set-new-handler.md

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_set_new_handler

Transfers control to your error-handling mechanism if the new operator fails to allocate memory.

Syntax

_PNH _set_new_handler( _PNH pNewHandler );

Parameters

pNewHandler
Pointer to the application-supplied memory handling function. An argument of 0 causes the new handler to be removed.

Return Value

Returns a pointer to the previous exception handling function registered by _set_new_handler, so that the previous function can be restored later. If no previous function has been set, the return value can be used to restore the default behavior; this value can be NULL.

Remarks

The C++ _set_new_handler function specifies an exception-handling function that gains control if the new operator fails to allocate memory. If new fails, the run-time system automatically calls the exception-handling function that was passed as an argument to _set_new_handler. _PNH, defined in New.h, is a pointer to a function that returns type int and takes an argument of type size_t. Use size_t to specify the amount of space to be allocated.

There is no default handler.

_set_new_handler is essentially a garbage-collection scheme. The run-time system retries allocation each time your function returns a nonzero value and fails if your function returns 0.

An occurrence of the _set_new_handler function in a program registers the exception-handling function specified in the argument list with the run-time system:

// set_new_handler1.cpp
By default, this function's global state is scoped to the application. To change this, see [Global state in the CRT](../global-state.md).

#include <new.h>

int handle_program_memory_depletion( size_t )
{
   // Your code
}

int main( void )
{
   _set_new_handler( handle_program_memory_depletion );
   int *pi = new int[BIG_NUMBER];
}

You can save the function address that was last passed to the _set_new_handler function and reinstate it later:

   _PNH old_handler = _set_new_handler( my_handler );
   // Code that requires my_handler
   // . . .
   _set_new_handler( old_handler )
   // Code that requires old_handler
   // . . .

The C++ _set_new_mode function sets the new handler mode for malloc. The new handler mode indicates whether, on failure, malloc is to call the new handler routine as set by _set_new_handler. By default, malloc does not call the new handler routine on failure to allocate memory. You can override this default behavior so that, when malloc fails to allocate memory, malloc calls the new handler routine in the same way that the new operator does when it fails for the same reason. To override the default, call:

_set_new_mode(1);

early in your program or link with Newmode.obj.

If a user-defined operator new is provided, the new handler functions are not automatically called on failure.

For more information, see new and delete in the C++ Language Reference.

There is a single _set_new_handler handler for all dynamically linked DLLs or executables; even if you call _set_new_handler your handler might be replaced by another or that you are replacing a handler set by another DLL or executable.

Requirements

Routine	Required header
_set_new_handler	<new.h>

For more compatibility information, see Compatibility.

Example

In this example, when the allocation fails, control is transferred to MyNewHandler. The argument passed to MyNewHandler is the number of bytes requested. The value returned from MyNewHandler is a flag indicating whether allocation should be retried: a nonzero value indicates that allocation should be retried, and a zero value indicates that allocation has failed.

// crt_set_new_handler.cpp
// compile with: /c
#include <stdio.h>
#include <new.h>
#define BIG_NUMBER 0x1fffffff

int coalesced = 0;

int CoalesceHeap()
{
   coalesced = 1;  // Flag RecurseAlloc to stop
   // do some work to free memory
   return 0;
}
// Define a function to be called if new fails to allocate memory.
int MyNewHandler( size_t size )
{
   printf("Allocation failed. Coalescing heap.\n");

   // Call a function to recover some heap space.
   return CoalesceHeap();
}

int RecurseAlloc() {
   int *pi = new int[BIG_NUMBER];
   if (!coalesced)
      RecurseAlloc();
   return 0;
}

int main()
{
   // Set the failure handler for new to be MyNewHandler.
   _set_new_handler( MyNewHandler );
   RecurseAlloc();
}

Allocation failed. Coalescing heap.

This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.

See also

Memory Allocation
calloc
free
realloc