AIO_READ(2) FreeBSD System Calls Manual AIO_READ(2)
NAME
aio_read, aio_read2, aio_readv - asynchronous read from a file (REALTIME)
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <aio.h>
int
aio_read(struct aiocb *iocb);
int
aio_read2(struct aiocb *iocb, int flags);
#include <sys/uio.h>
int
aio_readv(struct aiocb *iocb);
DESCRIPTION
The aio_read(), aio_read2() and aio_readv() system calls allow the
calling process to read from the descriptor iocb->aio_fildes. The
syscalls return immediately after the read request has been enqueued to
the descriptor; the read may or may not have completed at the time the
call returns.
For the aio_read() and aio_readv() calls, the read begins at the offset
iocb->aio_offset.
The aio_read() call will read iocb->aio_nbytes into the buffer pointed to
by iocb->aio_buf, whereas aio_readv() reads the data into the
iocb->aio_iovcnt buffers specified by the members of the iocb->aio_iov
array. For aio_readv() the iovec structure is defined in readv(2).
The aio_read2() call takes the flags argument. If flags is passed as
zero, the call behaves identically to aio_read(). The following flags
can be specified by logical or:
AIO_OP2_FOFFSET The read occurs at the file descriptor offset, which is
advanced by the operation as done by the read(2)
syscall. The iocb->aio_offset field is ignored.
AIO_OP2_VECTORED Similar to aio_readv(), the read 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 could not be enqueued (generally due to invalid
arguments), then the call returns without having enqueued the request.
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_read(), aio_read2(), and aio_readv() functions return the value 0
if successful; otherwise the value -1 is returned and the global variable
errno is set to indicate the error.
DIAGNOSTICS
None.
ERRORS
The aio_read(), aio_read2(), and aio_readv() 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 read 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_read() or aio_readv() 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.
[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.
[EOVERFLOW] The file is a regular file, iocb->aio_nbytes is
greater than zero, the starting offset in
iocb->aio_offset is before the end of the file, but is
at or beyond the iocb->aio_fildes offset maximum.
If the request is successfully enqueued, but subsequently cancelled or an
error occurs, the value returned by the aio_return() system call is per
the read(2) system call, and the value returned by the aio_error() system
call is either one of the error returns from the read(2) system call, or
one of:
[EBADF] The iocb->aio_fildes argument is invalid for reading.
[ECANCELED] The request was explicitly cancelled 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), aio_write(2), sigevent(3), siginfo(3), aio(4)
STANDARDS
The aio_read() system call is expected to conform to the IEEE Std 1003.1
("POSIX.1") standard. The aio_read2() and aio_readv() system calls are
FreeBSD extensions, and should not be used in portable code.
HISTORY
The aio_read() system call first appeared in FreeBSD 3.0. The
aio_readv() system call first appeared in FreeBSD 13.0. The aio_read2()
system call first appeared in FreeBSD 14.1.
AUTHORS
This manual page was written by Terry Lambert <terry@whistle.com>.
BUGS
Invalid information in iocb->_aiocb_private may confuse the kernel.
FreeBSD 15.1-STABLE-HBSD March 27, 2025 AIO_READ(2)