7.21. ioctl VIDIOC_EXPBUF

7.21.1. Name

VIDIOC_EXPBUF - Export a buffer as a DMABUF file descriptor.

7.21.2. Synopsis

int ioctl(int fd, VIDIOC_EXPBUF, struct v4l2_exportbuffer *argp)

7.21.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct v4l2_exportbuffer.

7.21.4. Description

This ioctl is an extension to the memory mapping I/O method, therefore it is available only for V4L2_MEMORY_MMAP buffers. It can be used to export a buffer as a DMABUF file at any time after buffers have been allocated with the ioctl VIDIOC_REQBUFS ioctl.

To export a buffer, applications fill struct v4l2_exportbuffer. The type field is set to the same buffer type as was previously used with struct v4l2_requestbuffers type. Applications must also set the index field. Valid index numbers range from zero to the number of buffers allocated with ioctl VIDIOC_REQBUFS (struct v4l2_requestbuffers count) minus one. For the multi-planar API, applications set the plane field to the index of the plane to be exported. Valid planes range from zero to the maximal number of valid planes for the currently active format. For the single-planar API, applications must set plane to zero. Additional flags may be posted in the flags field. Refer to a manual for open() for details. Currently only O_CLOEXEC, O_RDONLY, O_WRONLY, and O_RDWR are supported. All other fields must be set to zero. In the case of multi-planar API, every plane is exported separately using multiple ioctl VIDIOC_EXPBUF calls.

After calling ioctl VIDIOC_EXPBUF the fd field will be set by a driver. This is a DMABUF file descriptor. The application may pass it to other DMABUF-aware devices. Refer to DMABUF importing for details about importing DMABUF files into V4L2 nodes. It is recommended to close a DMABUF file when it is no longer used to allow the associated memory to be reclaimed.

7.21.5. Examples

int buffer_export(int v4lfd, enum v4l2_buf_type bt, int index, int *dmafd)
{
    struct v4l2_exportbuffer expbuf;

    memset(&expbuf, 0, sizeof(expbuf));
    expbuf.type = bt;
    expbuf.index = index;
    if (ioctl(v4lfd, VIDIOC_EXPBUF, &expbuf) == -1) {
        perror("VIDIOC_EXPBUF");
        return -1;
    }

    *dmafd = expbuf.fd;

    return 0;
}

int buffer_export_mp(int v4lfd, enum v4l2_buf_type bt, int index,
    int dmafd[], int n_planes)
{
    int i;

    for (i = 0; i < n_planes; ++i) {
        struct v4l2_exportbuffer expbuf;

        memset(&expbuf, 0, sizeof(expbuf));
        expbuf.type = bt;
        expbuf.index = index;
        expbuf.plane = i;
        if (ioctl(v4lfd, VIDIOC_EXPBUF, &expbuf) == -1) {
            perror("VIDIOC_EXPBUF");
            while (i)
                close(dmafd[--i]);
            return -1;
        }
        dmafd[i] = expbuf.fd;
    }

    return 0;
}

v4l2_exportbuffer

struct v4l2_exportbuffer

__u32	type	Type of the buffer, same as struct v4l2_format type or struct v4l2_requestbuffers type, set by the application. See v4l2_buf_type
__u32	index	Number of the buffer, set by the application. This field is only used for memory mapping I/O and can range from zero to the number of buffers allocated with the ioctl VIDIOC_REQBUFS and/or ioctl VIDIOC_CREATE_BUFS ioctls.
__u32	plane	Index of the plane to be exported when using the multi-planar API. Otherwise this value must be set to zero.
__u32	flags	Flags for the newly created file, currently only O_CLOEXEC, O_RDONLY, O_WRONLY, and O_RDWR are supported, refer to the manual of open() for more details.
__s32	fd	The DMABUF file descriptor associated with a buffer. Set by the driver.
__u32	reserved[11]	Reserved field for future use. Drivers and applications must set the array to zero.

7.21.6. Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.

EINVAL

A queue is not in MMAP mode or DMABUF exporting is not supported or flags or type or index or plane fields are invalid.