NAME

aio_read — asynchronous read from a file (REALTIME)

LIBRARY

POSIX Real-time Library (librt, -lrt)

SYNOPSIS

#include <aio.h>

int
aio_read(struct aiocb *aiocbp);

DESCRIPTION

The aio_read() 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 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 aio_read() system call.

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

RESTRICTIONS

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 aio_read() 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.

RETURN VALUES

The aio_read() function returns 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() system call will fail if:

[EAGAIN]

The request was not queued because of system resource limitations.

The following conditions may be synchronously detected when the aio_read() 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, and will return -1, and aio_error() must be called to determine the actual value that would have been returned in errno.

[EBADF]

The aiocbp->aio_fildes argument is invalid.

[EINVAL]

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.

[EOVERFLOW]

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:

[EBADF]

The aiocbp->aio_fildes argument is invalid for reading.

[ECANCELED]

The request was explicitly cancelled via a call to aio_cancel().

[EINVAL]

The offset aiocbp->aio_offset would be invalid.

SEE ALSO

siginfo(2), aio(3)

STANDARDS

The aio_read() system call is expected to conform to the IEEE Std 1003.1-2001 (“POSIX.1”) standard.

HISTORY

The aio_read() system call first appeared in NetBSD 5.0.