C6263

warning C6263: using _alloca in a loop; this can quickly overflow stack

This warning indicates that calling _alloca inside a loop to allocate memory can cause stack overflow. _alloca allocates memory from the stack, but that memory is only freed when the calling function exits. Stack, even in user-mode, is limited, and failure to commit a page of stack causes a stack overflow exception. The _resetstkoflw function recovers from a stack overflow condition, allowing a program to continue instead of failing with a fatal exception error. If the _resetstkoflw function is not called, there is no guard page after the previous exception. The next time that there is a stack overflow, there are no exceptions at all and the process terminates without warning.

You should avoid calling _alloca inside a loop if either the allocation size or the iteration count is unknown because it might cause stack overflow. In these cases, consider other options such as, heap memory, or C++ Standard Library classes.

Example

The following code generates this warning:

#include <windows.h>  
#include <malloc.h>  
#include <excpt.h>  
#include <stdio.h>  
  
#define MAX_SIZE 50  
  
void f ( int size )  
{  
  char* cArray;  
  __try  
  {  
    for(int i = 0; i < MAX_SIZE; i++)  
    {  
      cArray = (char *)_alloca(size);  
  
     // process cArray...  
    }  
  }  
  __except(GetExceptionCode() == STATUS_STACK_OVERFLOW ?   
              EXCEPTION_EXECUTE_HANDLER : EXCEPTION_CONTINUE_SEARCH )  
  {  
     // code...  
     puts("Allocation Failed");  
    _resetstkoflw();  
  }  
}  

The following code uses malloc( ) to correct this warning:

#include <windows.h>  
#define MAX_SIZE 50  
  
void f ( int size )  
{  
  char* cArray;  
  
  for(int i = 0; i < MAX_SIZE; i++)  
  {  
     cArray = (char *) malloc(size);  
     if (cArray != NULL)  
     {  
       // process cArray...  
       free(cArray);  
     }  
  }  
}  

See Also

malloc
_alloca
_malloca
C++ Standard Library