NAME

aio_write, aio_writev —

asynchronous write to a file (REALTIME)

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <aio.h>

int
aio_write(struct aiocb *iocb);

#include <sys/uio.h>

int
aio_writev(struct aiocb *iocb);

DESCRIPTION

The aio_write() and aio_writev() system calls allow the calling process to write to the descriptor iocb->aio_fildes. aio_write() will write iocb->aio_nbytes from the buffer pointed to by iocb->aio_buf, whereas aio_writev() gathers the data from the iocb->aio_iovcnt buffers specified by the members of the iocb->aio_iov array. Both syscalls return immediately after the write request has been enqueued to the descriptor; the write may or may not have completed at the time the call returns. If the request could not be enqueued, generally due to invalid arguments, the call returns without having enqueued the request.

For aio_writev() the iovec structure is defined in writev(2).

If O_APPEND is set for iocb->aio_fildes, write operations append to the file in the same order as the calls were made. If O_APPEND is not set for the file descriptor, the write operation will occur at the absolute position from the beginning of the file plus iocb->aio_offset.

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 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 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).

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.

The asynchronous I/O control buffer iocb should be zeroed before the aio_write() or aio_writev() system 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.

RETURN VALUES

The aio_write() and aio_writev() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

ERRORS

The aio_write() and aio_writev() system calls will fail if:

[EAGAIN]: The request was not queued because of system resource limitations.
[EFAULT]: Part of aio_iov points outside the process's allocated address space.
[EINVAL]: The asynchronous notification method in iocb->aio_sigevent.sigev_notify is invalid or not supported.
[EOPNOTSUPP]: Asynchronous write 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_write() or aio_writev() system call is made, or asynchronously, at any time thereafter. If they are detected at call time, aio_write() or aio_writev() 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, or is not opened for writing.
[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.

If the request is successfully enqueued, but subsequently canceled or an error occurs, the value returned by the aio_return() system call is per the write(2) system call, and the value returned by the aio_error() system call is either one of the error returns from the write(2) system call, or one of:

[EBADF]: The iocb->aio_fildes argument is invalid for writing.
[ECANCELED]: The request was explicitly canceled via a call to aio_cancel().
[EINVAL]: The offset iocb->aio_offset would be invalid.

STANDARDS

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

The aio_writev() system call is a FreeBSD extension, and should not be used in portable code.

HISTORY

The aio_write() system call first appeared in FreeBSD 3.0. The aio_writev() system call first appeared in FreeBSD 13.0.

AUTHORS

This manual page was written by Wes Peters <wes@softweyr.com>.

BUGS

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