V4L2驱动的移植与应用（三）

三、V4L2的demo

capture.c是官方示例程序。

capture.c 程序中的 process_image 函数：

capture.c 程序主要是用来演示怎样使用 v4l2 接口，并没有对采集到的视频帧数据做任何实际的处理，仅仅用 process_image 函数表示了处理图像的代码位置。

process_image 函数只有一个参数，就是存储视频帧的内存的地址指针，但是在真正的应用中，通常还需要知道该指针指向的数据的大小。

因此可以修改函数，改成 void process_image ( const void * p, int len ) ，但是每次调用 process_image 的时候，第 2 个参数该传递什么值？

考虑到程序中对 buffer 的定义

struct buffer {

void * start;

size_t length};

如果将 buffer.length 作为第 2 个参数传递到修改后的 process_image 函数中，这样做是不正确的。 process_image 需要的第二个参数应该是每帧图像的大小，仔细阅读代码后会发现， buffer.length 并不一定就等于图像帧的大小。 (buffer 的大小，还需要考虑其他的一些因素，比如内存对齐等 )。

capture.c只是一个示例程序，仅仅是演示怎样使用v4l2中最基本的接口。尤其是在main函数中的那几个函数调用，表明了在使用v4l2时的最基本的一个流程，包括 open_device，init_device，start_capturing，mainloop，stop_capturing，uninit_device，close_device。在写程序的时候，可以充分的利用这几个基本模块，把他们分散在不同的代码位置上，灵活的调用，有兴趣的可以看一下gstreamer中v4l2src的源代码或者其他的大型程序的相关部分。

总之一句话，capture.c仅仅是一个演示程序，不要局限于它的代码结构，要灵活的使用。

下面是capture.c的源代码：

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <assert.h>

#include <getopt.h> /* getopt_long() */

#include <fcntl.h> /* low-level i/o */

#include <unistd.h>

#include <errno.h>

#include <malloc.h>

#include <sys/stat.h>

#include <sys/types.h>

#include <sys/time.h>

#include <sys/mman.h>

#include <sys/ioctl.h>

#include <asm/types.h> /* for videodev2.h */

#include <linux/videodev2.h>

#define CLEAR(x) memset (&(x), 0, sizeof (x))

typedef enum {

IO_METHOD_READ, IO_METHOD_MMAP, IO_METHOD_USERPTR,

} io_method;

struct buffer {

void * start;

size_t length;//buffer's length is different from cap_image_size

};

static char * dev_name = NULL;

static io_method io = IO_METHOD_MMAP;//IO_METHOD_READ;//IO_METHOD_MMAP;

static int fd = -1;

struct buffer * buffers = NULL;

static unsigned int n_buffers = 0;

static FILE * outf = 0;

static unsigned int cap_image_size = 0;//to keep the real image size!!

static void errno_exit(const char * s) {

fprintf(stderr, "%s error %d, %s/n", s, errno, strerror(errno));

exit(EXIT_FAILURE);

}

static int xioctl(int fd, int request, void * arg) {

int r;

r = ioctl(fd, request, arg);

while (-1 == r && EINTR == errno);

return r;

}

static void process_image(const void * p, int len) {

// static char[115200] Outbuff ;

fputc('.', stdout);

if (len > 0) {

fputc('.', stdout);

fwrite(p, 1, len, outf);

}

fflush(stdout);

}

static int read_frame(void) {

struct v4l2_buffer buf;

unsigned int i;

switch (io) {

case IO_METHOD_READ:

if (-1 == read(fd, buffers[0].start, buffers[0].length)) {

switch (errno) {

case EAGAIN:

return 0;

case EIO:

/* Could ignore EIO, see spec. */

/* fall through */

default:

errno_exit("read");

}

// printf("length = %d/r", buffers[0].length);

// process_image(buffers[0].start, buffers[0].length);

printf("image_size = %d,/t IO_METHOD_READ buffer.length=%d/r",

cap_image_size, buffers[0].length);

process_image(buffers[0].start, cap_image_size);

break;

case IO_METHOD_MMAP:

