C26100

 

For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

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.

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.

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.

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