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Date: Mon, 3 Feb 2020 23:00:13 +0100
From: Sebastian Kemper <sebastian_ml@....net>
To: musl@...ts.openwall.com
Subject: Re: Cross-compiling and -D_LARGEFILE64_SOURCE

On Mon, Feb 03, 2020 at 04:44:45PM -0500, Rich Felker wrote:
> On Mon, Feb 03, 2020 at 09:44:17PM +0100, Sebastian Kemper wrote:
> > Hello list,
> >
> > I'm working on updating apr in OpenWrt. During configure it uses TRY_RUN
> > a lot, so we have a lot of configure variables defined to point it into
> > the right direction.
> >
> > With one variable I have some trouble: apr_cv_use_lfs64=yes. The trouble
> > is I don't know whether or not to set it.
>
> Can you clarify where that came from? It sounds like it wasn't
> detected by configure but something OpenWrt's build scripts are
> forcing. Is that right?

Hi Rich!

Historically OpenWrt had

TARGET_CPPFLAGS += -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -D_GNU_SOURCE

_and_ apr_cv_use_lfs64=yes as configure var. I think this was working
around some bug or the other, I don't know which. Then these got removed
(not by me). A few weeks ago I added myself as maintainer of apr and was
poking around, and saw that buildroot also sets apr_cv_use_lfs64=yes. So
I thought we should use it too. But afterwards I had some doubts about
it and poked around some more. And here we are :D

>
> > It basically forces -D_LARGEFILE64_SOURCE to be included in the
> > CPPFLAGS. So my first impulse was to force it to "yes". But then I saw
> > some build logs from Alpine Linux:
>
> _LARGEFILE64_SOURCE is a horrible horrible thing you never want, so if
> you have the opportunity to omit it, do. We're actually trying to get
> rid of this stuff completely from the headers. It was originally put
> there just as a workaround for a lot of bad GNU software trying to use
> the "64"-suffixed symbols based on invalid configure tests (link-only)
> which broke things badly when no declaration for the functions
> existed.

Remove it I shall, thank you very much!

>
> > https://build.alpinelinux.org/buildlogs/build-edge-x86/main/apr/apr-1.7.0-r0.log
> >
> > <snip>
> > checking whether the compiler provides atomic builtins... yes
> > checking whether to enable -D_LARGEFILE64_SOURCE... no
> > configure: Configured for Linux 4.14
> > <snip>
> >
> > So Alpine is not cross-compiling here, and the result is "no". And this
> > particular log is for i586 (I thought - before seeing this log - that
> > maybe -D_LARGEFILE64_SOURCE is needed for non-64-bit platforms).
>
> It's not. It's legacy junk that should never be used. It's to make
> visible the "64"-suffixed functions on systems where the standard
> functions are limited to 32-bit offsets, which is never the case on
> musl.
>
> > The TRY_RUN that apr uses to find out whether to add
> > -D_LARGEFILE64_SOURCE is pasted at the bottom of this mail. If I run
> > configure on my x86_64 glibc laptop it says "no". In the Alpine logs it
> > also says "no". If I search the Internet I find some old pastes where
> > the outcome was "yes", though. It kind of looks like the answer over
> > time changed from "yes" to "no", for no apparent reason (at least for
> > me).
> >
> > If it sets -D_LARGEFILE64_SOURCE it propagates the same flag to its
> > users via apr-1-config. In the apr sources I see the define also being
> > used occasionally.
> >
> > include/arch/unix/apr_arch_file_io.h:
> >
> > #if APR_HAS_LARGE_FILES && defined(_LARGEFILE64_SOURCE)
> > #define stat(f,b) stat64(f,b)
> > #define lstat(f,b) lstat64(f,b)
> > #define fstat(f,b) fstat64(f,b)
> > #define lseek(f,o,w) lseek64(f,o,w)
> > #define ftruncate(f,l) ftruncate64(f,l)
> > typedef struct stat64 struct_stat;
> > #else
> > typedef struct stat struct_stat;
> > #endif
>
> OK, this is going to break badly. It's also completely bogus (bug in
> APR). *All* feature test macros are conveyed *from* the application
> *to* libc. #define of them belongs only in the application and
> #ifdef/#if defined() of them only in libc headers.
>
> > file_io/unix/open.c:
> >
> > #if APR_HAS_LARGE_FILES && defined(_LARGEFILE64_SOURCE)
> >     oflags |= O_LARGEFILE;
> > #elif defined(O_LARGEFILE)
> >     if (flag & APR_FOPEN_LARGEFILE) {
> >         oflags |= O_LARGEFILE;
> >     }
> > #endif
> >
> > I obviously would like LFS support, but I don't know about this
> > configure variable. I guess the correct answer must be one of these:
> >
> > "Don't use it"
> > "Use it"
> > "Use it if condition X applies"
>
> Don't use it. :-)
>

Thanks Rich! Again I came here for final wisdom and wasn't disappointed
;)

Kind regards,
Seb

> > I guess this is a bit off-topic maybe. But I'm really running out of
> > ideas :) So if anybody can share some insight I'd really appreciate it.
> >
> > Kind regards,
> > Seb
> >
> >    AC_CACHE_CHECK([whether to enable -D_LARGEFILE64_SOURCE], [apr_cv_use_lfs64], [
> >    apr_save_CPPFLAGS=$CPPFLAGS
> >    CPPFLAGS="$CPPFLAGS -D_LARGEFILE64_SOURCE"
> >    AC_TRY_RUN([
> > #include <sys/types.h>
> > #include <sys/stat.h>
> > #include <fcntl.h>
> > #include <stdlib.h>
> > #include <stdio.h>
> > #include <unistd.h>
> >
> > void main(void)
> > {
> >     int fd, ret = 0;
> >     struct stat64 st;
> >     off64_t off = 4242;
> >
> >     if (sizeof(off64_t) != 8 || sizeof(off_t) != 4)
> >        exit(1);
> >     if ((fd = open("conftest.lfs", O_LARGEFILE|O_CREAT|O_WRONLY, 0644)) < 0)
> >        exit(2);
> >     if (ftruncate64(fd, off) != 0)
> >        ret = 3;
> >     else if (fstat64(fd, &st) != 0 || st.st_size != off)
> >        ret = 4;
> >     else if (lseek64(fd, off, SEEK_SET) != off)
> >        ret = 5;
> >     else if (close(fd) != 0)
> >        ret = 6;
> >     else if (lstat64("conftest.lfs", &st) != 0 || st.st_size != off)
> >        ret = 7;
> >     else if (stat64("conftest.lfs", &st) != 0 || st.st_size != off)
> >        ret = 8;
> >     unlink("conftest.lfs");
> >
> >     exit(ret);
> > }], [apr_cv_use_lfs64=yes], [apr_cv_use_lfs64=no], [apr_cv_use_lfs64=no])
> >    CPPFLAGS=$apr_save_CPPFLAGS])
> >    if test "$apr_cv_use_lfs64" = "yes"; then
> >       APR_ADDTO(CPPFLAGS, [-D_LARGEFILE64_SOURCE])
> >    fi
>
> OK, the configure tests that need to run something are a big problem
> if you're cross-compiling, so you probably need to force it to no. The
> above should be factored into two tests, one compile-only and the
> other execution, so that the compile-only one is all that's needed
> when it's sufficient to give a conclusive result.
>
> Rich

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