CLEAR (buf);

buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

buf.memory = V4L2_MEMORY_MMAP;

if (-1 == xioctl(fd, VIDIOC_DQBUF, &buf)) {

switch (errno) {

case EAGAIN:

return 0;

case EIO:

/* Could ignore EIO, see spec. */

/* fall through */

default:

errno_exit("VIDIOC_DQBUF");

}

assert(buf.index < n_buffers);

// printf("length = %d/r", buffers[buf.index].length);

// process_image(buffers[buf.index].start, buffers[buf.index].length);

printf("image_size = %d,/t IO_METHOD_MMAP buffer.length=%d/r",

cap_image_size, buffers[0].length);

process_image(buffers[0].start, cap_image_size);

if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))

errno_exit("VIDIOC_QBUF");

break;

case IO_METHOD_USERPTR:

CLEAR (buf);

buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

buf.memory = V4L2_MEMORY_USERPTR;

if (-1 == xioctl(fd, VIDIOC_DQBUF, &buf)) {

switch (errno) {

case EAGAIN:

return 0;

case EIO:

/* Could ignore EIO, see spec. */

/* fall through */

default:

errno_exit("VIDIOC_DQBUF");

}

for (i = 0; i < n_buffers; ++i)

if (buf.m.userptr == (unsigned long) buffers[i].start && buf.length

== buffers[i].length)

break;

assert(i < n_buffers);

// printf("length = %d/r", buffers[i].length);

// process_image((void *) buf.m.userptr, buffers[i].length);

printf("image_size = %d,/t IO_METHOD_USERPTR buffer.length=%d/r",

cap_image_size, buffers[0].length);

process_image(buffers[0].start, cap_image_size);

if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))

errno_exit("VIDIOC_QBUF");

break;

}

return 1;

}

static void mainloop(void) {

unsigned int count;

count = 100;

while (count-- > 0) {

for (;;) {

fd_set fds;

struct timeval tv;

int r;

FD_ZERO(&fds);

FD_SET(fd, &fds);

/* Timeout. */

tv.tv_sec = 2;

tv.tv_usec = 0;

r = select(fd + 1, &fds, NULL, NULL, &tv);

if (-1 == r) {

if (EINTR == errno)

continue;

errno_exit("select");

}

if (0 == r) {

fprintf(stderr, "select timeout/n");

exit(EXIT_FAILURE);

}

if (read_frame())

break;

/* EAGAIN - continue select loop. */

}

static void stop_capturing(void) {

enum v4l2_buf_type type;

switch (io) {

case IO_METHOD_READ:

/* Nothing to do. */

break;

case IO_METHOD_MMAP:

case IO_METHOD_USERPTR:

type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == xioctl(fd, VIDIOC_STREAMOFF, &type))

errno_exit("VIDIOC_STREAMOFF");

break;

}

static void start_capturing(void) {

unsigned int i;

enum v4l2_buf_type type;

switch (io) {

case IO_METHOD_READ:

/* Nothing to do. */

break;

case IO_METHOD_MMAP:

for (i = 0; i < n_buffers; ++i) {

struct v4l2_buffer buf;

CLEAR (buf);

buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

buf.memory = V4L2_MEMORY_MMAP;

buf.index = i;

if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))

errno_exit("VIDIOC_QBUF");

}

type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == xioctl(fd, VIDIOC_STREAMON, &type))

errno_exit("VIDIOC_STREAMON");

break;

case IO_METHOD_USERPTR:

for (i = 0; i < n_buffers; ++i) {

struct v4l2_buffer buf;

CLEAR (buf);

buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

buf.memory = V4L2_MEMORY_USERPTR;

buf.index = i;

buf.m.userptr = (unsigned long) buffers[i].start;

buf.length = buffers[i].length;

if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))

errno_exit("VIDIOC_QBUF");

}

type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == xioctl(fd, VIDIOC_STREAMON, &type))

errno_exit("VIDIOC_STREAMON");

break;

