Question on internal workings of Allocate

[wallyg]

When I call Allocate like

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          dim as any ptr	p = Allocate(some value)

The system must store the requested amount and/or the amount allocated to this area. This would be required for

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          Delete p

To work properly.

My question is whether I can get access to this value(s), for reading purposes only. If so how can I safely access this value(s)?

Thank you.

[Imortis]

You shouldn't be mixing Allocate/DeAllocate and New/Delete. If you use Allocate, also use deallocate. If you use New. also use Delete.

Allocate allocates memory of the size you request and spits out a pointer. DaAllocate takes a pointer and marks it invalid. It doesn't destroy anything, which is why the manual says to clear a string before dallocating a pointer to a string.

New and Delete require a pointer to a type, so the type can can be determined with "TypeOf" but that is not really required for new/delete.

What exactly are you trying to do? I fear you are getting into a mess.

[wallyg]

Sorry, I meant Deallocate not Delete.

I am just wondering since when you call Deallocate, you only indicate the start of the block of memory to return to the system. I figured the system would need to know the amount of memory to return so it must be stored somewhere.

Therefore subsequent to the Allocate call, given a memory address that was allocated, can I find out how much was originally allocated?

[jevans4949]

I seem to remember that I discovered the length is in the qword before the pointer which you get back. Don't think this is publicised anywhere You could test fairly easily whether this is the case.

[coderJeff]

Therefore subsequent to the Allocate call, given a memory address that was allocated, can I find out how much was originally allocated?

fbc's allocate/callocate/deallocate/reallocate eventually call malloc/calloc/free/realloc in the C runtime (CRT).

For windows and linux, something like:

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extern "c"
	#if defined(__FB_WIN32__)
		#ifdef __FB_64BIT__
			declare function getBufferSize cdecl alias "_msize" _
				( byval p as any ptr ) as ulongint
		#else
			declare function getBufferSize cdecl alias "_msize" _
				( byval p as any ptr ) as ulong
		#endif
	#elseif defined(__FB_LINUX__)
		#ifdef __FB_64BIT__
			declare function getBufferSize cdecl alias "malloc_usable_size" _
				( byval p as any ptr ) as ulongint
		#else
			declare function getBufferSize cdecl alias "malloc_usable_size" _
				( byval p as any ptr ) as ulong
		#endif
	#else
		#error getBufferSize not implemented
	#endif
end extern

'' test

var a = allocate(100)
var b = allocate(200)
var c = allocate(999)

print getBufferSize(a) 
print getBufferSize(b)
print getBufferSize(c)

sleep

But, there's no guarantee that the CRT implements this and it is different on every platform.

If you search the forum for "_msize" or "malloc_usable_size" and you can see a couple of other places this was mentioned.