C++ Dereference the Non-allocated Memory but Without Segmentation Fault

Question

I have encountered a problem which I don't understand, the following is my code:

#include <iostream>
#include <stdio.h>
#include <string.h>
#include <cstdlib>
using namespace std;

int main(int argc, char **argv)
{
    char *format = "The sum of the two numbers is: %d";
    char *presult;
    int sum = 10;
    presult = (char *)calloc(sizeof(format) + 20, 1);   //allocate 24 bytes

    sprintf(presult, format, sum);  // after this operation, 
                                    // the length of presult is 33
    cout << presult << endl;

    presult[40] = 'g';   //still no segfault here...
    delete(presult);
}

I compiled this code on different machines. On one machine the sizeof(format) is 4 bytes and on another, the sizeof(format) is 8 bytes; (On both machines, the char only takes one byte, which means sizeof(*format) equals 1)

However, no matter on which machine, the result is still confusing to me. Because even for the second machine, the allocated memory for use is just 20 + 8 which is 28 bytes and obviously the string has a length of 33 meaning that at least 33 bytes are needed. But there is NO segmentation fault occurring after I run this program. As you can see, even if I tried to dereference the presult at position 40, the program doesn't crash and show any segfault information.

Could anyone help to explain why? Thank you so much.

Answer 1

Accessing unallocated memory is undefined behavior, meaning you might get a segfault (if you're lucky) or you might not.

Or your program is free to display kittens on the screen.

Speculating on why something happens or doesn't happen in undefined behavior land is usually counter-productive, but I'd imagine what's happening to you is that the OS is actually assigning your application a larger block of memory than it's asking for. Since your application isn't trying to dereference anything outside that larger block, the OS doesn't detect the problem, and therefore doesn't kill your program with a segmentation fault.