8 bit os_f handles

[David J. Ruck]

On Tue 28 Mar, Daniel Ellis wrote:
> If anyone tries using code form the library on a machine with a
> perfectly legal filesystem that allocates filehandles above 255 then
> strange things will occur and they won't have the faintest reason
> why.  This will cause lack of confidence in the OS and the software.

Exactly. This problem is can lead to the one of the worse insiduous
errors I've come accross. Not all code sequences will fail with
32 bit handles because of the way the compiler keeps variables in
registers. Take this example using the current OSLib:-

	os_f f = osfind_openin(osfind_NO_PATH, info.filename, NULL);
	osgbpb_read(f, (byte*)&decomp, sizeof(bits));
	osfind_close(f);
    
This compiles to:-
	
	SUB     sp,sp,#4
	MOV     a3,#0
	MOV     a1,#3
	LDR     a2,[v1,#0]
	BL      osfind_openin
	MOV     v4,a1
	MOV     a2,sp
	MOV     a3,#4
	BL      osgbpb_read
	MOV     a1,v4
	BL      &00009C58
	
|osfind_openin|
	ORR     a1,a1,#&40         ; ="@" (64)
	SWI     OS_Find
	MOVS    pc,lr
|osgbpb_read|
	STMFD   sp!,{v1}
	MOV     a4,a3
	MOV     a3,a2
	MOV     a2,a1
	MOV     a1,#4
	SWI     OS_GBPB
	MOV     a1,a4
	LDMFD   sp!,{v1}
	MOVS    pc,lr
|osfind_close|
	MOV     a2,a1
	MOV     a1,#0
	SWI     OS_Find
	MOVS    pc,lr

As you can see because OS_Lib returns values in regsiters and those
regsiters are passed directly to the other filing system functions, the 32bit
file handles are maintained.

BUT if mode code is inserted between osfind_openin() and osgbpb_read(), and
the function block is suffienctly complex to cause the os_f handle to be
backed off on to the automatic varibale storage on the stack (or if it were a
global or part of a stucture) it will be stored and retrieved using byte
instructions, therefor truncating it to 8 bits.

So code which happend to work with 32bit handles can be broken by changing
* non releated code in the same function
* compiler optimisation level / debug options / version

Please belive me you dont want to have to track down these types of problems
with your code - you will not have a clue why certain filing system code
works, others dont, and changing the slightest thing can alter it.

Cheers
---Dave

-- 
______________________________________________________________________

  David J. Ruck     Phone: 07974 108301     Email: druck at freeuk.com
______________________________________________________________________