Subject: Re: question about the casting in printf
To: None <tech-toolchain@NetBSD.org>
From: der Mouse <mouse@Rodents.Montreal.QC.CA>
Date: 02/08/2007 11:00:37
> unsigned long long ull = 0x0000000100000003; //MyLIB FreeBSD Linux
> printf(¿%Luâ¿, ull); //4294967299 4294967299
> printf(¿%lu¿, ull); //1 3 3
> printf(¿%lu¿, (unsigned long)ull); //3 3 3
I'm not sure what happened to the double quotes in the printf calls;
they seem to have been characters with no Latin-1 equivalents (on
investigating the original, I see they were Unicode 0x201c and 0x201d,
directed left and right double quotes). This will not work; C requires
the double quotes around strings to be undirected double quotes.
(Usually won't work, at least; while compilers that accept directed
double quote characters there might exist, I find it doubtful.)
Assuming that's just an artifact of including the code sample in the
email in a broken way....
> "ull" is a 8-byte value, if we want print it in unsigned long form,
> we will get "0x00000001" without the casting first, "0x00000003" if
> casting first.
This depends on the way your machine passes arguments and, for many
common argument-passing conventions, its endianness as well. Based on
the output you describe, you appear to be talking about a big-endian
> I think the behavior is correct since argument type in va_arg() is
> unsigned long (because of "%lu"), it only handles first 4 bytes,
Yes (well, assuming 4-byte unsigned longs), but...
> so we will get "0x00000001" without casting first.
...the first 4 bytes of what?
Depending on how the C implementation passes arguments, it may not be
the first 4 bytes of the unsigned long long argument you passed. Even
if it is, which 4 bytes are first depends on how arguments are passed,
and, usually, on the endianness of the machine.
> But why I can get "0x00000003" on FreeBSD/Linux platform!?? I don't
If you're running on the same hardware, I can only assume it does
argument passing differently.
I don't see any point in worrying over why results you get differ when
you're doing things whose results are undefined (or
implementation-defined and you're comparing implementations which
define them in different ways). I think the former is the case here,
though I'd have to check with a real C expert to be certain.
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