how to retrieve errno from an os error?

Question

This question is divided into two parts.

Part one:

I run this ...

 1 (handler-case (posix:kill 1 0)
 2   (error (the-condition) (prin1 (type-of the-condition)) (terpri)
 3                          (princ the-condition) (terpri)))

... and get this output:

SYSTEM::SIMPLE-OS-ERROR
UNIX error 1 (EPERM): Operation not permitted

I can use #'princ-to-string and parse the string to get the error number. But is there a more direct way to retrieve errno? Something like #'file-error-pathname, but for errno instead?

Part two:

Where in the documentation could I have found the answer to Part one?

Answer 1

Released version

The released version 2.49 does not have an accessor for the errno. You can get it, however, thusly:

[4]> (setq c (nth-value 1 (ignore-errors (posix:kill 1 0))))
#<SYSTEM::SIMPLE-OS-ERROR #x00000002002634A1>
[5]> (describe c)

#<SYSTEM::SIMPLE-OS-ERROR #x000000020021AF69> is an instance of the CLOS class #1=#<STANDARD-CLASS SYSTEM::SIMPLE-OS-ERROR>.
Slots:
  SYSTEM::$FORMAT-CONTROL     = 
"UNIX error ~S (EPERM): Operation not permitted
"
  SYSTEM::$FORMAT-ARGUMENTS   = (1)

 "UNIX error ~S (EPERM): Operation not permitted
" is a simple 1 dimensional array (vector) of CHARACTERs, of size 47 (a ISO-8859-1 string).

 (1) is a list of length 1.

[6]> (car (slot-value c 'SYSTEM::$FORMAT-ARGUMENTS))
1

Development version

The dev version in the tip of the mercurial repo has os-error instead:

[1]> (setq c (nth-value 1 (ignore-errors (posix:kill 1 0))))
#<OS-ERROR #x0000000200253301>
[2]> (describe c)

#<OS-ERROR #x0000000200253301> is an instance of the CLOS class #1=#<STANDARD-CLASS OS-ERROR>.
Slots:
  SYSTEM::$CODE   = 1

 1 is an integer, uses 1 bit, is represented as a fixnum.

[6]> (apropos "os-error")
OS-ERROR                                   class
OS-ERROR-CODE                              function
EXT::OS-ERROR-CODE-1                    

[10]> (os-error-code c)
1