s->buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
s->buf.memory = V4L2_MEMORY_MMAP;
I must confess that I'm rather unfamiliar with V4L2 APIs. I fathom in the above two lines, the first one is to establish a buffer type, the second line is to pass the pointer of the video device dedicated memory to the buffer.
Problem is, I don't know if I'm making the right guess and I need someone to explain in detail how the second line works.
The second line establishes how the buffer will be used. The legal values are:
Table 3.5. enum v4l2_memory
V4L2_MEMORY_MMAP 1 The buffer is used for memory mapping I/O.
V4L2_MEMORY_USERPTR 2 The buffer is used for user pointer I/O.
V4L2_MEMORY_OVERLAY 3 [Not Yet Implemented]
V4L2_MEMORY_DMABUF 4 The buffer is used for DMA shared buffer I/O.
The code you included does not actually allocate the buffers. Here is a somewhat more complete code fragment showing the mmaping of buffers:
CLEAR(req);
req.count = 2;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;
xioctl(fd, VIDIOC_REQBUFS, &req);
buffers = calloc(req.count, sizeof(*buffers));
for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
CLEAR(buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers;
xioctl(fd, VIDIOC_QUERYBUF, &buf);
buffers[n_buffers].length = buf.length;
buffers[n_buffers].start = v4l2_mmap(NULL, buf.length,
PROT_READ | PROT_WRITE, MAP_SHARED,
fd, buf.m.offset);
if (MAP_FAILED == buffers[n_buffers].start) {
perror("mmap");
exit(EXIT_FAILURE);
}
}