}

static void uninit_device(void) {

unsigned int i;

switch (io) {

case IO_METHOD_READ:

free(buffers[0].start);

break;

case IO_METHOD_MMAP:

for (i = 0; i < n_buffers; ++i)

if (-1 == munmap(buffers[i].start, buffers[i].length))

errno_exit("munmap");

break;

case IO_METHOD_USERPTR:

for (i = 0; i < n_buffers; ++i)

free(buffers[i].start);

break;

}

free(buffers);

}

static void init_read(unsigned int buffer_size) {

buffers = calloc(1, sizeof(*buffers));

if (!buffers) {

fprintf(stderr, "Out of memory/n");

exit(EXIT_FAILURE);

}

buffers[0].length = buffer_size;

buffers[0].start = malloc(buffer_size);

if (!buffers[0].start) {

fprintf(stderr, "Out of memory/n");

exit(EXIT_FAILURE);

}

static void init_mmap(void) {

struct v4l2_requestbuffers req;

CLEAR (req);

req.count = 4;

req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

req.memory = V4L2_MEMORY_MMAP;

if (-1 == xioctl(fd, VIDIOC_REQBUFS, &req)) {

if (EINVAL == errno) {

fprintf(stderr, "%s does not support "

"memory mapping/n", dev_name);

exit(EXIT_FAILURE);

} else {

errno_exit("VIDIOC_REQBUFS");

}

if (req.count < 2) {

fprintf(stderr, "Insufficient buffer memory on %s/n", dev_name);

exit(EXIT_FAILURE);

}

buffers = calloc(req.count, sizeof(*buffers));

if (!buffers) {

fprintf(stderr, "Out of memory/n");

exit(EXIT_FAILURE);

}

for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {

struct v4l2_buffer buf;

CLEAR (buf);

buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

buf.memory = V4L2_MEMORY_MMAP;

buf.index = n_buffers;

if (-1 == xioctl(fd, VIDIOC_QUERYBUF, &buf))

errno_exit("VIDIOC_QUERYBUF");

buffers[n_buffers].length = buf.length;

buffers[n_buffers].start = mmap(NULL /* start anywhere */, buf.length,

PROT_READ | PROT_WRITE /* required */,

MAP_SHARED /* recommended */, fd, buf.m.offset);

if (MAP_FAILED == buffers[n_buffers].start)

errno_exit("mmap");

}

static void init_userp(unsigned int buffer_size) {

struct v4l2_requestbuffers req;

unsigned int page_size;

page_size = getpagesize();

buffer_size = (buffer_size + page_size - 1) & ~(page_size - 1);

CLEAR (req);

req.count = 4;

req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

req.memory = V4L2_MEMORY_USERPTR;

if (-1 == xioctl(fd, VIDIOC_REQBUFS, &req)) {

if (EINVAL == errno) {

fprintf(stderr, "%s does not support "

"user pointer i/o/n", dev_name);

exit(EXIT_FAILURE);

} else {

errno_exit("VIDIOC_REQBUFS");

}

buffers = calloc(4, sizeof(*buffers));

if (!buffers) {

fprintf(stderr, "Out of memory/n");

exit(EXIT_FAILURE);

}

for (n_buffers = 0; n_buffers < 4; ++n_buffers) {

buffers[n_buffers].length = buffer_size;

buffers[n_buffers].start = memalign(/* boundary */page_size,

buffer_size);

if (!buffers[n_buffers].start) {

fprintf(stderr, "Out of memory/n");

exit(EXIT_FAILURE);

}

static void init_device(void) {

struct v4l2_capability cap;

struct v4l2_cropcap cropcap;

struct v4l2_crop crop;

struct v4l2_format fmt;

unsigned int min;

if (-1 == xioctl(fd, VIDIOC_QUERYCAP, &cap)) {

if (EINVAL == errno) {

fprintf(stderr, "%s is no V4L2 device/n", dev_name);

exit(EXIT_FAILURE);

