posix_spawn(p) returns before the spawned process is fully up and running.  As a 
result, the spawned process can fail to receive signals.  The attached test case 
illustrates the problem.  It spawns a sleep process and then tries to kill it. 
On exit, the sleep process is still running.

The following excerpts from the strace output show the issue: The SIGTERM signal 
is sent after the main program has forked a subprocess (and posix_spawnp has 
returned), but before the forked subprocess has exec'd the sleep process:

   559   32069 [main] spawn_test 4125 vfork: stub called
   257   48437 [main] spawn_test 4125 dofork: 4126 = fork()
   754    9511 [main] spawn_test 4126 dofork: 0 = fork()
    66   48503 [main] spawn_test 4125 kill0: kill (4126, 15)
    44    9555 [main] spawn_test 4126 find_exec: find_exec (/usr/bin/sleep)
    42   10835 [main] spawn_test 4126 spawnve: spawnve (/usr/bin/sleep, sleep, 
0x8000281A0)
    45    3149 [main] sleep 4126 child_info::ready: signalled 0x164 that I was ready
  6475   21055 [main] spawn_test 4126! child_info::sync: pid 45028, WFMO 
returned 0, exit_code 0x103, res 1
--- Process 45028 (pid: 4126) thread 41444 created

Ken