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AIO_WRITE(2) System Calls Manual AIO_WRITE(2)

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

Standard C Library (libc, -lc)

#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);

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

The () 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 () and () 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).

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.

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.

The aio_write(), aio_write2(), and aio_writev() system calls will fail if:

[]
The request was not queued because of system resource limitations.
[]
Part of aio_iov points outside the process's allocated address space.
[]
The asynchronous notification method in iocb->aio_sigevent.sigev_notify is invalid or not supported.
[]
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.

[]
The iocb->aio_fildes argument is invalid, or is not opened for writing.
[]
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:

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

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

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.

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.

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

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

February 1, 2024 dev