xref: /freebsd/lib/libsys/aio_write.2 (revision 1b37af092211f9947d9f089f452232b791b9eafc)

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.Dd March 27, 2025
.Dt AIO_WRITE 2
.Os
.Sh NAME
.Nm aio_write ,
.Nm aio_write2 ,
.Nm aio_writev
.Nd asynchronous write to a file (REALTIME)
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In aio.h
.Ft int
.Fn aio_write "struct aiocb *iocb"
.Ft int
.Fn aio_write2 "struct aiocb *iocb" "int flags"
.In sys/uio.h
.Ft int
.Fn aio_writev "struct aiocb *iocb"
.Sh DESCRIPTION
The
.Fn aio_write ,
.Fn aio_write2 ,
and
.Fn aio_writev
system calls allow the calling process to write
to the descriptor
.Fa 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.
.Pp
The
.Fn aio_write
call will write
.Fa iocb->aio_nbytes
from the buffer pointed to by
.Fa iocb->aio_buf ,
whereas
.Fn aio_writev
gathers the data from the
.Fa iocb->aio_iovcnt
buffers specified by the members of the
.Fa iocb->aio_iov
array.
.Pp
If the request could not be enqueued, generally due
to invalid arguments, the call returns without having enqueued the
request.
.Pp
For
.Fn aio_writev
the
.Fa iovec
structure is defined in
.Xr writev 2 .
.Pp
If
.Dv O_APPEND
is set for
.Fa iocb->aio_fildes ,
write operations append to the file in the same order as the calls were
made.
If
.Dv O_APPEND
is not set for the file descriptor, the write operation for
.Fn aio_write
will occur at
the absolute position from the beginning of the file plus
.Fa iocb->aio_offset .
.Pp
The
.Fn aio_write2
call takes the
.Fa flags
argument.
If
.Fa flags
is passed as zero, the call behaves identically to
.Fn aio_write .
The following flags can be specified by logical or:
.Bl -tag -width AIO_OP2_VECTORED
.It AIO_OP2_FOFFSET
The write for non
.Dv O_APPEND
file descriptors occurs at the file descriptor offset,
which is advanced by the operation as done by the
.Xr write 2
syscall.
The
.Fa iocb->aio_offset
field is ignored.
.It AIO_OP2_VECTORED
Similar to
.Fn aio_writev ,
the write buffers are specified by the
.Fa aiocb->aio_iov
array.
.El
.Pp
The
.Fa iocb
pointer may be subsequently used as an argument to
.Fn aio_return
and
.Fn aio_error
in order to determine return or error status for the enqueued operation
while it is in progress.
.Pp
If the request is successfully enqueued, the value of
.Fa iocb->aio_offset
can be modified during the request as context, so this value must not
be referenced after the request is enqueued.
.Pp
The
.Fa iocb->aio_sigevent
structure can be used to request notification of the operation's
completion as described in
.Xr aio 4 .
.Sh RESTRICTIONS
The Asynchronous I/O Control Block structure pointed to by
.Fa iocb
and the buffer that the
.Fa iocb->aio_buf
member of that structure references must remain valid until the
operation has completed.
.Pp
The asynchronous I/O control buffer
.Fa iocb
should be zeroed before the
system calls to avoid passing bogus context information to the kernel.
.Pp
Modifications of the Asynchronous I/O Control Block structure or the
buffer contents are not allowed while the request is queued.
.Pp
If the file offset in
.Fa iocb->aio_offset
is past the offset maximum for
.Fa iocb->aio_fildes ,
no I/O will occur.
.Sh RETURN VALUES
.Rv -std aio_write aio_writev
.Sh ERRORS
The
.Fn aio_write ,
.Fn aio_write2 ,
and
.Fn aio_writev
system calls will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The request was not queued because of system resource limitations.
.It Bq Er EFAULT
Part of
.Fa aio_iov
points outside the process's allocated address space.
.It Bq Er EINVAL
The asynchronous notification method in
.Fa iocb->aio_sigevent.sigev_notify
is invalid or not supported.
.It Bq Er EOPNOTSUPP
Asynchronous write operations on the file descriptor
.Fa iocb->aio_fildes
are unsafe and unsafe asynchronous I/O operations are disabled.
.El
.Pp
The following conditions may be synchronously detected when the
.Fn aio_write ,
.Fn aio_write2 ,
or
.Fn 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
.Va errno
appropriately; otherwise the
.Fn aio_return
system call must be called, and will return -1, and
.Fn aio_error
must be called to determine the actual value that would have been
returned in
.Va errno .
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa iocb->aio_fildes
argument
is invalid, or is not opened for writing.
.It Bq Er EINVAL
The offset
.Fa iocb->aio_offset
is not valid, the priority specified by
.Fa iocb->aio_reqprio
is not a valid priority, or the number of bytes specified by
.Fa iocb->aio_nbytes
is not valid.
.El
.Pp
If the request is successfully enqueued, but subsequently canceled
or an error occurs, the value returned by the
.Fn aio_return
system call is per the
.Xr write 2
system call, and the value returned by the
.Fn aio_error
system call is either one of the error returns from the
.Xr write 2
system call, or one of:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa iocb->aio_fildes
argument
is invalid for writing.
.It Bq Er ECANCELED
The request was explicitly canceled via a call to
.Fn aio_cancel .
.It Bq Er EINVAL
The offset
.Fa iocb->aio_offset
would be invalid.
.El
.Sh SEE ALSO
.Xr aio_cancel 2 ,
.Xr aio_error 2 ,
.Xr aio_return 2 ,
.Xr aio_suspend 2 ,
.Xr aio_waitcomplete 2 ,
.Xr sigevent 3 ,
.Xr siginfo 3 ,
.Xr aio 4
.Sh STANDARDS
The
.Fn aio_write
system call
is expected to conform to the
.St -p1003.1
standard.
.Pp
The
.Fn aio_write2
and
.Fn aio_writev
system calls are
.Fx
extensions,
and should not be used in portable code.
.Sh HISTORY
The
.Fn aio_write
system call first appeared in
.Fx 3.0 .
The
.Fn aio_writev
system call first appeared in
.Fx 13.0 .
The
.Fn aio_write2
system call first appeared in
.Fx 14.1 .
.Sh AUTHORS
This manual page was written by
.An Wes Peters Aq Mt wes@softweyr.com .
.Sh BUGS
Invalid information in
.Fa iocb->_aiocb_private
may confuse the kernel.