Re: [musl] errno not set to EBADF when reading from invalid descriptor

[Rich Felker <dalias@libc.org>]

On Thu, Jun 25, 2020 at 01:21:05PM +0000, Stefan Ciotec wrote:
> Hi,
> 
> We are using MUSL C-library 1.1.22.
> According to the POSIX standard, EOF should be returned and errno
> should be set to EBADF for the read group of functions (i.e.
> fgetc(), getc(), getc_unlocked()) when attempting to read from a
> stream with an invalid file descriptor open for reading.
> However, in our tests with MUSL, we discovered that EOF is returned,
> but errno is not set to EBADF (it's 0 instead), for the following
> code:

I think you're misreading the standard. Per ISO C, it's undefined
behavior to call a read function on a FILE stream not opened for read
or update. The POSIX "shall fail" text you're looking at applies when
the FILE stream is open for read or update but the underlying fd is
not open for reading; this can happen with fdopen, when inheriting an
unsuitable fd for stdin via exec, or when using dup2 to replace
fileno(f) for some already-opened FILE with a reference to a different
open file that was opened for write only.

Note that the POSIX text is not very well aligned with the C text, but
the DESCRIPTION in POSIX refers to "the input stream pointed to by
stream". This reflects that it's a constraint, and passing an
inappropriate stream pointer is a constraint violation.

Note that there are lots of other reasons you can't safely use stdio
read functions on a stream you don't know is suitable for it.
Switching from writing to reading without a successful flush produces
UB, and switching the other direction without a successful seek does,
even if the FILE stream is open for both.

Rich