} else {

errno_exit("VIDIOC_QUERYCAP");

}

if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {

fprintf(stderr, "%s is no video capture device/n", dev_name);

exit(EXIT_FAILURE);

}

switch (io) {

case IO_METHOD_READ:

if (!(cap.capabilities & V4L2_CAP_READWRITE)) {

fprintf(stderr, "%s does not support read i/o/n", dev_name);

exit(EXIT_FAILURE);

}

break;

case IO_METHOD_MMAP:

case IO_METHOD_USERPTR:

if (!(cap.capabilities & V4L2_CAP_STREAMING)) {

fprintf(stderr, "%s does not support streaming i/o/n", dev_name);

exit(EXIT_FAILURE);

}

break;

}

//not all capture support crop!!!!!!!

/* Select video input, video standard and tune here. */

printf("-#-#-#-#-#-#-#-#-#-#-#-#-#-/n");

CLEAR (cropcap);

cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (0 == xioctl(fd, VIDIOC_CROPCAP, &cropcap)) {

crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

#ifndef CROP_BY_JACK

crop.c = cropcap.defrect; /* reset to default */

#else

crop.c.left = cropcap.defrect.left;

crop.c.top = cropcap.defrect.top;

crop.c.width = 352;

crop.c.height = 288;

#endif

printf("----->has ability to crop!!/n");

printf("cropcap.defrect = (%d, %d, %d, %d)/n", cropcap.defrect.left,

cropcap.defrect.top, cropcap.defrect.width,

cropcap.defrect.height);

if (-1 == xioctl(fd, VIDIOC_S_CROP, &crop)) {

switch (errno) {

case EINVAL:

/* Cropping not supported. */

break;

default:

/* Errors ignored. */

break;

}

printf("-----!!but crop to (%d, %d, %d, %d) Failed!!/n",

crop.c.left, crop.c.top, crop.c.width, crop.c.height);

} else {

printf("----->sussess crop to (%d, %d, %d, %d)/n", crop.c.left,

crop.c.top, crop.c.width, crop.c.height);

}

} else {

/* Errors ignored. */

printf("!! has no ability to crop!!/n");

}

printf("-#-#-#-#-#-#-#-#-#-#-#-#-#-/n");

printf("/n");

crop finished!

//set the format

CLEAR (fmt);

fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

fmt.fmt.pix.width = 640;

fmt.fmt.pix.height = 480;

//V4L2_PIX_FMT_YVU420, V4L2_PIX_FMT_YUV420 — Planar formats with 1/2 horizontal and vertical chroma resolution, also known as YUV 4:2:0

//V4L2_PIX_FMT_YUYV — Packed format with 1/2 horizontal chroma resolution, also known as YUV 4:2:2

fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;//V4L2_PIX_FMT_YUV420;//V4L2_PIX_FMT_YUYV;

fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;

{

printf("-#-#-#-#-#-#-#-#-#-#-#-#-#-/n");

printf("=====will set fmt to (%d, %d)--", fmt.fmt.pix.width,

fmt.fmt.pix.height);

if (fmt.fmt.pix.pixelformat == V4L2_PIX_FMT_YUYV) {

printf("V4L2_PIX_FMT_YUYV/n");

} else if (fmt.fmt.pix.pixelformat == V4L2_PIX_FMT_YUV420) {

printf("V4L2_PIX_FMT_YUV420/n");

} else if (fmt.fmt.pix.pixelformat == V4L2_PIX_FMT_NV12) {

printf("V4L2_PIX_FMT_NV12/n");

}

if (-1 == xioctl(fd, VIDIOC_S_FMT, &fmt))

errno_exit("VIDIOC_S_FMT");

{

printf("=====after set fmt/n");

printf(" fmt.fmt.pix.width = %d/n", fmt.fmt.pix.width);

printf(" fmt.fmt.pix.height = %d/n", fmt.fmt.pix.height);

printf(" fmt.fmt.pix.sizeimage = %d/n", fmt.fmt.pix.sizeimage);

cap_image_size = fmt.fmt.pix.sizeimage;

printf(" fmt.fmt.pix.bytesperline = %d/n", fmt.fmt.pix.bytesperline);

printf("-#-#-#-#-#-#-#-#-#-#-#-#-#-/n");

printf("/n");

