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NAME

     aio_read -- asynchronous read from a file (REALTIME)

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <aio.h>

     int
     aio_read(struct aiocb *iocb);

DESCRIPTION

     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
     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 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.

RESTRICTIONS

     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.

RETURN VALUES

     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.

DIAGNOSTICS

     None.

ERRORS

     The aio_read() system call will fail if:

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

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

     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
                        aio_cancel().

     [EINVAL]           The offset iocb->aio_offset would be invalid.

SEE ALSO

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

STANDARDS

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

HISTORY

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

AUTHORS

     This manual page was written by Terry Lambert <terry@whistle.com>.

BUGS

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