---
description: "Learn more about: Warning C26101"
title: Warning C26101
ms.date: 11/04/2016
f1_keywords: ["C26101"]
helpviewer_keywords: ["C26101"]
ms.assetid: 86046553-09ec-40ce-82b3-fd641928f0b0
---
# Warning C26101

> Failing to use interlocked operation properly for variable '*var*'.

Windows APIs offer various interlocked operations. Annotation `_Interlocked_` specifies that a variable should only be accessed through an interlocked operation. Warning C26101 is issued when a variable access isn't consistent with the `_Interlocked_` annotation.

## Example

The following example generates warning C26101 because there's a violation of the `_Interlocked_` contract.

```cpp
CRITICAL_SECTION cs;
typedef struct _DATA
{
    _Interlocked_ LONG data;
} DATA;

void Safe(DATA* p)
{
    InterlockedIncrement(&p->data); // OK
}

void Unsafe(DATA* p)
{
    p->data += 1; // Warning C26101
    EnterCriticalSection(&cs);
    LeaveCriticalSection(&cs);
}
```