---
name: "aio_read"
section: "2"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/aio_read"
---

# aio\_read(2)

asynchronous read from a file (REALTIME)

## NAME

**aio\_read** – asynchronous read from a file (REALTIME)

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<aio.h\>**

*int*\

**aio\_read**(*struct aiocb \*iocb*);

## DESCRIPTION

The **aio\_read**() system call allows the calling process to read *iocb-\>aio\_nbytes* from the descriptor *iocb-\>aio\_fildes* beginning at the offset *iocb-\>aio\_offset* into the buffer pointed to by *iocb-\>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 *iocb-\>aio\_reqprio*.

The *iocb-\>aio\_lio\_opcode* argument is ignored by the **aio\_read**() system call.

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.

## 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. For this reason, use of auto (stack) variables for these objects is discouraged.

The asynchronous I/O control buffer *iocb* 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 *iocb-\>aio\_offset* is past the offset maximum for *iocb-\>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.

- \[ENOSYS\]

  The **aio\_read**() system call is not supported.

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 *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)](/read/2.md) system call, and the value returned by the **aio\_error**() system call is either one of the error returns from the [read(2)](/read/2.md) 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_cancel/2.md), [aio\_error(2)](/aio_error/2.md), [aio\_return(2)](/aio_return/2.md), [aio\_suspend(2)](/aio_suspend/2.md), [aio\_waitcomplete(2)](/aio_waitcomplete/2.md), [aio\_write(2)](/aio_write/2.md), [aio(4)](/aio/4.md)

## STANDARDS

The **aio\_read**() system call is expected to conform to the IEEE Std 1003.1 (“POSIX.1”) standard.

## HISTORY

The **aio\_read**() system call first appeared in FreeBSD 3.0.

## AUTHORS

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

## BUGS

Invalid information in *iocb-\>\_aiocb\_private* may confuse the kernel.