AIO_READ(2) System Calls Manual AIO_READ(2)

aio_read, aio_readv — asynchronous read from a file (REALTIME)

Standard C Library (libc, -lc)

#include <aio.h>

int
aio_read(struct aiocb *aiocbp);

#include <sys/uio.h>

int
aio_readv(struct aiocb *aiocbp);

The () system call allows the calling process to read aiocbp->aio_nbytes from the descriptor aiocbp->aio_fildes, beginning at the offset aiocbp->aio_offset, into the buffer pointed to by aiocbp->aio_buf.

The () system call allows the calling process to read data from multiple buffers in a single operation, performing vectored I/O similar to readv(2). For aio_readv(), the aiocbp->aio_iov field points to an array of struct iovec structures, and aiocbp->aio_iovcnt specifies the number of elements in the array.

The () function can also be used with () by setting the aio_lio_opcode field to LIO_READV.

For both functions, the calling process must ensure that the aiocbp->aio_buf. The call returns 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.

If _POSIX_PRIORITIZED_IO is defined, and the descriptor supports it, then the enqueued operation is submitted at a priority equal to that of the calling process minus aiocbp->aio_reqprio.

The aiocbp->aio_lio_opcode argument is ignored by the () and aio_readv() system calls.

The aiocbp 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 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 aiocbp->aio_offset can be modified during the request as context, so this value must not be referenced after the request is enqueued.

All asynchronous I/O control buffers contain a sigevent structure in the aio_sigevent field which can be used to request notification when an operation completes.

For SIGEV_KEVENT notifications, the sigev_notify field should be set to SIGEV_KEVENT, and its sigevent's sigev_signo field should contain the descriptor of the kqueue that the event should be attached to. The posted kevent will contain:

ident asynchronous I/O control buffer pointer
filter
udata value stored in aiocbp->aio_sigevent.sigev_value

The Asynchronous I/O Control Block structure pointed to by aiocbp and the buffer that the aiocbp->aio_buf member of that structure references must remain valid until the operation has completed. For this reason, use of auto (stack) variables for these objects is discouraged.

The asynchronous I/O control buffer aiocbp should be zeroed before the () or aio_readv() call to avoid passing bogus context information to the kernel.

Modifications of the Asynchronous I/O Control Block structure or the buffer contents after the request has been enqueued, but before the request has completed, are not allowed.

If the file offset in aiocbp->aio_offset is past the offset maximum for aiocbp->aio_fildes, no I/O will occur.

The aio_read() 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.

None.

The aio_read() and aio_readv() system calls will fail if:

[]
Because of system resource limitations, the request was not queued.
[]
The aio_read() and aio_readv() system calls are not supported.

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, aio_read() returns -1 and sets errno appropriately. Otherwise, the aio_return() system call must be called. It will return -1; aio_error() must then be called to determine the actual value that would have been returned in errno.

[]
Failed to transfer the asynchronous I/O control buffer aiocbp into the kernel's memory space, possibly due to an invalid or incorrect address.
[]
The aiocbp->aio_fildes argument is invalid.
[]
The offset aiocbp->aio_offset is not valid, the priority specified by aiocbp->aio_reqprio is not a valid priority, or the number of bytes specified by aiocbp->aio_nbytes is not valid.

The aio_readv() calls may also return one of the following errors:

[]
Part of the aio_iov points outside the process's allocated address space.
[]
aio_iovcnt was less than or equal to 0, or greater than AIO_UIO_MAXIOV.
[]
One of the iov_len values in the aio_iov array was negative.
[]
The sum of the iov_len values in the aio_iov array overflowed a 32-bit integer.
[]
The file is a regular file, aiocbp->aio_nbytes is greater than zero, the starting offset in aiocbp->aio_offset is before the end of the file, but is at or beyond the aiocbp->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:

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

aio_cancel(2), aio_error(2), aio_return(2), aio_suspend(2), aio_write(2), aio(4)

The aio_read() system call is expected to conform to the IEEE Std 1003.1 (“POSIX.1”) standard. The aio_readv() system call is an extension to the IEEE Std 1003.1 (“POSIX.1”) standard.

The aio_read() system call first appeared in FreeBSD 3.0. The aio_readv() system call first appeared in Mac OS X 27.0.

This manual page was written by Terry Lambert ⟨terry@whistle.com⟩.

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

November 17, 1998 Mac OS X 12