C26100

warning C26100: Race condition. Variable <var> should be protected by lock <lock>.

The _Guarded_by_ annotation in the code specifies the lock to use to guard a shared variable. Warning C26100 is generated when the guard contract is violated.

Example

The following example generates warning C26100 because there is a violation of the _Guarded_by_ contract.

CRITICAL_SECTION gCS;

_Guarded_by_(gCS) int gData;

typedef struct _DATA {
    _Guarded_by_(cs) int data;
    CRITICAL_SECTION cs;
} DATA;

void Safe(DATA* p) {
    EnterCriticalSection(&p->cs);
    p->data = 1; // OK
    LeaveCriticalSection(&p->cs);
    EnterCriticalSection(&gCS);
    gData = 1; // OK
    LeaveCriticalSection(&gCS);
}

void Unsafe(DATA* p) {
    EnterCriticalSection(&p->cs);
    gData = 1; // Warning C26100 (wrong lock)
    LeaveCriticalSection(&p->cs);
}

The contract violation occurs because an incorrect lock is used in the function Unsafe. In this case, gCS is the correct lock to use.

Example

Occasionally a shared variable only has to be guarded for write access but not for read access. In that case, use the _Write_guarded_by_ annotation, as shown in the following example.

CRITICAL_SECTION gCS;

_Guarded_by_(gCS) int gData;

typedef struct _DATA2 {
    _Write_guarded_by_(cs) int data;
    CRITICAL_SECTION cs;
} DATA2;

int Safe2(DATA2* p) {
    // OK: read does not have to be guarded
    int result = p->data;
    return result;
}

void Unsafe2(DATA2* p) {
    EnterCriticalSection(&gCS);
    // Warning C26100 (write has to be guarded by p->cs)
    p->data = 1;
    LeaveCriticalSection(&gCS);
}

This example also generates warning C26100 because it uses an incorrect lock in the function Unsafe2.