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Top Forums Programming pass a pointer-to-pointer, or return a pointer? Post 302273937 by redoubtable on Tuesday 6th of January 2009 09:04:53 AM
Old 01-06-2009
The actual difference is that the first example copies the address of your pmem pointer to the stack and the second example doesn't. The second example only works because you're using malloc() which allocates memory from the heap, otherwise you could have unexpected behavior because you would be returning a local variable (which is cleared upon function completion).

IMHO the second option would be faster because there's one less parameter to be copied to the stack and there's no desreferencing of the pointer's address to obtain the data. Anyway, you should feel no significant speed difference in any of the alternatives.
 

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STACK(9)						   BSD Kernel Developer's Manual						  STACK(9)

NAME
STACK -- stack macros SYNOPSIS
#include <sys/param.h> type STACK_ALLOC(sp, size); type STACK_MAX(sp, size); type STACK_ALIGN(sp, bytes); type STACK_GROW(sp, size); type STACK_SHRINK(sp, size); DESCRIPTION
A stack is an area of memory with a fixed origin but with a variable size. A stack pointer points to the most recently referenced location on the stack. Initially, when the stack has a size of zero, the stack pointer points to the origin of the stack. When data items are added to the stack, the stack pointer moves away from the origin. The STACK_ALLOC() macro returns a pointer to allocated stack space of some size. Given the returned pointer sp and size, STACK_MAX() returns the maximum stack address of the allocated stack space. The STACK_ALIGN() macro can be used to align the stack pointer sp by the specified amount of bytes. Two basic operations are common to all stacks: a data item is added (``push'') to the location pointed by sp or a data item is removed (``pop'') from the stack. The stack pointer must be subsequently adjusted by the size of the data item. The STACK_GROW() and STACK_SHRINK() macros adjust the stack pointer sp by given size. A stack may grow either up or down. The described macros take this into account by using the __MACHINE_STACK_GROWS_UP preprocessor define. SEE ALSO
param(3), queue(3) BSD
April 8, 2011 BSD
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