Technical Q&A QA1259

Empty Memory Objects

Q: What does `malloc(0)` do? What about `free(NULL)`?

A: These seemingly simple questions have remarkably complex answers, especially if you generalize them to cover other Mac OS memory management APIs. Table 1 and Table 2 list some commonly-used routines and their behavior in these cases.

**Table 1** Allocating a block of zero bytes
Routine	Platform	Returns
malloc	ANSI C specification	either NULL or a pointer to a zero-length block
malloc	Mac OS X System framework	a pointer to a zero-length block
malloc	CodeWarrior MSL	NULL [1] [2]
CFAllocatorAllocate	all	NULL [3]
NewPtr	all	a pointer to a zero-length block
NewHandle	all	a handle to a zero-length block
MPAllocateAligned	Mac OS X	a pointer to a zero-length block
MPAllocateAligned	traditional Mac OS	NULL
OTAllocMem[InContext]	all	a pointer to a zero-length block
operator new []	ANSI C++ specification	a pointer to a zero-length block
operator new []	all	a pointer to a zero-length block

**Table 2** Freeing a NULL pointer
Routine	Platform	Behavior
free	ANSI C specification	does nothing
free	all	does nothing [1]
CFAllocatorDeallocate	all	does nothing [4]
DisposePtr	Mac OS X	sets MemErr to noErr
DisposePtr	traditional Mac OS, PowerPC	sets MemErr to noErr
DisposePtr	traditional Mac OS, 68K	illegal
DisposeHandle	Mac OS X	sets MemErr to nilHandleErr
DisposeHandle	traditional Mac OS	sets MemErr to noErr
MPFree	Mac OS X	illegal [5]
MPFree	traditional Mac OS	does nothing
OTFreeMem	all	does nothing
operator delete	ANSI C++ specification	does nothing
operator delete	all	does nothing

Notes:

MSL information obtained from CodeWarrior Pro 8.3 MSL.
You can alter this behavior by setting the _MSL_MALLOC_0_RETURNS_NON_NULL compile-time variable and rebuilding MSL.
This behavior is part of CFAllocatorAllocate and is independent of the actual allocator used.
This behavior is part of CFAllocatorDeallocate and is independent of the actual allocator used.
On current versions of Mac OS X (10.2 and earlier), this will not crash but instead print an a warning message to the console (r. 3227311) .

Document Revision History

Date	Notes
2011-07-26	Reformatted content and made minor editorial changes.
2003-04-21	New document that describes how two memory management edge cases are handled by the common Mac OS memory allocators.

Empty Memory Objects

Q: What does malloc(0) do? What about free(NULL)?

Document Revision History

Q: What does `malloc(0)` do? What about `free(NULL)`?