portable way to trap SIGCHLD?

[Eric Blake]

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The coreutils 5.3.0 testsuite has a test (tests/install/trap) that wants
to trap SIGCHLD to detect an infinite loop in install(1) when SIGCHLD was
ignored.  But SIGCHLD is not one of the portable signal numbers (on the
systems I have access to, it is 18 on Solaris and 20 on cygwin).  Note
that coreutils kill(1), bash's builtin kill, and zsh's builtin kill all
have the ability to convert a single signal between name and number:
$ kill -l CHLD
20
$ kill -l 20
CHLD

Ash does not accept signal names, but uses the system's kill (which for my
cygwin system is from coreutils):
$ trap '' CHLD
trap: bad signal CHLD
$ trap -l
trap: bad option -l
$ type kill
kill is /usr/bin/kill
$ kill -l CHLD
20

Solaris /bin/sh has a builtin kill with limited capabilities, but also has
/bin/kill that does what is desired:
$ type kill
kill is a shell builtin
$ kill -l CHLD
kill: bad signal
$ /bin/kill -l CHLD
18

ksh and pdksh use a builtin that can only do one direction of the mapping:
$ type kill
kill is a shell builtin
$ kill -l CHLD
ksh: kill: CHLD: bad number
$ kill -l 20
CHLD

Are there any systems where there are no accessible versions of kill(1)
which can convert either a single signal name to number, or vice versa?
I'm hoping that something like the following can be used to portably trap
an otherwise non-portable signal number, and welcome any help from the
list to turn this into something worthy of adding to autoconf.texi:

# My attempt to portably trap SIGCHLD
for k in kill /bin/kill /usr/bin/kill
do
  chld=`$kill -l CHLD 2> /dev/null`
  test -n "$chld" && break;
done
if test -z "$chld" ; then
  for i in 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 \
           21 22 23 24 25 26 27 28 29 30 31 32
  do
    if test x"`kill -l $i 2> /dev/null`" = xCHLD ; then
      chld=$i
      break
    fi
  done
fi
if test -n "$chld" ; then
  trap '' $chld
else
  echo Failed to find value of SIGCHLD
  exit 1
fi

Of course, for the coreutils testsuite, the solution is simple - coreutils
bundles kill(1), so it can use the just-built kill to determine SIGCHLD in
the tests/install/trap test, but that is besides the point for the general
question.

- --
Life is short - so eat dessert first!

Eric Blake             address@hidden
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