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Inside Macintosh: Memory /

Chapter 2 - Memory Manager / Memory Manager Reference

Memory Manager Routines / Changing the Sizes of Relocatable and Nonrelocatable Blocks

GetPtrSize

You can use the GetPtrSize function to find out the logical size of the nonrelocatable block corresponding to a pointer.

FUNCTION GetPtrSize (p: Ptr): Size;

p

A pointer to a nonrelocatable block.

DESCRIPTION

The GetPtrSize function returns the logical size, in bytes, of the nonrelocatable block pointed to by p. In case of an error, GetPtrSize returns 0.

ASSEMBLY-LANGUAGE INFORMATION

The registers on entry and exit for GetPtrSize are

Registers on entry
A0	Pointer to the nonrelocatable block

Registers on exit
D0	If >=0, number of bytes in nonrelocatable block
	If <0, result code

The trap dispatcher sets the condition codes before returning from a trap by testing the low-order word of register D0 with a TST.W instruction. Because the block size returned in D0 by _GetPtrSize is a full 32-bit long word, the word-length test sets the condition codes incorrectly in this case. To branch on the contents of D0, use your own TST.L instruction on return from the trap to test the full 32 bits of the register.

RESULT CODES

noErr	0	No error
memWZErr	-111	Attempt to operate on a free block