flocking a socket

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi,

     I'd like to flock() a socket passed to a child process 
via a filehandle (STDOUT).

     I've tried doing this in the child:

flock(STDOUT, 2);    # LOCK_EX = 2 - grab lock
syswrite(STDOUT, "$message");
flock(STDOUT, 8);    # LOCK_UN = 8 - free lock
[download]


But flock() doesn't work, I think because all children 
share the same sockethandle - flock() is always 
successful in gaining a lock - it seems flock() does 
not work across child processes.

One solution I've heard of is to open a new filehandle in 
each child - and flock on this per-child filehandle.

Does anyone know how to do that to a socket?

Is something like this feasible?

open(CHILDHANDLE);      # not sure how to do this?
*CHILDHANDLE = *STDOUT; # unique handle on parent socket

flock(CHILDHANDLE, 2);    # LOCK_EX = 2 - grab lock
syswrite(CHILDHANDLE, "$message");
flock(CHILDHANDLE, 8);    # LOCK_UN = 8 - free lock
[download]


Any perls of wisdom would be much appreciated...

Nige

Comment on flocking a socket Select or Download Code

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Re: flocking a socket by kschwab (Vicar) on Jan 25, 2002 at 19:54 UTC
The following text from my Solaris man page for flock() would seem to indicate that you can't do this: Locks are on files, not file descriptors. That is, file descriptors duplicated through dup(2) or fork(2) do not result in multiple instances of a lock, but rather multiple references to a single lock. If a process holding a lock on a file forks and the child explicitly unlocks the file, the parent will lose its lock. Locks are not inherited by a child process. I'm guessing that you are trying to coordinate reads or writes between the parent and child. I think that restating the higher-level problem you are trying to solve would be more helpful.	[reply]
Re: flocking a socket by clintp (Curate) on Jan 25, 2002 at 19:45 UTC
This code: `flock(HANDLE, 2);` [download] Probably isn't doing what you think it is. flock() has a return value, and it's important. Try this: `flock(HANDLE, 2) \|\| warn "flock failed: $!";` [download] I won't even begin to nag you about fcntl constants, you've got a harsh lesson ahead of you already. :)	[reply] [d/l] [select]