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POlocks/condition variables issue
 

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    <p>So in the program I'm writing I have three functions, let's call them A, B and C for simplicities sake. Each function needs to access resource X to work. </p> <p>The restrictions are that A and B are not allowed to run concurrently and must be synchronized appropriately. C is allowed however to run concurrently with either A or B. </p> <p>This is causing me some issues. </p> <p>First I tried putting locks on A and B so when A is called it acquires the lock to X and then releases it and same as B. This way A and B run sequentially. However if I use locks in this manner C is unable to run concurrently with A or B. </p> <p>So I tried using condition variables where A had to wait for a signal from B if B was running and B had to wait for a signal from A, but this sort of interdependent calling didn't seem to work either. </p> <p>Don't know what to do. </p>
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    1. COIf (**and only if**) access from C to the resource doesn't need to be serialized then you do not need to put any lock on C BUT this is a pretty strange situation (to run concurrently doesn't mean that they can access a shared resource in the same moment). Please post more details about what you want to do and what OS/language you're using
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    2. COI'm writing my code in C on my mac. So the shared resource here is a branch of a bank, functions A and B are withdrawal and deposit respectively, and C is transfer. I am not supposed to be able to concurrently do a withdrawal and deposit from the same branch (even on different accounts in the branch) but I can do transfers concurrently within the branch as long as the accounts doing the transferring are not the same as the ones handling a deposit/withdrawal. This has to do with keeping the total branch balance from being corrupted.
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    3. COSo just do not put any lock for transferring (but you'll have to lock somehow at a lower level for account). How to implement this depends on what you're using (is this an example and you access a database directly? you talk with some kind of service?). Personally I wouldn't use a double lock so I would use a account-level locking (for A, B and C) expanding to _any account_ for A and B. Because these operations comes from human users you may think to use a mechanism like the _optimistic lock_ commonly used in DB for A/B vs C and a simple sw lock for A and B.
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