Frees a block of memory in the heap (debug version only).

void _free_dbg( 
   void *userData,
   int blockType 


Pointer to the allocated memory block to be freed.


Type of allocated memory block to be freed: _CLIENT_BLOCK, _NORMAL_BLOCK, or _IGNORE_BLOCK.

The _free_dbg function is a debug version of the free function. When _DEBUG is not defined, each call to _free_dbg is reduced to a call to free. Both free and _free_dbg free a memory block in the base heap, but _free_dbg accommodates two debugging features: the ability to keep freed blocks in the heap's linked list to simulate low memory conditions and a block type parameter to free specific allocation types.

_free_dbg performs a validity check on all specified files and block locations before performing the free operation. The application is not expected to provide this information. When a memory block is freed, the debug heap manager automatically checks the integrity of the buffers on either side of the user portion and issues an error report if overwriting has occurred. If the _CRTDBG_DELAY_FREE_MEM_DF bit field of the _crtDbgFlag flag is set, the freed block is filled with the value 0xDD, assigned the _FREE_BLOCK block type, and kept in the heap's linked list of memory blocks.

If an error occurs in freeing the memory, errno is set with information from the operating system on the nature of the failure. For more information, see errno, _doserrno, _sys_errlist, and _sys_nerr.

For information about how memory blocks are allocated, initialized, and managed in the debug version of the base heap, see CRT Debug Heap Details. For information about the allocation block types and how they are used, see Types of blocks on the debug heap. For information about the differences between calling a standard heap function and its debug version in a debug build of an application, see Debug Versions of Heap Allocation Functions.


Required header



For more compatibility information, see Compatibility in the Introduction.

For a sample of how to use _free_dbg, see crt_dbg2.

Not applicable. To call the standard C function, use PInvoke. For more information, see Platform Invoke Examples.