# TIP 128: Ability to Install a Custom Memory Allocator
	Author:         Christophe Cap <udragon@users.sourceforge.net>
	Author:         Mike Jackson <hhyJackson1275@who-got-mail.com>
	State:          Rejected
	Type:           Project
	Vote:           Done
	Created:        13-Mar-2003
	Post-History:   
	Tcl-Version:    8.6
-----

# Abstract

This TIP alters Tcl to allow embedded uses of the Tcl library \(and any
extensions\) to either use the Tcl memory allocators as their main
allocator \(especially in C\+\+\) or to set the memory allocator that Tcl
uses for itself through _ckalloc\(\)_.

# Background

A while ago I was experiencing troubles when allocating images
\([image create photo]\) while memory was already exhausted, my app
crashed \(due to known bug item \#698571, which is in the HEAD by now!\)
This shouldn't happen anyway since my application had it's new handler
set.

Tracing down the source of the allocators I noticed that Tcl uses
_HeapAlloc\(\)_ \(on Win32\) to allocate its memory.  Why not use
_malloc\(\)_?

# New/Malloc Handler 

It would be nice to be able to catch memory allocation errors with a
custom new handler.

A solution could be to replace _HeapAlloc\(\)_ \(on Win32\) and other
platform specific memory handlers should be replaced by _malloc\(\)_.

This way a new handler can by set through _set\_new\_handler\(\)_.

Note that the Microsoft VC\+\+ compiler has some ANSI incompatibility in
that it uses _\_set\_new\_handler\(\)_ rather than _set\_new\_handler\(\)_.
We would naturally conceal this platform difference.

For example:

	#include <new> 
	
	//
	// New handler for Microsoft Visual C++ compiler
	//
	
	#ifdef _MSC_VER
	#include <new.h>
	
	int __cdecl _newHandler(size_t size )
	{
	   // Do whatever
	
	   return 0;
	}
	
	#else
	
	//
	// Ansi C/C++ new handler
	//
	
	void __cdecl _newHandler( void )
	{
	   // Do whatever
	}
	#endif
	
	void sethandlers(void)
	{
	// Microsoft compiler 
	#ifdef _MSC_VER  
	
	   _set_new_handler (_newHandler); // Setup new handler
	   _set_new_mode( 1 ); // Re-route malloc failures to new handler !
	
	// Ansi compiler 
	
	#else 
	
	   set_new_handler (_newHandler); // ANSI new handler
	
	#endif
	}

# Tcl Implementation 

The above suggested solution could work for some compilers, but may
not for all \(some compilers might not support setting a malloc failure
callback.\)  Therefore a Tcl custom new handler functionality could be
implemented that handles Tcl specific memory allocation failures.

Something like: _Tcl\_SetMemHandler\(\)_?

# Copyright

This document has been placed in the public domain.

