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AIO_READ(2)               FreeBSD System Calls Manual              AIO_READ(2)

     aio_read - asynchronous read from a file (REALTIME)

     Standard C Library (libc, -lc)

     #include <time.h>
     #include <aio.h>

     aio_read(struct aiocb *iocb);

     The aio_read() function 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 is ignored by the aio_read() 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 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.  For this reason, use of
     auto (stack) variables for these objects is discouraged.

     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 after the request has been enqueued, but before the
     request has completed, are not allowed.

     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() function will fail if:

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

     [ENOSYS]           The aio_read() call is not supported.

     The following conditions may be synchronously detected when the
     aio_read() 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() function must be called, and
     will return -1, and aio_error() must be called to determine the actual
     calue that would have been returned in errno.

     [EBADF]            iocb-_aio_fildes 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() function is per the
     read(2) call, and the value returned by the aio_error() function is
     either one of the error returns from the read(2) call, or one of:

     [EBADF]            iocb-_aio_fildes is invalid for reading.

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

     [EINVAL]           The offset iocb-_aio_offset would be invalid.

     The aio_read() call is expected to conform to the IEEE Std 1003.2
     (``POSIX.2'') standard.

     The aio_read function first appeared in FreeBSD 3.0.

     This manual page was written by Terry Lambert <>.

     The value of iocb-_aio_offset is ignored.  Invalid information in
     iocb-__aiocb_private may confuse the kernel.

FreeBSD 11.0-PRERELEASE        November 17, 1998       FreeBSD 11.0-PRERELEASE


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