File locking and pthreads

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Hi,
I'm developing a multithreaded TFTP server for an exam, but i'm running into a lot of problems in finding a way to lock files (just as fcntl, flock and lockf do for processes) that works for pthreads. I think this is a typical problem, so i hope someone can suggest a "simple" solution.

Posted 1-Apr-11 8:12am

tuccio

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tuccio 1-Apr-11 16:15pm

pthread's rwlocks and mutexes require processes to share memory in order to work, that doesn't look simple to be honest

also that would require me to identify files on the filesystem, and mapping them with their own rwlock.. so i'd need a data structure to manage these locks, wouldn't i? or maybe i'm making it too complicated?

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CPallini · Accepted Answer · 2011-04-01T09:13:00

Solution 1

Just use the provided POSIX thread synchronization mechanism (like mutex, condition variables).

Posted 1-Apr-11 9:13am

CPallini

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Sergey Alexandrovich Kryukov 1-Apr-11 17:01pm

Sure; and the real problem is good architecture which I (even I :-) would not volunteer to teach in the format of quick Questions/Answers. So what... a 5 I guess. :-)
--SA

CPallini 1-Apr-11 17:57pm

Thank you. Teaching the good architecture could be material for an article about.

tuccio 1-Apr-11 19:15pm

Well, maybe i could do this, but i was wondering.. can i allocate a std::map on a shared memory segment i get with shmget, or i need to implement my own data structure?

lijiantao 1-Apr-11 23:36pm

yes, you can use std::map

tuccio 2-Apr-11 7:10am

well.. how? i mean, and how can i be sure that no memory will be allocated out of my data segment? i guess that any "new" would be a problem, isn't it?

tuccio 2-Apr-11 12:32pm

ok, i've been told i should use an allocator, but that looks complicated to be honest

CPallini 2-Apr-11 16:08pm

Why do you need memory shared among processes? Threads of the same process already shares memory.

tuccio 2-Apr-11 17:13pm

because i'm expressly required not to allow multiple server instances to access the same files when one of them is writing

maybe i could find a way to combine flock and rwlocks safely

CPallini 3-Apr-11 4:08am

Why did you need multiple server instances while the server is multithreaded?

tuccio 3-Apr-11 9:20am

you should ask my professor :D

i see you're italian, so i just paste: "Non e' consentito cioe' accesso ne' in lettura ne' in scrittura da parte di altre istanze del server tftp allo stesso file mentre il file viene scritto."

i'm expressly required to do so :/

CPallini 3-Apr-11 12:12pm

OK, but then why should you have more than one instance of the server running?
However, at the very end, you may use a mutext lock the access to the file (the mutex acts either on threads of the same process and of other processes).