Corrections to aio_return and aio_error

The first issue I tackled involved the implementation of aio_return and aio_error. According to the specification for aio_return():

“The aio_return() function may be called exactly once to retrieve the return status of a given asynchronous operation; thereafter, if the same aiocb structure is used in a call to aio_return() or aio_error(), an error may be returned. When the aiocb structure referred to by aiocbp is used to submit another asynchronous operation, then aio_return() may be successfully used to retrieve the return status of that operation.”

This aspect was not considered in the initial implementation, where both methods simply returned a private field of the aiocbp structure. I solved the problem by adding a new private field to the aiocb struct, called return_status, to track whether a request associated with an aiocb has already been returned. return_status can have two values: AIO_RETURNED or AIO_NOTRETURNED.

I then modified the existing code in this way:

• In the aio_enqueue() method, I set return_status to AIO_NOTRETURNED so that every time a new request for an aiocb is enqueued, it can be returned.
• In aio_error(), I added a check for return_status. If return_status is AIO_RETURNED, the function returns an error. Otherwise, it returns the value of the error_code field.
• In aio_return(), I added a check like the one in aio_error(), with the additional step of setting return_status to AIO_RETURNED when returning the value in return_value.

During this modification, I also noticed that the methods lacked a NULL pointer check for the input, so I added these checks where necessary.