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Date: Tue, 16 Mar 2021 01:09:16 +0300 (MSK)
From: Alexander Monakov <>
cc: Dominic Chen <>
Subject: Re: Issue with fread() and unaligned readv()

On Mon, 15 Mar 2021, Rich Felker wrote:

> On Mon, Mar 15, 2021 at 05:39:43PM -0400, Dominic Chen wrote:
> > Not sure this counts as a problem in musl or the application, but
> > I've been debugging a return error of EINVAL from `fread(&buf, 8,
> > 16, f)`, where `f = fopen("/proc/self/pagemap", "r")`. Internally,
> > musl converts this into a call to `readv(f->fd, iov, 2)`, where `iov
> > = {{iov_base = buf, iov_len = 127}, {iov_base = f->buf, iov_len =
> > 1024}}`. However, it turns out that the kernel VFS read
> > implementation inside `pagemap_read` checks that both the file
> > position and count are divisible by PM_ENTRY_BYTES (8 on x86_64),
> > otherwise it rejects the read with EINVAL. In comparison, glibc's
> > `_IO_file_xsgetn` does appear to try to maintain read alignment,
> > although I haven't looked at it in detail.
> You can't use stdio to read or write special files/devices that depend
> on the reads or writes happening in particular units, because the
> relationship between stdio operations and the underlying
> buffer-fill/flush operations on the underlying fd is unspecified. It's
> really unfortunate that the kernel lies that procfs files are regular
> files but doesn't give them regular-file semantics, but you really
> need to use direct operations on the fd in the units the interface
> requires, rather than stdio, to work with these files.

Where does iov_len = 127 for the first iov tuple come from, though?
>From fread arguments I'd expect 8 * 16 = 128.

If musl always does such off-by-one, it is an efficiency issue (forces
a copy with mismatching source/dest alignment).


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