error-double-free.md

title	description	ms.date	f1_keywords	helpviewer_keywords
Error: double-free	Source examples and live debug screenshots for double free errors.	03/02/2021	double-free	double-free error AddressSanitizer error double-free

Error: double-free

Address Sanitizer Error: Deallocation of freed memory

In C, you can call free erroneously. In C++, you can call delete more than once. In these examples, we show errors with delete, free, and HeapCreate.

Example C++ - double operator delete

// example1.cpp
// double-free error
int main() {

    int *x = new int[42];
    delete [] x;

    // ... some complex body of code

    delete [] x;
    return 0;
}

To build and test this example, run these commands in a Visual Studio 2019 version 16.9 or later developer command prompt:

cl example1.cpp /fsanitize=address /Zi
devenv /debugexe example1.exe

Resulting error - double operator delete

:::image type="content" source="media/double-free-example-1.png" alt-text="Screenshot of debugger displaying double-free error in example 1.":::

Example 'C' - double free

// example2.cpp
// double-free error
#include <stdlib.h>
#include <string.h>

int main(int argc, char** argv) {

    char* x = (char*)malloc(10 * sizeof(char));
    memset(x, 0, 10);
    int res = x[argc];
    free(x);

    // ... some complex body of code

    free(x + argc - 1);  // Boom!
    return res;
}

To build and test this example, run these commands in a Visual Studio 2019 version 16.9 or later developer command prompt:

cl example2.cpp /fsanitize=address /Zi
devenv /debugexe example2.exe

Resulting error - double free

:::image type="content" source="media/double-free-example-2.png" alt-text="Screenshot of debugger displaying double-free error in example 2.":::

Example - Windows HeapCreate double HeapFree

// example3.cpp
// double-free error
#include <Windows.h>
#include <stdio.h>

int main() {
    void* newHeap = HeapCreate(0, 0, 0);
    void* newAlloc = HeapAlloc(newHeap, 0, 100);

    HeapFree(newHeap, 0, newAlloc);
    HeapFree(newHeap, 0, newAlloc);
    printf("failure\n");
    return 1;
}

To build and test this example, run these commands in a Visual Studio 2019 version 16.9 or later developer command prompt:

cl example3.cpp /fsanitize=address /Zi
devenv /debugexe example3.exe

Resulting error - Windows HeapCreate double HeapFree

:::image type="content" source="media/double-free-example-3.png" alt-text="Screenshot of debugger displaying double-free error in example 3.":::

See also

AddressSanitizer overview
AddressSanitizer known issues
AddressSanitizer build and language reference
AddressSanitizer runtime reference
AddressSanitizer shadow bytes
AddressSanitizer cloud or distributed testing
AddressSanitizer debugger integration
AddressSanitizer error examples