_IO_file_underflow_mmap read call for atime

[Ulrich Drepper]

Roland McGrath wrote:

> If we assume that the mmap implementations we are using in fact also meet
> the stricter criterion of updating st_atime on each read fault, then it is
> safe to just use mmap and st_atime will meet the POSIX stdio rules.  I
> haven't experimented on Linux, but this is certainly how mmap behaves on
> GNU/Hurd.

The Linux code modifies atime when it's mapped.  So we'd need a symbol 
MMAP_ATIME_AT_ACCESS which is defined for Hurd but not for Linux.  The 
read call is only performed when the symbol is not defined.

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