AIO_WRITE(2) FreeBSD System Calls Manual AIO_WRITE(2)
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.
SEE ALSO
aio_cancel(2), aio_error(2), aio_return(2), aio_suspend(2),
aio_waitcomplete(2), sigevent(3), siginfo(3), aio(4)
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.
FreeBSD 15.1-STABLE-HBSD March 27, 2025 AIO_WRITE(2)