );
Sleep(1);
g--;
}
while(1);
}
int main ()
{
if(SIG_ERR == signal(SIGSEGV, signal_handler) ) {printf("error seting signal handler %d\n",SIGSEGV);}
p = (unsigned long long*)calloc(10,8);
if( p!= NULL)
{
printf("successfully allocated memory\n");
}
else
{
EXIT_FAILURE;
}
do
{
printf("Virtual Adress:%llu is %llu\n",p+g,*(p+g) );
Sleep(2);
g++;
}
while(1);
return 0;
}