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Technical Reference: Base Operating System and Extensions, Volume 1

aio_read or aio_read64 Subroutine


Reads asynchronously from a file.


Standard C Library (libc.a)


#include <aio.h>

int aio_read( FileDescriptor, aiocbp)
int FileDescriptor;
struct aiocb *aiocbp;

int aio_read64( FileDescriptor, aiocbp)
int FileDescriptor;
struct aiocb64 *aiocbp;


The aio_read subroutine reads asynchronously from a file. Specifically, the aio_read subroutine reads from the file associated with the FileDescriptor parameter into a buffer.

The aio_read64 subroutine is similar to the aio_read subroutine execpt that it takes an aiocb64 reference parameter. This allows the aio_read64 subroutine to specify offsets in excess of OFF_MAX (2 gigbytes minus 1).

In the large file enabled programming environment, aio_read is redefined to be aio_read64 .

The details of the read are provided by information in the aiocb structure, which is pointed to by the aiocbp parameter. This information includes the following fields:

aio_buf Indicates the buffer to use.
aio_nbytes Indicates the number of bytes to read.

When the read request has been queued, the aio_read subroutine updates the file pointer specified by the aio_whence and aio_offset fields in the aiocb structure as if the requested I/O were already completed. It then returns to the calling program. The aio_whence and aio_offset fields have the same meaning as the whence and offset parameters in the lseek (lseek, llseek or lseek64 Subroutine) subroutine. The subroutine ignores them for file objects that are not capable of seeking.

If an error occurs during the call, the read request is not queued. To determine the status of a request, use the aio_error (aio_error or aio_error64 Subroutine) subroutine.

To have the calling process receive the SIGIO signal when the I/O operation completes, set the AIO_SIGNAL bit in the aio_flag field in the aiocb structure.

Note: The SIGIO signal is replaced by real-time signals when they are available. The event structure in the aiocb structure is currently not in use but is included for future compatibility.


FileDescriptor Identifies the object to be read as returned from a call to open.
aiocbp Points to the asynchronous I/O control block structure associated with the I/O operation. The aiocb and the aiocb64 structures are defined in the aio.h file and contains the following members:

int            aio_whence
off_t          aio_offset
char           *aio_buf
size_t         aio_nbytes
int            aio_flag
aio_handle_t   aio_handle 

Execution Environment

The aio_read and aio_read64 subroutines can be called from the process environment only.

Return Values

When the read request queues successfully, the aio_read subroutine returns a value of 0. Otherwise, it returns a value of -1 and sets the global variable errno to identify the error.

Return codes can be set to the following errno values:

EAGAIN Indicates that the system resources required to queue the request are not available. Specifically, the transmit queue may be full, or the maximum number of opens may be reached.
EBADF Indicates that the FileDescriptor parameter is not valid.
EFAULT Indicates that the address specified by the aiocbp parameter is not valid.
EINVAL Indicates that the aio_whence field does not have a valid value, or that the resulting pointer is not valid.

Note: Other error codes defined in the sys/errno.h file can be returned by aio_error if an error during the I/O operation is encountered.

Implementation Specifics

The aio_read and aio_read64 subroutines are part of Base Operating System (BOS) Runtime.

Related Information

The aio_cancel or aio_cancel64 (aio_cancel or aio_cancel64 Subroutine) subroutine, aio_error or aio_error64 (aio_error or aio_error64 Subroutine) subroutine, aio_return or aio_return64 (aio_return or aio_return64 Subroutine) subroutine, aio_suspend or aio_suspend64 (aio_suspend or aio_suspend64 Subroutine) subroutine, aio_write (aio_write or aio_write64 Subroutine) subroutine, lio_listio or lio_listo64 (lio_listio or lio_listio64 Subroutine) subroutine.

The Asynchronous I/O Overview and the Communications I/O Subsystem: Programming Introduction in AIX 5L Version 5.1 Kernel Extensions and Device Support Programming Concepts.

The Input and Output Handling Programmer's Overview in AIX 5L Version 5.1 General Programming Concepts: Writing and Debugging Programs describes the files, commands, and subroutines used for low-level, stream, terminal, and asynchronous I/O interfaces.

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