When/how is global "errno" variable from reent.c to be used?

[Grant Edwards]

On 2020-10-16, Richard Damon <Richard@Damon-Family.org> wrote:

> First, newlib considers itself as part of 'the implementation', as its
> purpose is to provide the 'standard library' for an implementation, thus
> it can do what it wants here (as long as the net result meets the
> specifications).

Right. I didn't mean to imply that newlib having a globally visible
integer variable named 'errno' was wrong. I was just asking what it
was for.

> I would have to dig into the actual implementation details to tell
> for sure, but first check if when the statement int errno; is
> compiled, is errno currently a macro, and after replacing that macro
> with its definition what the statement actually defines, it might be
> creating the pointer that is used to access the effective errno.

Sorry, I don't really understand that sentence. The file
newlib/libc/reent/reent.c defines a globally visible integer variable
named 'errno'. I was asking what that globally visible 'int errno' was
for. I've done some additional code browsing, and it appears to be
used by other files in newlib/libc/reent/ to provide fake "reentrant"
<whatever>_r() versions of system calls for which there aren't actual
_r() versions.

Except I don't see how those _r() functions could actaully _be_
rentrant, since they all use a single shared, global, errno variable.
The description/comments claiming they are rentrant seems to be
deceptive.  For example from readr.c (edited for time and to fit your
screen):

    /* Reentrant versions of read system call. */
    [...]
    /* We use the errno variable used by the system dependent layer.  */
    #undef errno
    extern int errno;
    /*
    [...]
    DESCRIPTION
    	This is a reentrant version of <<read>>.  It
    	takes a pointer to the global data block, which holds
    	<<errno>>.
    */
    
    _ssize_t _read_r (struct _reent *ptr, int fd, void *buf, size_t cnt)
    {
      _ssize_t ret;
    
      errno = 0;
      if ((ret = (_ssize_t)_read (fd, buf, cnt)) == -1 && errno != 0)
        ptr->_errno = errno;
      return ret;
    }

I just don't see how that function can be called reentrant.

> It could also be creating a global variable just to limit the breakage
> that programs that do it wrong incur

I don't think so. It appears to actually be used when making calls to
the system-dependent layer.

> (though, I might prefer getting the link error errno not defined
> rather than code that ends up checking the wrong errno).

Agreed.