Posixaio

Posixaio refers to the POSIX Asynchronous Input/Output (AIO) interface and, in some Unix-like systems, to a user-space library that implements that interface (often packaged as libposixaio or similar). The POSIX AIO API provides a framework for issuing input/output requests that complete asynchronously, allowing an application to continue processing while I/O is performed in the background.

The API is defined by the POSIX standard and lives primarily in the aio.h header. The central data structure is struct aiocb, which describes an individual I/O operation. Key fields include aio_fildes (the file descriptor), aio_offset (the file offset), aio_buf (the user buffer), aio_nbytes (the number of bytes to transfer), aio_sigevent (notification upon completion), and optional fields for request priority and other controls. Completion notification is typically achieved via the sigevent mechanism, which can deliver a signal, invoke a thread, or perform no notification (polling or manual waiting).

Core functions in POSIX AIO include aio_read and aio_write to submit asynchronous read and write requests, aio_fsync to flush I/O, aio_cancel to cancel outstanding requests, aio_error to check the completion status, aio_return to obtain the transfer result, and aio_suspend or lio_listio for waiting on one or more operations. The lio_listio function supports issuing and waiting on a list of I/O requests.

Implementation notes: POSIX AIO is optional in some systems and features or performance can vary. On Linux, POSIX AIO may be backed by the kernel’s AIO interface or map to newer asynchronous I/O mechanisms such as io_uring in modern environments. Some applications prefer alternative asynchronous I/O approaches (e.g., event-driven I/O or io_uring) for portability and efficiency.

See also: POSIX standards, aio.h, struct aiocb, sigevent, asynchronous I/O in Unix-like systems.