1. bytes_to_int bug?
I recently needed to read some binary integers from a file, so
I used the routine:
function read_16bit( handle )
integer b1, b2
b1 = getc( handle )
b2 = getc( handle )
return (b2*256) + b1
end function
This worked fine most of the time, on most files, but in some cases
would return something like -254, which was clearly not right. This
was because it was reading two #FFFFs, which it read as -1. So I
changed the code to:
function read_16bit( handle )
integer b1, b2
b1 = getc( handle )
b2 = getc( handle )
return bytes_to_int( { b1, b2, 0, 0 } )
end function
But this still did not work. Finally, I changed it to:
function read_16bit( handle )
integer b1, b2
b1 = getc( handle )
b2 = getc( handle )
return and_bits(b2*256, b1)
end function
Which returned a reasonable result.
My question: is there a bug in bytes_to_int (and, by
implication, ints_to_byte)?
-- David
2. Re: bytes_to_int bug?
David Cuny wrote:
> function read_16bit( handle )
> integer b1, b2
> b1 = getc( handle )
> b2 = getc( handle )
> return (b2*256) + b1
> end function
> This worked fine most of the time, on most files, but in
> some cases would return something like -254, which was
> clearly not right. This was because it was reading
> two #FFFFs, which it read as -1.
getc() should always return a number between 0 and 255
*except* when it is at end-of-file, where it will return -1.
Are you sure that you weren't at end-of-file? I wrote
two #FF bytes to a file and read them back with your function.
It gave me 65535 (#FFFF) as it should.
> So I changed the code to:
> function read_16bit( handle )
> integer b1, b2
> b1 = getc( handle )
> b2 = getc( handle )
> return bytes_to_int( { b1, b2, 0, 0 } )
> end function
bytes_to_int({b1,b2,0,0}) should give the same result. It
simply multiplies b2 by 256 and adds b1.
Regards,
Rob Craig
Rapid Deployment Software
