How to byte swap an IEEE Float?
in [ADA]
|
From: Björn on 28 Jun 2005 16:43 > Sure. You know, I meant representation = bits and bytes, implying same > length. It seems the OP already has the float type (IEEE_Float_32), > 32-bit = 4-byte long, and the data are floats of this length, only > different in byte order. > > OP, if the data are of a different length, then you have to unchecked > convert first to a float type of that length, and then normal convert > to the final type (risking Constraint_Error). All the data in the file are of 32 bits float so that should be ok. The target for the values is at the moment x86 (although I do expect it to work on PowerPC as well so I check System.Default_Bit_Order before doing any byte swapping). My dirty workaround for the moment was to read the data as a string. I was merly wondering which is the normal/"best" way to deal with this, since I expect it to be a fairly common task. A packed byte array does however seem like a much better idea than handling it as a string. Regards Björn
From: Stephen Leake on 28 Jun 2005 19:35 "Björn" <ssh9614(a)hotmail.com> writes: > I need to read some float values from file that have been written in > big-endian byte order from a c-program. See SAL.Network_Order.Gen_Scalar_64 at http://www.toadmail.com/~ada_wizard/ada/sal.html for a convenient way to do this. > The simple swapping procedure that I have just interchanges the byte > order of type IEEE_Float_32 to get little-endian. Ok. > The problem is that for some values (eg. 33.229000) it is a "NaN" > when doing IEEE_Float_32'Read and I get a constraint error (invalid > data) from stream_io when the value is read. How do I get around > this? Hmm. Are you saying the original value is _not_ a NaN? If so, then your swap implementation is faulty; try mine. -- -- Stephe
From: Marius Amado Alves on 28 Jun 2005 20:13 On 28 Jun 2005, at 21:43, Björn wrote: >> Sure. You know, I meant representation = bits and bytes, implying same >> length. It seems the OP already has the float type (IEEE_Float_32), >> 32-bit = 4-byte long, and the data are floats of this length, only >> different in byte order. >> >> OP, if the data are of a different length, then you have to unchecked >> convert first to a float type of that length, and then normal convert >> to the final type (risking Constraint_Error). > > All the data in the file are of 32 bits float so that should be ok. > The target for the values is at the moment x86 (although I do expect it > to work on PowerPC as well so I check System.Default_Bit_Order before > doing any byte swapping). My dirty workaround for the moment was to > read the data as a string. I was merly wondering which is the > normal/"best" way to deal with this, since I expect it to be a > fairly common task. A packed byte array does however seem like a much > better idea than handling it as a string. It is better. Because it's portable. Probably you got away with a String because on your system a String happens to be a packed array of bytes, but this is not guaranteed for all systems by the standard.
From: Björn on 29 Jun 2005 06:00 > See SAL.Network_Order.Gen_Scalar_64 at > http://www.toadmail.com/~ada_wizard/ada/sal.html for a convenient way > to do this. Thanks! I will check it out. > Hmm. Are you saying the original value is _not_ a NaN? If so, then > your swap implementation is faulty; try mine. No the original value _is_ a NaN when read as a float on x86 (the subject on this thread was perhaps a bit misleading) BR Björn
From: Damien on 29 Jun 2005 16:10
Bjýrn a ýcrit : > I need to read some float values from file that have been written in > big-endian byte order from a c-program. The simple swapping procedure > that I have just interchanges the byte order of type IEEE_Float_32 to > get little-endian. The problem is that for some values (eg. 33.229000) > it is a "NaN" when doing IEEE_Float_32'Read and I get a constraint > error (invalid data) from stream_io when the value is read. How do I > get around this? > I had to deal with byte swapping for floats and doubles between Intel and PPC. This was C++ and I discovered that swapping had to be done in a neutral form (byte array) before interpretation to float or double. Initialy I directly swapped floats and doubles, and for certain values some bits were changed. So a function with such a signature should never be defined: function Swap (X : Float) return Float; It seems that all bit patterns can not be legal floats. I interpreted this as a normalisation of the float, but this is not sure at all. So, to swap a float or a double X, do this : 1) convert X to a byte array Y (right size) 2) swap Y from host 1 format to network or archive format 3) send Y 4) read Y 5) swap Y from network or archive format to host 2 format 6) convert Y to X I don't know if this is related to your problem, but it seems similar, and it may help. It was C++, but those issues are language independent. Damien Carbonne |