Non-volatile errno

[ricaljasan]

Chapter Error Reporting, section Checking for Errors defines errno as:

@deftypevr {Variable} {volatile int} errno

and continues on to say:

Since @code{errno} is declared @code{volatile}, it might be changed
asynchronously by a signal handler; see @ref{Defining Handlers}.

but errno is not declared volatile.  On a system built with 2.19:

$ find /usr/include -name errno.h | xargs grep 'volatile.*errno'
$ find /usr/include -name errno.h | xargs grep 'int errno'
/usr/include/errno.h:extern int errno;

In the current git:
$ find . -name errno.h | xargs grep 'volatile.*errno'
$ find . -name errno.h | xargs grep 'int errno'
...
./stdlib/errno.h:extern int errno;

I assume extern is taken for granted since I don't see any other
variables declared extern in the manual.

Also, I'm not sure about the correctness of the subsequent description.
 Is it true that a signal handler can asynchronously change errno
*because* it is declared volatile?  If so, does removing the volatile
declaration warrant removing the comment about signal handlers, or
should that be reworded to say something else?

Thank you,
Rical Jasan

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