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     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 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

     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

     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


     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 error.




     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.


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


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


     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.