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.