C6308

warning C6308: 'realloc' may return null pointer: assigning a null pointer to <variable>, which is passed as an argument to 'realloc', will cause the original memory block to be leaked

This warning indicates a memory leak that is the result of the incorrect use of a reallocation function. Heap reallocation functions do not free the passed buffer if reallocation is unsuccessful. To correct the defect, assign the result of the reallocation function to a temporary, and then replace the original pointer after successful reallocation.

Example

The following sample code generates this warning:

#include <malloc.h>  
#include <windows.h>  
  
void f( )  
{  
  char *x;  
  x = (char *) malloc(10);  
  if (x != NULL)  
  {  
    x = (char *) realloc(x, 512);  
    // code...  
    free(x);  
  }     
}  

To correct this warning, use the following code:

#include <malloc.h>  
#include <windows.h>  
  
void f()  
{  
  char *x, *tmp;  
  
  x = (char *) malloc(10);  
  
  if (x != NULL)  
  {  
    tmp = (char *) realloc(x,512);  
    if (tmp != NULL)   
    {  
      x = tmp;  
    }  
    free(x);  
  }  
}  

This warning might generate noise if there is a live alias to the buffer-to-be-reallocated at the time of the assignment of the result of the reallocation function.

To avoid these kinds of problems altogether, use the mechanisms that are provided by the C++ Standard Template Library (STL). These include shared_ptr, unique_ptr, and vector. For more information, see Smart Pointers and C++ Standard Library.

See Also

C6014