Synchronization problem with posix_spawn

Ken Brown kbrown@cornell.edu
Wed Jul 29 23:12:50 GMT 2020


On 7/29/2020 4:17 PM, Ken Brown via Cygwin wrote:
> 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

I just took a look at the source, and I see that posix_spawn was taken from 
FreeBSD.  Does FreeBSD have the same problem?  Should applications just be aware 
of this issue and insert a sleep after posix_spawn before sending signals?

Ken


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