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;
do
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;
}
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