NAME

aio_write, aio_write2, aio_writev — asynchronous write to a file (REALTIME)

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <aio.h>

int
aio_write(struct aiocb *iocb);

int
aio_write2(struct aiocb *iocb, int flags);

#include <sys/uio.h>

int
aio_writev(struct aiocb *iocb);

DESCRIPTION

The aio_write(), aio_write2(), and aio_writev() system calls allow the calling process to write to the descriptor iocb->aio_fildes. The 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.

The aio_write() call 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.

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 for aio_write() will occur at the absolute position from the beginning of the file plus iocb->aio_offset.

The aio_write2() call takes the flags argument. If flags is passed as zero, the call behaves identically to aio_write(). The following flags can be specified by logical or:

AIO_OP2_FOFFSET: The write for non O_APPEND file descriptors occurs at the file descriptor offset, which is advanced by the operation as done by the write(2) syscall. The iocb->aio_offset field is ignored.
AIO_OP2_VECTORED: Similar to aio_writev(), the write buffers are specified by the aiocb->aio_iov array.

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 system calls 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(), aio_write2(), 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(), aio_write2(), or aio_writev() system call is made, or asynchronously, at any time thereafter. If they are detected at call time, the calls return -1 and set 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_write2() and aio_writev() system calls are FreeBSD extensions, 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. The aio_write2() system call first appeared in FreeBSD 14.1.

AUTHORS

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

BUGS

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