aio_read or aio_read64 Subroutine

The aio_read or aio_read64 subroutine includes information for the POSIX AIO aio_read subroutine (as defined in the IEEE std 1003.1-2001), and the Legacy AIO aio_read subroutine.

POSIX AIO aio_read Subroutine

Purpose

Asynchronously reads a file.

Library

Standard C Library (libc.a)

Syntax

#include <aio.h>

int aio_read (aiocbp)
struct aiocb *aiocbp;

Description

The aio_read subroutine reads aio_nbytes from the file associated with aio_fildes into the buffer pointed to by aio_buf. The subroutine returns when the read request has been initiated or queued to the file or device (even when the data cannot be delivered immediately).

The aiocbp value may be used as an argument to the aio_error and aio_return subroutines in order to determine the error status and return status, respectively, of the asynchronous operation while it is proceeding. If an error condition is encountered during queuing, the function call returns without having initiated or queued the request. The requested operation takes place at the absolute position in the file as given by aio_offset , as if the lseek subroutine were called immediately prior to the operation with an offset equal to aio_offset and a whence equal to SEEK_SET. After a successful call to enqueue an asynchronous I/O operation, the value of the file offset for the file is unspecified.

The aio_lio_opcode field is ignored by the aio_read subroutine.

If prioritized I/O is supported for this file, the asynchronous operation is submitted at a priority equal to the scheduling priority of the process minus aiocbp->aio_reqprio.

The aiocbp parameter points to an aiocb structure. If the buffer pointed to by aio_buf or the control block pointed to by aiocbp becomes an illegal address prior to asynchronous I/O completion, the behavior is undefined.

Simultaneous asynchronous operations using the same aiocbp produce undefined results.

If synchronized I/O is enabled on the file associated with aio_fildes, the behavior of this subroutine is according to the definitions of synchronized I/O data integrity completion and synchronized I/O file integrity completion.

For any system action that changes the process memory space while an asynchronous I/O is outstanding, the result of that action is undefined.

For regular files, no data transfer occurs past the offset maximum established in the open file description.

If you use the aio_read or aio_read64 subroutine with a file descriptor obtained from a call to the shm_open subroutine, it will fail with EINVAL.

Parameters

aiocb Structure

int               aio_fildes
off_t             aio_offset
char             *aio_buf
size_t            aio_nbytes
int               aio_reqprio
struct sigevent   aio_sigevent
int               aio_lio_opcode

Execution Environment

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

Return Values

The aio_read subroutine returns 0 to the calling process if the I/O operation is successfully queued. Otherwise, it returns a -1 and sets the errno global variable to indicate the error.

Error Codes

Each of the following conditions may be detected synchronously at the time of the call to the aio_read subroutine, or asynchronously. If any of the conditions below are detected synchronously, the aio_read subroutine returns -1 and sets the errno global variable to the corresponding value. If any of the conditions below are detected asynchronously, the return status of the asynchronous operation is set to -1, and the error status of the asynchronous operation is set to the corresponding value.

Purpose

Legacy AIO aio_read Subroutine

Reads asynchronously from a file.

Library

Standard C Library (libc.a)

Syntax

#include <aio.h>

Description

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 .

If you use the aio_read or aio_read64 subroutine with a file descriptor obtained from a call to the shm_open subroutine, it will fail with EINVAL.

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:

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.

#define _AIO_AIX_SOURCE
#include <sys/aio.h>

->xlc ... -D_AIO_AIX_SOURCE ... legacy_aio_program.c 

Since prioritized I/O is not supported at this time, the aio_reqprio field of the structure is not presently used.

Parameters

aiocb Structure

struct aiocb
{
       int                 aio_whence;
       off_t               aio_offset;
       char                *aio_buf;
       ssize_t             aio_return;
       int                 aio_errno;
       size_t              aio_nbytes;
       union {
              int          reqprio;
              struct {
                     int   version:8;
                     int   priority:8;
                     int   cache_hint:16;
              } ext;
       } aio_u1;
       int                 aio_flag;
       int                 aio_iocpfd;
       aio_handle_t        aio_handle;
}

#define aio_reqprio        aio_u1.reqprio
#define aio_version        aio_u1.ext.version
#define aio_priority       aio_u1.ext.priority
#define aio_cache_hint     aio_u1.ext.cache_hint

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:

When using I/O Completion Ports with AIO Requests, return codes can also be set to the following errno values: