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The ioctl VIDIOC_EXPBUF exports the dma memory as a filedescriptor. I want so transfer these memory over the network using sendfile (because it is really fast compared to send() or write()).

But sendfile() does not work on these filedescriptor. sendfile() returns with Error -1 and Errno is: Illegal seek.

How to get this working?

int buffer_export(int *vfd, int index, int *dmafd)
{
    struct v4l2_exportbuffer expbuf;
    memset(&expbuf, 0, sizeof(expbuf));
    expbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    expbuf.index = index;
    if (ioctl(*vfd, VIDIOC_EXPBUF, &expbuf) == -1) {
        perror("VIDIOC_EXPBUF");
        return -1;
    }
    *dmafd = expbuf.fd;
    return 0;
}

Here the transfer code.

int *vfd;

//Capture Image

int dmafd, err, bytes_send;
int frame_size = 721920;
off_t offset   = 0;

err = buffer_export( vfd, 0, &dmafd);
// returns 0

err = sendfile(socket_fd, dmafd ,&offset, frame_size);
//returns with -1

Update:

Browsing the sendfile source I came across the error occurs while checking the input fd.

retval = -ESPIPE; //=-29
if( !(in.file->f_mode & FMODE_PREAD) )
   goto in_fd; //error

f_mode=5; which is 

/* file is open for reading */
#define FMODE_READ      ((__force fmode_t)0x1)
/* file is open for writing */
#define FMODE_WRITE     ((__force fmode_t)0x2)
/* file is seekable */
#define FMODE_LSEEK     ((__force fmode_t)0x4)
/* file can be accessed using pread */
#define FMODE_PREAD     ((__force fmode_t)0x8)
/* file can be accessed using pwrite */

fails

john s.
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  • At a glance it would appear your device (graphics adapter?) driver does not implement enough for `sendfile()`. Are you ready for some driver hacking? Do you even have access to the source for it? – John Zwinck May 29 '17 at 13:26
  • @JohnZwinck I am using the v4l2 capture interface and got all the needed source. I do not know what to start with.. – john s. May 30 '17 at 06:12

0 Answers0