这个函数返回一个信号量集标识符，它与一个参数的键值关联在一起。
This function returns the semaphore set identifier associated with the argument key.
semid是信号量集的标识符。
key是一个惟一的标识符，不同的进程使用它来标识这个信号量集。
The key is a unique identifier that is used by different processes to identify this semaphore set.
semflg值是在返回信号量集标识符之前对访问权限进行验证时设置的。
The semflg value is set while creation is verified for the access permissions before returning semaphore set identifier.
semflg值是在返回信号量集标识符之前对访问权限进行验证时设置的。
The semflg value is set while creation is verified for the access permissions before returning semaphore set identifier.
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