Ubuntu Manpage: io_submit - submit asynchronous I/O blocks for processing

NAME

       io_submit - submit asynchronous I/O blocks for processing

LIBRARY

       Standard C library (libc, -lc)

       Alternatively, Asynchronous I/O library (libaio, -laio); see NOTES.

SYNOPSIS

       #include <linux/aio_abi.h>          /* Defines needed types */

       int io_submit(aio_context_t ctx_id, long nr, struct iocb **iocbpp);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION

Note: this page describes the raw Linux system call interface. The wrapper function
provided by libaio uses a different type for the ctx_id argument. See NOTES.

The io_submit() system call queues nr I/O request blocks for processing in the AIO context
ctx_id. The iocbpp argument should be an array of nr AIO control blocks, which will be
submitted to context ctx_id.

The iocb (I/O control block) structure defined in linux/aio_abi.h defines the parameters
that control the I/O operation.

#include <linux/aio_abi.h>

struct iocb {
__u64 aio_data;
__u32 PADDED(aio_key, aio_rw_flags);
__u16 aio_lio_opcode;
__s16 aio_reqprio;
__u32 aio_fildes;
__u64 aio_buf;
__u64 aio_nbytes;
__s64 aio_offset;
__u64 aio_reserved2;
__u32 aio_flags;
__u32 aio_resfd;
};

The fields of this structure are as follows:

aio_data
This data is copied into the data field of the io_event structure upon I/O
completion (see io_getevents(2)).

aio_key
This is an internal field used by the kernel. Do not modify this field after an
io_submit() call.

aio_rw_flags
This defines the R/W flags passed with structure. The valid values are:

RWF_APPEND (since Linux 4.16)
Append data to the end of the file. See the description of the flag of the
same name in pwritev2(2) as well as the description of O_APPEND in open(2).
The aio_offset field is ignored. The file offset is not changed.

RWF_DSYNC (since Linux 4.13)
Write operation complete according to requirement of synchronized I/O data
integrity. See the description of the flag of the same name in pwritev2(2)
as well the description of O_DSYNC in open(2).

RWF_HIPRI (since Linux 4.13)
High priority request, poll if possible

RWF_NOWAIT (since Linux 4.14)
Don't wait if the I/O will block for operations such as file block
allocations, dirty page flush, mutex locks, or a congested block device
inside the kernel. If any of these conditions are met, the control block is
returned immediately with a return value of -EAGAIN in the res field of the
io_event structure (see io_getevents(2)).

RWF_SYNC (since Linux 4.13)
Write operation complete according to requirement of synchronized I/O file
integrity. See the description of the flag of the same name in pwritev2(2)
as well the description of O_SYNC in open(2).

aio_lio_opcode
This defines the type of I/O to be performed by the iocb structure. The valid
values are defined by the enum defined in linux/aio_abi.h:

enum {
IOCB_CMD_PREAD = 0,
IOCB_CMD_PWRITE = 1,
IOCB_CMD_FSYNC = 2,
IOCB_CMD_FDSYNC = 3,
IOCB_CMD_POLL = 5,
IOCB_CMD_NOOP = 6,
IOCB_CMD_PREADV = 7,
IOCB_CMD_PWRITEV = 8,
};

aio_reqprio
This defines the requests priority.

aio_fildes
The file descriptor on which the I/O operation is to be performed.

aio_buf
This is the buffer used to transfer data for a read or write operation.

aio_nbytes
This is the size of the buffer pointed to by aio_buf.

aio_offset
This is the file offset at which the I/O operation is to be performed.

aio_flags
This is the set of flags associated with the iocb structure. The valid values are:

IOCB_FLAG_RESFD
Asynchronous I/O control must signal the file descriptor mentioned in
aio_resfd upon completion.

IOCB_FLAG_IOPRIO (since Linux 4.18)
Interpret the aio_reqprio field as an IOPRIO_VALUE as defined by
linux/ioprio.h.

aio_resfd
The file descriptor to signal in the event of asynchronous I/O completion.

RETURN VALUE

       On success, io_submit() returns the number of iocbs submitted (which may be less than  nr,
       or 0 if nr is zero).  For the failure return, see NOTES.

ERRORS

       EAGAIN Insufficient resources are available to queue any iocbs.

       EBADF  The file descriptor specified in the first iocb is invalid.

       EFAULT One of the data structures points to invalid data.

       EINVAL The  AIO  context  specified by ctx_id is invalid.  nr is less than 0.  The iocb at
              *iocbpp[0] is not properly initialized, the operation specified is invalid for  the
              file descriptor in the iocb, or the value in the aio_reqprio field is invalid.

       ENOSYS io_submit() is not implemented on this architecture.

       EPERM  The  aio_reqprio  field  is  set with the class IOPRIO_CLASS_RT, but the submitting
              context does not have the CAP_SYS_ADMIN capability.

VERSIONS

       The asynchronous I/O system calls first appeared in Linux 2.5.

STANDARDS

       io_submit() is Linux-specific and should not be used in programs that are intended  to  be
       portable.

NOTES

       glibc  does  not  provide  a  wrapper  for  this  system  call.  You could invoke it using
       syscall(2).  But instead, you probably  want  to  use  the  io_submit()  wrapper  function
       provided by libaio.

       Note  that the libaio wrapper function uses a different type (io_context_t) for the ctx_id
       argument.  Note also that  the  libaio  wrapper  does  not  follow  the  usual  C  library
       conventions  for  indicating  errors:  on  error  it  returns  a negated error number (the
       negative of one of the values listed in ERRORS).   If  the  system  call  is  invoked  via
       syscall(2),  then  the return value follows the usual conventions for indicating an error:
       -1, with errno set to a (positive) value that indicates the error.