need the unsigned value from dl.call()

[eliss]

I'm using dl.call() to call a C function in an external library. It's
working great so far except for one function, which returns an
unsigned int in the C version. However, in python it returns a signed
value to me. How can I get the unsigned value from this? I haven't
brushed up on my two's complement in a while, so I was hoping someone
could give me a hand.

Thanks

eliss

 

[Larry Bates]

I'm using dl.call() to call a C function in an external library. It's
working great so far except for one function, which returns an
unsigned int in the C version. However, in python it returns a signed
value to me. How can I get the unsigned value from this? I haven't
brushed up on my two's complement in a while, so I was hoping someone
could give me a hand.

Thanks

eliss

It is returning 32 bits. If the sign bit (bit 32) is on it appears as a
negative number. Test for negative and multiply the absolute value * 2.
That should get you the unsigned value you want in a long.

-Larry

 

[Diez B. Roggisch]

It is returning 32 bits. If the sign bit (bit 32) is on it appears as a
negative number. Test for negative and multiply the absolute value * 2.
That should get you the unsigned value you want in a long.

Erm... Nope.

All bits set is -1 - so according to your recipe, that would be abs(-1) * 2
= 2

I'd suggest this formula:

if value < 0:
value = 2^32 + value + 1

Diez

 

[Larry Bates]

Erm... Nope.

All bits set is -1 - so according to your recipe, that would be abs(-1) * 2
= 2

I'd suggest this formula:

if value < 0:
value = 2^32 + value + 1

Diez

Thanks for the correction. You are of course correct.

-Larry

 

[eliss]

Thanks for the correction. You are of course correct.

-Larry

Hi thanks for the responses but I'm afraid I don't see how either
formula works.

Lets say I get the return value of -5, which is 1011b when it should
be 11. Then according to the formula it would be 2^4-5+1=12

But it should be 11...

 

[eliss]

Hi thanks for the responses but I'm afraid I don't see how either
formula works.

Lets say I get the return value of -5, which is 1011b when it should
be 11. Then according to the formula it would be 2^4-5+1=12

But it should be 11...

Seems like the simple formula of:

if value < 0:
value = 2^32 + value

might just work. Thanks

 

[Gabriel Genellina]

Hi thanks for the responses but I'm afraid I don't see how either
formula works.

Lets say I get the return value of -5, which is 1011b when it should
be 11. Then according to the formula it would be 2^4-5+1=12

But it should be 11...

Yes, both formulae were wrong, omit the "+1" in the last one.
Another way is to use a bitwise and (&) with a number whose bits are all
1's.
For 4 bits (your example), you need 1111b = 0xF

py> -5 & 0xF
11

For a 32bit number, you have to use x & 0xFFFFFFFF (or 0xFFFFFFFFL on
older Python versions)
4294967291L

 

[MRAB]

Seems like the simple formula of:

if value < 0:
value = 2^32 + value

might just work. Thanks

You're working in Python so that should be:

if value < 0:
value = 2 ** 32 + value

because ^ is exclusive-or.

 

[Dennis Lee Bieber]

Hi thanks for the responses but I'm afraid I don't see how either
formula works.

Lets say I get the return value of -5, which is 1011b when it should
be 11. Then according to the formula it would be 2^4-5+1=12

But it should be 11...

A signed 4-bit value can't represent an 11...

From glimpses at the thread, I suspect two separate ideas have
somehow conjoined... The first being the 2s complement, in which, as I
recall my CS classes, the conversion from negative to positive value is
to first 1s complement the value (flip the bits), and then add 1

1011 -5
0100 ones complement
+ 0001
= 0101 twos complement +5

But you aren't searching for the twos complement of some negative
number... You want the /positive/ number equivalent to an extended
integer that has one bit more range.

01011 +11

which IS different from sign extending that -5

11011 (5-bit) -5
00100 ones complement
+ 00001
= 00101 +5
--
Wulfraed Dennis Lee Bieber KD6MOG
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HTTP://wlfraed.home.netcom.com/
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