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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. Tables 1 and 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:

  1. MSL information obtained from CodeWarrior Pro 8.3 MSL.
  2. You can alter this behavior by setting the _MSL_MALLOC_0_RETURNS_NON_NULL compile-time variable and rebuilding MSL.
  3. This behavior is part of CFAllocatorAllocate and is independent of the actual allocator used.
  4. This behavior is part of CFAllocatorDeallocate and is independent of the actual allocator used.
  5. 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).

[Apr 14, 2003]

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