io_submit(2): submit asynchronous I/O

io_submit — submit asynchronous I/O blocks for processing

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

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_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.

Conforming to

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 function 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.

Colophon

This page is part of release 5.04 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

Referenced By

aio(7), ev(3), fcntl(2), io_cancel(2), io_destroy(2), io_getevents(2), ioping(1), io_setup(2), stress-ng(1), syscalls(2), systemd.exec(5).

2019-10-10 Linux Programmer's Manual