On Sat, 2017-09-02 at 21:47:15 +0200, Boud Roukema wrote: > On Sat, 2 Sep 2017, Adam Borowski wrote: > > > AC_SYS_LARGEFILE on i386 (linux) for mpgrafic-0.3.15-1 apparently does > > > *not* > > > always do the right thing
> > Nope, you use pwrite() not pwrite64() -- on i386 you need the latter to get > > LFS; they're aliases only on amd64, x32 and so on. That's not correct, neither the new changes to the description of the tag. When you define _FILE_OFFSET_BITS=64 the interfaces are transparently mapped to the 64-bit variants and no other code change is required. So you *must* not be switching from foo to foo64 in the call sites. This is IMO the preferred way to enable LFS. The other way to enable LFS is to define _LARGEFILE64_SOURCE, which makes visible the 64-bit variants in parallel to the 32-bit default interfaces, so you get foo and foo64, and you can call either from the code. This might be useful in case you are for example writing a library and need to support entry points for both 32-bit and 64-bit functions for backwards compatibility. Otherwise I'd strongly discourage its use. > OK, so config.h + p(read|write)64 should hopefully solve the mpgrafic > bug. I initially tried hardwiring pwrite64 and pread64 (compiling on > amd64), but got "warning: implicit declaration of function ‘pwrite64’ > [-Wimplicit-function-declaration]". My understanding is that > _FILE_OFFSET_BITS and __USE_FILE_OFFSET64 are *not* set for amd64 by > AC_SYS_LARGEFILE, and instead only set for some systems, such as i386. > The following should correctly work together with AC_SYS_LARGEFILE, it > seems to me. > > Does this look OK? > > #ifndef __USE_FILE_OFFSET64 > i_stat=pwrite(fd,buffer,size,offset); > #else > i_stat=pwrite64(fd,buffer,size,offset); > #endif > > #ifndef __USE_FILE_OFFSET64 > i_stat = pread(fd,buffer,size,offset); > #else > i_stat = pread64(fd,buffer,size,offset); > #endif Nope, that looks wrong. The __USE_* macros are internal implementation details, and applications should not be using those. In addition you should just be using pread() and pwrite(), which will call the correct interfaces. > > But in any case, even if it had been indeed a false positive, it'd be a > > different bug. > > I think I should post a lintian bug for improving the info field for > the LFS unsafe detection - I spent quite a bit of time searching for how > to handle this, and having more detailed information could help other > debian contributors respond more easily to built-without-LFS-support > warnings. That patch needs to be reverted. I can provide a new patch explaining in more detail this though and giving more pointers perhaps. Thanks, Guillem