}

cap_image_size = fmt.fmt.pix.sizeimage;

/* Note VIDIOC_S_FMT may change width and height. */

printf("-#-#-#-#-#-#-#-#-#-#-#-#-#-/n");

/* Buggy driver paranoia. */

min = fmt.fmt.pix.width * 2;

if (fmt.fmt.pix.bytesperline < min)

fmt.fmt.pix.bytesperline = min;

min = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height;

if (fmt.fmt.pix.sizeimage < min)

fmt.fmt.pix.sizeimage = min;

printf("After Buggy driver paranoia/n");

printf(" >>fmt.fmt.pix.sizeimage = %d/n", fmt.fmt.pix.sizeimage);

printf(" >>fmt.fmt.pix.bytesperline = %d/n", fmt.fmt.pix.bytesperline);

printf("-#-#-#-#-#-#-#-#-#-#-#-#-#-/n");

printf("/n");

switch (io) {

case IO_METHOD_READ:

init_read(fmt.fmt.pix.sizeimage);

break;

case IO_METHOD_MMAP:

init_mmap();

break;

case IO_METHOD_USERPTR:

init_userp(fmt.fmt.pix.sizeimage);

break;

}

static void close_device(void) {

if (-1 == close(fd))

errno_exit("close");

fd = -1;

}

static void open_device(void) {

struct stat st;

if (-1 == stat(dev_name, &st)) {

fprintf(stderr, "Cannot identify '%s': %d, %s/n", dev_name, errno,

strerror(errno));

exit(EXIT_FAILURE);

}

if (!S_ISCHR(st.st_mode)) {

fprintf(stderr, "%s is no device/n", dev_name);

exit(EXIT_FAILURE);

}

fd = open(dev_name, O_RDWR /* required */| O_NONBLOCK, 0);

if (-1 == fd) {

fprintf(stderr, "Cannot open '%s': %d, %s/n", dev_name, errno,

strerror(errno));

exit(EXIT_FAILURE);

}

static void usage(FILE * fp, int argc, char ** argv) {

fprintf(fp, "Usage: %s [options]/n/n"

"Options:/n"

"-d | --device name Video device name [/dev/video0]/n"

"-h | --help Print this message/n"

"-m | --mmap Use memory mapped buffers/n"

"-r | --read Use read() calls/n"

"-u | --userp Use application allocated buffers/n"

"", argv[0]);

}

static const char short_options[] = "d:hmru";

static const struct option long_options[] = { { "device", required_argument,

NULL, 'd' }, { "help", no_argument, NULL, 'h' }, { "mmap", no_argument,

NULL, 'm' }, { "read", no_argument, NULL, 'r' }, { "userp",

no_argument, NULL, 'u' }, { 0, 0, 0, 0 } };

int main(int argc, char ** argv) {

dev_name = "/dev/video0";

outf = fopen("out.yuv", "wb");

for (;;) {

int index;

int c;

c = getopt_long(argc, argv, short_options, long_options, &index);

if (-1 == c)

break;

switch (c) {

case 0: /* getopt_long() flag */

break;

case 'd':

dev_name = optarg;

break;

case 'h':

usage(stdout, argc, argv);

exit(EXIT_SUCCESS);

case 'm':

io = IO_METHOD_MMAP;

break;

case 'r':

io = IO_METHOD_READ;

break;

case 'u':

io = IO_METHOD_USERPTR;

break;

default:

usage(stderr, argc, argv);

exit(EXIT_FAILURE);

}

open_device();

init_device();

start_capturing();

mainloop();

printf("/n");

stop_capturing();

fclose(outf);

uninit_device();

close_device();

exit(EXIT_SUCCESS);

return 0;

}

（完）

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