Re: how to detect the compile is 32 bits or 64 bits?
- From: Tim Prince <timothyprince@xxxxxxxxxxxxx>
- Date: Sat, 05 Aug 2006 16:37:02 GMT
Walter Roberson wrote:
In article <0f3Bg.2584$9T3.1111@xxxxxxxxxxxxxxxxxxxxxxxxxx>,OP didn't explain what the reason for non-portable code might be. Certainly, there wasn't anything about finding the largest efficient data types.
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.
So, your interpretation of 32- bit vs 64-bit is based on whether there is an efficient implementation of 64-bit int. It's certainly likely, but not assured, that a compiler would choose (sizeof)int in accordance with efficiency. For example, most Windows compilers have 32-bit int and long even on platforms which have efficient 64- and (limited) 128-bit instructions, where linux for the same platform supports 64-bit int.
If this is your goal, you need a configure script which checks existence of various candidate data types, which don't necessarily appear in <stdint.h>, and tests their efficiency.
.
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- how to detect the compile is 32 bits or 64 bits?
- From: steve yee
- Re: how to detect the compile is 32 bits or 64 bits?
- From: Tim Prince
- Re: how to detect the compile is 32 bits or 64 bits?
- From: Walter Roberson
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