[GHC] #7229: Detecting if a process was killed by a signal is impossible

[GHC]

#7229: Detecting if a process was killed by a signal is impossible
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        Reporter:  benmachine         |            Owner:
            Type:  bug                |           Status:  new
        Priority:  high               |        Milestone:  7.8.1
       Component:  libraries/process  |          Version:
      Resolution:                     |         Keywords:
Operating System:  Unknown/Multiple   |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown       |       Difficulty:  Unknown
       Test Case:                     |       Blocked By:
        Blocking:                     |  Related Tickets:
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Comment (by andersk):

 Wikipedia is confused.  If you go actually read the
 [http://pubs.opengroup.org/onlinepubs/9699919799/functions/wait.html
 specification], you see

 > WEXITSTATUS(stat_val): … this macro evaluates to the low-order 8 bits of
 the status argument that the child process passed to _exit() or exit(), or
 the value the child process returned from main().

 and also

 > The status values are retrieved by macros, rather than given as specific
 bit encodings as they are in most historical implementations (and thus
 expected by existing programs). This was necessary to eliminate a
 limitation on the number of signals an implementation can support that was
 inherent in the traditional encodings.

 Here’s the
 [https://sourceware.org/git/?p=glibc.git;a=blob;f=bits/waitstatus.h;hb=refs/tags/glibc-2.18#l27
 definition] from glibc:
 {{{
 /* If WIFEXITED(STATUS), the low-order 8 bits of the status.  */
 #define __WEXITSTATUS(status)   (((status) & 0xff00) >> 8)

 /* If WIFSIGNALED(STATUS), the terminating signal.  */
 #define __WTERMSIG(status)      ((status) & 0x7f)
 }}}
 which is to say that bits 8-15 of the wait()/waitpid() status are bits 0-7
 of the exit() status (the rest of the exit() status being thrown away),
 and bits 0-6 of the wait()/waitpid() status are the signal number.  This
 agrees with the Perl/Python encoding (because, of course, that’s where the
 Perl/Python encoding came from).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7229#comment:17>
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