Re: how to detect the compile is 32 bits or 64 bits?
- From: roberson@xxxxxxxxxxxxxxxxxx (Walter Roberson)
- Date: Sat, 5 Aug 2006 16:23:52 +0000 (UTC)
In article <0f3Bg.2584$9T3.1111@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
Tim Prince <tprince@xxxxxxxxxxxxxxxxxxx> wrote:
steve yee wrote:
i want to detect if the compile is 32 bits or 64 bits in the source
code itself. so different code are compiled respectively. how to do
this?
You invite bugs, as well as taking the question off the topic of
standard C, if you write source code which has to change.
Not necessarily.
The usual
question is about (sizeof)<various pointer types>. If your question is
simply about (sizeof)int, you create problems by assuming it is
determined by whether you have a 32- or 64-bit platform. If you insist
on unions of pointers and ints, knowing whether it is 32 or 64 bits
won't save you. More so, if you are one of those who writes code which
depends on (sizeof)(size_t x).
You appear to have read a fair bit into the poster's question
that I don't think is justified by what the poster wrote.
Suppose I have an algorithm, such as a cryptography algorithm, that
operates on chunks of bits at a time. The algorithm is the same
(except perhaps for a few constants) whether I'm computing with 32 or
64 bits, but the chunk size differs for the two cases. In such a case,
I -could- write the code using only the minimum guaranteed size,
but on most platforms it would be noticably more efficient to use
the larger chunk size *if the platform supports it*. The constants
for the algorithm could probably be computed at run-time and a
generic algorithm used, but in the real world, having the constants
available at compile time increases compiler optimization opportunities
leading to faster code.
In C, it is not an error to write int x = 40000; for use on
platforms that have an int of at least 17 bits. It is not maximally
portable, but it is not an error -- and the OP was asking for a
a compile-time method of selecting such code for platforms that allow it,
dropping back to smaller value assumptions when that is all the
platform supports.
--
Okay, buzzwords only. Two syllables, tops. -- Laurie Anderson
.
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