AIO_READ(2)                  BSD System Calls Manual                 AIO_READ(2)

     aio_read — asynchronous read from a file (REALTIME)

     Standard C Library (libc, -lc)

     #include <aio.h>

     aio_read(struct aiocb *iocb);

     The aio_read() 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

     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 aio_read() system call.

     The iocb pointer may be subsequently used as an argument to aio_return()
     and aio_error() 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
     aio_read() 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


     The aio_read() system call will fail if:

     [EAGAIN]           The request was not queued because of system resource

     [EINVAL]           The asynchronous notification method in
                        iocb->aio_sigevent.sigev_notify is invalid or not

     [EOPNOTSUPP]       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.

     [EBADF]            The iocb->aio_fildes argument is invalid.

     [EINVAL]           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.

     [EOVERFLOW]        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:

     [EBADF]            The iocb->aio_fildes argument is invalid for reading.

     [ECANCELED]        The request was explicitly cancelled via a call to

     [EINVAL]           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.

BSD                              August 19, 2016                             BSD