Re: getpagesize.2

[Alejandro Colomar <alx@kernel.org>]

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Hi Elliott!

On Tue, Mar 26, 2024 at 03:27:27PM -0700, enh wrote:
> i'm not sure a lot of the text in getpagesize.2 is very helpful? for
> example:
> 
>        Portable applications should employ sysconf(_SC_PAGESIZE) instead
>        of getpagesize():
> 
>            #include <unistd.h>
>            long sz = sysconf(_SC_PAGESIZE);
> 
>        (Most systems allow the synonym _SC_PAGE_SIZE for _SC_PAGESIZE.)
> 
> is this a good recommendation _unless_ you've found yourself on a system
> that doesn't have getpagesize()? on Android i'm advising getpagesize()
> because it's (a) attribute const and (b) inherently cheaper than sysconf().
> (though obviously, "if it really matters, stick it in a static".)
> 
> afaict the only motivation for the "portable applications" advice is that
> HP-UX removed getpagesize() at some point? but given that HP-UX has been
> kept "alive" in an artificial coma since 2007... does anyone writing code
> in 2024 need to worry about that enough for this to be good advice?

I hope not.  It's dead, AFAIC.

> getpagesize() is available on Android, *BSD, iOS/macOS, glibc/musl, and
> even mingw!
> 
>        Whether getpagesize() is present as a Linux system call depends
>        on the architecture.  If it is, it returns the kernel symbol
>        PAGE_SIZE, whose value depends on the architecture and machine
>        model.  Generally, one uses binaries that are dependent on the
>        architecture but not on the machine model, in order to have a
>        single binary distribution per architecture.  This means that a
>        user program should not find PAGE_SIZE at compile time from a
>        header file, but use an actual system call, at least for those
>        architectures (like sun4) where this dependency exists.
> 
> should this just say alpha, sparc32, and sparc64 instead of being vague? (i

Yep, I'd appreciate that, after the current text (not replacing it).

> suspect anyone who doesn't default to searching the kernel source would
> turn to the man pages to answer the "which architectures?" question :-)
> though i'm not sure why anyone who doesn't read the kernel source would
> care whether getpagesize() should really be in man2 or man3...)
> 
>        glibc 2.0 fails because its getpagesize() returns a statically
>        derived value, and does not use a system call.  Things are OK in
>        glibc 2.1.
> 
> 1997 called, and wondered whether we're helping anyone by documenting a bug
> that's been fixed longer than several folks on my team have been alive? :-)

Agree.

Do you intend to send patches?  Otherwise, let me know, and I'll prepare
something.

Have a lovely night!
Alex

-- 
<https://www.alejandro-colomar.es/>
Looking for a remote C programming job at the moment.

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