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AIO_READ(2) System Calls Manual AIO_READ(2)


aio_readasynchronous read from a file (REALTIME)


Standard C Library (libc, -lc)


#include <aio.h>

aio_read(struct aiocb *iocb);


The () system call allows the calling process to read iocb->aio_nbytes from the descriptor iocb->aio_fildes beginning at the offset iocb->aio_offset into the buffer pointed to by iocb->aio_buf. The call returns immediately after the read request has been enqueued to the descriptor; the read may or may not have completed at the time the call returns.

If _POSIX_PRIORITIZED_IO is defined, and the descriptor supports it, then the enqueued operation is submitted at a priority equal to that of the calling process minus iocb->aio_reqprio.

The iocb->aio_lio_opcode argument is ignored by the () system call.

The iocb pointer may be subsequently used as an argument to () and () in order to determine return or error status for the enqueued operation while it is in progress.

If the request could not be enqueued (generally due to invalid arguments), then the call returns without having enqueued the request.

If the request is successfully enqueued, the value of iocb->aio_offset can be modified during the request as context, so this value must not be referenced after the request is enqueued.

The iocb->aio_sigevent structure can be used to request notification of the operation's completion as described in aio(4).


The Asynchronous I/O Control Block structure pointed to by iocb and the buffer that the iocb->aio_buf member of that structure references must remain valid until the operation has completed.

The asynchronous I/O control buffer iocb should be zeroed before the () call to avoid passing bogus context information to the kernel.

Modifications of the Asynchronous I/O Control Block structure or the buffer contents are not allowed while the request is queued.

If the file offset in iocb->aio_offset is past the offset maximum for iocb->aio_fildes, no I/O will occur.


The aio_read() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.




The aio_read() system call will fail if:

The request was not queued because of system resource limitations.
The asynchronous notification method in iocb->aio_sigevent.sigev_notify is invalid or not supported.
Asynchronous read operations on the file descriptor iocb->aio_fildes are unsafe and unsafe asynchronous I/O operations are disabled.

The following conditions may be synchronously detected when the aio_read() system call is made, or asynchronously, at any time thereafter. If they are detected at call time, aio_read() returns -1 and sets errno appropriately; otherwise the aio_return() system call must be called, and will return -1, and aio_error() must be called to determine the actual value that would have been returned in errno.

The iocb->aio_fildes argument is invalid.
The offset iocb->aio_offset is not valid, the priority specified by iocb->aio_reqprio is not a valid priority, or the number of bytes specified by iocb->aio_nbytes is not valid.
The file is a regular file, iocb->aio_nbytes is greater than zero, the starting offset in iocb->aio_offset is before the end of the file, but is at or beyond the iocb->aio_fildes offset maximum.

If the request is successfully enqueued, but subsequently cancelled or an error occurs, the value returned by the aio_return() system call is per the read(2) system call, and the value returned by the aio_error() system call is either one of the error returns from the read(2) system call, or one of:

The iocb->aio_fildes argument is invalid for reading.
The request was explicitly cancelled via a call to aio_cancel().
The offset iocb->aio_offset would be invalid.


aio_cancel(2), aio_error(2), aio_return(2), aio_suspend(2), aio_waitcomplete(2), aio_write(2), sigevent(3), siginfo(3), aio(4)


The aio_read() system call is expected to conform to the IEEE Std 1003.1 (“POSIX.1”) standard.


The aio_read() system call first appeared in FreeBSD 3.0.


This manual page was written by Terry Lambert <>.


Invalid information in iocb->_aiocb_private may confuse the kernel.

August 19, 2016 Debian